Difference between DNA, Gene and Chromosome

DNA vs Gene vs Chromosome
Within the cell, DNA is complexed with histone proteins called chromatin. At the time of cell division, the chromatin condensed to form chromosome.

Gene vs Chromosome

In order to understand it clearly, Let us see how chromosomes are formed from double stranded DNA.
Level 1: Nucleotides are the building blocks of DNA. Nucleotides are joined by phosphodiester bond to form a DNA strand.

Level 2: Double stranded DNA helix is formed by the hydrogen bonding between nitrogenous bases of two strands. Double helical DNA is 2 nm or 20 A0 .

Level 3: Formation of chromatin:
This DNA is wrapped around histone proteins forming “beads on a string appearance”. Each unit comprising of DNA wrapped around histone octamer (8 histones) is called nucleosome. Now the structure is called chromatin (DNA + histone protein complex). This is the 10nm chromatin fibril.

Further condensation forms 30 nm chromatin fibril followed by non condensed loop and condensed loop formation ultimately forming the metaphase chromosome of 1400nm. Maximum condensation of chromosome occurs at the metaphase stage. Therefore the term ‘chromosome’ often refers to the metaphase chromosome.
Gene
‘Gene is a segment of DNA that codes for a functional protein and RNAs like tRNA, rRNA or ribozymes’.
Gene vs Chromosome


Gene
Chromosome
Gene is a segment of DNA on the chromosome that codes for a functional protein and RNAs like tRNA, rRNA or ribozymes’.
Chromosome is the structure formed by the condensation of chromatin during cell division.
Genes basically refers to the DNA fragment that directs the synthesis of a protein.
Chromosome consists of long DNA strand wrapped around histone proteins.
Gene is segment of DNA molecule made up of nucleotides.
A mitotic chromosome consists of a centromere, pair of telomeres and an origin of replication.
The position of each gene on a chromosome is called loci.
Chromosome is a long strand of DNA containing many genes.
Gene contain coding sequence called exons and non-coding sequence called introns on the chromosome that directs synthesis of a protein.
Chromosome is a long DNA strand  containing both coding (genes) and non-coding DNA (junk DNA or spacer DNA) between genes.
DNA vs Gene vs Chromosome
Within the cell, DNA is complexed with histone proteins called chromatin. At the time of cell division, the chromatin condensed to form chromosome.

Gene vs Chromosome

In order to understand it clearly, Let us see how chromosomes are formed from double stranded DNA.
Level 1: Nucleotides are the building blocks of DNA. Nucleotides are joined by phosphodiester bond to form a DNA strand.

Level 2: Double stranded DNA helix is formed by the hydrogen bonding between nitrogenous bases of two strands. Double helical DNA is 2 nm or 20 A0 .

Level 3: Formation of chromatin:
This DNA is wrapped around histone proteins forming “beads on a string appearance”. Each unit comprising of DNA wrapped around histone octamer (8 histones) is called nucleosome. Now the structure is called chromatin (DNA + histone protein complex). This is the 10nm chromatin fibril.

Further condensation forms 30 nm chromatin fibril followed by non condensed loop and condensed loop formation ultimately forming the metaphase chromosome of 1400nm. Maximum condensation of chromosome occurs at the metaphase stage. Therefore the term ‘chromosome’ often refers to the metaphase chromosome.
Gene
‘Gene is a segment of DNA that codes for a functional protein and RNAs like tRNA, rRNA or ribozymes’.
Gene vs Chromosome


Gene
Chromosome
Gene is a segment of DNA on the chromosome that codes for a functional protein and RNAs like tRNA, rRNA or ribozymes’.
Chromosome is the structure formed by the condensation of chromatin during cell division.
Genes basically refers to the DNA fragment that directs the synthesis of a protein.
Chromosome consists of long DNA strand wrapped around histone proteins.
Gene is segment of DNA molecule made up of nucleotides.
A mitotic chromosome consists of a centromere, pair of telomeres and an origin of replication.
The position of each gene on a chromosome is called loci.
Chromosome is a long strand of DNA containing many genes.
Gene contain coding sequence called exons and non-coding sequence called introns on the chromosome that directs synthesis of a protein.
Chromosome is a long DNA strand  containing both coding (genes) and non-coding DNA (junk DNA or spacer DNA) between genes.
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Difference between Genomics and Proteomics

Genomics vs Proteomics
Difference between genomics and proteomics

Genomics
Proteomics
Genomics is the study of genome of an organism. Genome represents the entire genes of an organism or a cell type
Proteomics is the study of proteome of an organism. Proteome refers to the entire protein set coded by the genome of an organism or a cell type
Genomics include mapping, sequencing and analysis of genome
Proteomics include characterization of all proteins of an organism or study of structure and function of proteins
Genomics can be broadly classified into structural and functional genomics
a)Structural genomics: is the study of the structure of all genes and its relative position on the chromosome
b)Functional genomics: study of function of all genes or the role of these genes in regulating metabolic activities of the cell
Proteomics can be classified into structural functional and expression proteomics
a)Structural proteomics: is the study of the structure of proteins and their location in the cell
b)Functional proteomics: study of function of all proteins which primarily include protein-protein interaction and interaction of proteins with other biomolecules
c) Expression proteomics: is the study of identification and quantification or expression level  of proteins of the cell at different developmental stages or at different environmental conditions
Techniques in genomics include
a) gene sequencing strategies like directed gene sequencing, whole genome short gun sequencing,
b)Construction of ESTs (expressed sequence Tags),
c) Identification of single nucleotide polymorphisms (SNPs),
d) Analysis and interpretation of sequenced data using different databases and software.
Techniques in proteomics include
a) protein extraction, electrophoretic separation, digestion of separated proteins into small fragments using trypsin, mass spectroscopy to find out amino acid sequences and finally protein identification using standard databases.
b) Protein 3D structure prediction using software.
c) Protein expression study using protein microarray.
Thrust areas in Genomics: Genome Sequencing projects of many organisms including Human Genome Project
Proteome database development like SWISS-2D PAGE and software development for computer aided drug design
Genomics vs Proteomics
Difference between genomics and proteomics

Genomics
Proteomics
Genomics is the study of genome of an organism. Genome represents the entire genes of an organism or a cell type
Proteomics is the study of proteome of an organism. Proteome refers to the entire protein set coded by the genome of an organism or a cell type
Genomics include mapping, sequencing and analysis of genome
Proteomics include characterization of all proteins of an organism or study of structure and function of proteins
Genomics can be broadly classified into structural and functional genomics
a)Structural genomics: is the study of the structure of all genes and its relative position on the chromosome
b)Functional genomics: study of function of all genes or the role of these genes in regulating metabolic activities of the cell
Proteomics can be classified into structural functional and expression proteomics
a)Structural proteomics: is the study of the structure of proteins and their location in the cell
b)Functional proteomics: study of function of all proteins which primarily include protein-protein interaction and interaction of proteins with other biomolecules
c) Expression proteomics: is the study of identification and quantification or expression level  of proteins of the cell at different developmental stages or at different environmental conditions
Techniques in genomics include
a) gene sequencing strategies like directed gene sequencing, whole genome short gun sequencing,
b)Construction of ESTs (expressed sequence Tags),
c) Identification of single nucleotide polymorphisms (SNPs),
d) Analysis and interpretation of sequenced data using different databases and software.
Techniques in proteomics include
a) protein extraction, electrophoretic separation, digestion of separated proteins into small fragments using trypsin, mass spectroscopy to find out amino acid sequences and finally protein identification using standard databases.
b) Protein 3D structure prediction using software.
c) Protein expression study using protein microarray.
Thrust areas in Genomics: Genome Sequencing projects of many organisms including Human Genome Project
Proteome database development like SWISS-2D PAGE and software development for computer aided drug design
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Difference between Male Urethra and Female Urethra

Human excretory system consists of two kidneys, a pair of ureters, a urinary bladder and a urethra.
The urethra is a membranous tube starting from the neck of the bladder up to the external urethral orifice.
Male Urethra vs Female Urethra

Male Urethra

Female Urethra
It is about 20 cm in length
It is about 4 cm in length
It has three regions: prostatic urethra(3-4cm), membranous(1cm) and penial(15cm)
It is not differentiated into regions
It opens out at the tip of the penis by urinogenital aperture
It opens by urinary aperture in front of vaginal aperture.
It carries urine as well as semen to the exterior
It carries urine only to the exterior
Human excretory system consists of two kidneys, a pair of ureters, a urinary bladder and a urethra.
The urethra is a membranous tube starting from the neck of the bladder up to the external urethral orifice.
Male Urethra vs Female Urethra

Male Urethra

Female Urethra
It is about 20 cm in length
It is about 4 cm in length
It has three regions: prostatic urethra(3-4cm), membranous(1cm) and penial(15cm)
It is not differentiated into regions
It opens out at the tip of the penis by urinogenital aperture
It opens by urinary aperture in front of vaginal aperture.
It carries urine as well as semen to the exterior
It carries urine only to the exterior
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Difference between Ammonotelism, Ureotelism and Uricotelism

 Ammonia, urea and uric acid are the main nitrogenous waste product in different animals. The major role of excretory system is the elimination of nitrogenous wastes. In animals, nitrogenous excretion is grouped into three categories based on the form in which nitrogenous wastes are removed;

1) Ammonotelism 2)Ureotelism and 3)Uricotelism
Difference between Ammonotelism, Ureotelism and Uricotelism:

Ammonotelism, Ureotelism and Uricotelism

Ammonotelism vs Ureotelism vs Uricotelism

Ammonotelism
Ureotelism
Uricotelism
Chief nitrogenous waste is ammonia
Chief nitrogenous waste is urea
Chief nitrogenous waste is uric acid
 Solubility: Easily  dissolves in liver cells by deamination of aminoacids
It is less soluble in water
It is almost insoluble in water.
Ammonia is formed by deamination of amino acids in liver cells
Urea is formed in liver and is produced from an amino acid arginine. Arginase is the enzyme involved. 
Potassium ureate reacts with water and carbon dioxide to form uric acid
Ammonia is very toxic
Urea is less toxic
Uric acid has very low toxicity
 Ammonia, urea and uric acid are the main nitrogenous waste product in different animals. The major role of excretory system is the elimination of nitrogenous wastes. In animals, nitrogenous excretion is grouped into three categories based on the form in which nitrogenous wastes are removed;

1) Ammonotelism 2)Ureotelism and 3)Uricotelism
Difference between Ammonotelism, Ureotelism and Uricotelism:

Ammonotelism, Ureotelism and Uricotelism

Ammonotelism vs Ureotelism vs Uricotelism

Ammonotelism
Ureotelism
Uricotelism
Chief nitrogenous waste is ammonia
Chief nitrogenous waste is urea
Chief nitrogenous waste is uric acid
 Solubility: Easily  dissolves in liver cells by deamination of aminoacids
It is less soluble in water
It is almost insoluble in water.
Ammonia is formed by deamination of amino acids in liver cells
Urea is formed in liver and is produced from an amino acid arginine. Arginase is the enzyme involved. 
Potassium ureate reacts with water and carbon dioxide to form uric acid
Ammonia is very toxic
Urea is less toxic
Uric acid has very low toxicity
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Difference between FISH and GISH

Fluorescence in situ hybridization (FISH) vs Genomic in situ hybridization (GISH)
In situ hybridization techniques, such as fluorescence in situ hybridization (FISH) and genomic in situ hybridization (GISH), is widely used to identify chromosome morphologies and sequences, amount and distribution of various types of chromatin in chromosomes, and genome organization during the metaphase stage of meiosis.
Difference between FIAH and GISH
Fluorescence in situ hybridization (FISH) is a laboratory technique for detecting and locating a specific DNA sequence or a gene on a chromosome within a person’s genome.
The technique relies on exposing chromosomes to a small DNA sequence called a probe that has a fluorescent molecule attached to it.
FISH helps scientist to visualize the location of particular gene to check for a variety of chromosomal abnormalities.
FISH procedure
Summary of steps
1. Cells cultured, harvested, prepared on microscopic slides and are denatured (now DNA is single stranded for probe attachment)
Cells on metaphase stage of division is selected (as maximum condensation on metaphase stage)
2. Fluorescently labeled hybridization probe is added
(The hybridization probe is a short fragment of DNA that has a fluorescent dye attached that enable scientist to visualize the site of probe attachment. A typical FISH probe would be 10 - 100 kb long)
3. If the DNA corresponding to the probe is present in the sample, then the fluorescently labeled probe will attach to the DNA and will be visible under a fluorescent microscope.
4. This allows deletions (no fluorescent spot at the expected position) and rearrangements (spot present, but in an unexpected chromosomal location) to be detected. Thus helps in diagnosis of genetic diseases.
FISH example
Figure. An example of FISH-treated metaphase chromosomes: Here, chromosomes 1, 2, and 4 were labeled yellow with FISH and the other chromosomes were stained red. Translocations between yellow and red chromosomes are detected. The left picture represents a normal cell (the numbers in the figure indicate chromosome numbers) and the right picture is an example of reciprocal translocation with two bi-color chromosomes (indicated by two arrows). http://www.rerf.jp/dept/genetics/fish_e.html
Applications:
  • To detect chromosomal aberrations or abnormalities in humans
  • To study somaclonal variation* in plants
  • In chromosome mapping
Genome in situ hybridization (GISH) is an in situ hybridization method that uses whole genomic DNA as a probe to study the relationships, divergence and evolution of the genome between different species.
clip_image006

Summary of Steps
  • Extraction of total genomic DNA of one of the species of interest (to be used as probe)
  • Chromosome preparations of the species 2 being studied
  • Repeated sequences in both species anneal quickly than the unique sequences of the genome
  • Thus helps in assessing genome relationship between species
Applications
  • GISH allows characterization of the genome and chromosome of hybrid plats and recombinant breeding lines
  • Helps in assessing phylogenetic relationship between different species of plants
Reference
· Devi, J., Ko, J. M., & Seo, B. B. (2005). FISH and GISH: Modern cytogenetic techniques. Indian Journal of Biotechnology, 4(3), 307.
· Younis, A., Ramzan, F., Hwang, Y. J., & Lim, K. B. (2015). FISH and GISH: molecular cytogenetic tools and their applications in ornamental plants. Plant Cell Reports, 34(9), 1477-1488.
· http://www.geneticseducation.nhs.uk/laboratory-process-and-testing-techniques/fish
· Snowdon, R. J., Köhler, A., Köhler, W., & Friedt, W. (1999). FISHing for new rapeseed lines: the application of molecular cytogenetic techniques to Brassica breeding. New horizons for an old crop. Proc 10th Int Rapeseed Congr. The Regional Institute, Gosford, NSW, Australia. (GISH image credit)
*Somaclonal variation: genetic variation in plants raised by tissue culture.
Fluorescence in situ hybridization (FISH) vs Genomic in situ hybridization (GISH)
In situ hybridization techniques, such as fluorescence in situ hybridization (FISH) and genomic in situ hybridization (GISH), is widely used to identify chromosome morphologies and sequences, amount and distribution of various types of chromatin in chromosomes, and genome organization during the metaphase stage of meiosis.
Difference between FIAH and GISH
Fluorescence in situ hybridization (FISH) is a laboratory technique for detecting and locating a specific DNA sequence or a gene on a chromosome within a person’s genome.
The technique relies on exposing chromosomes to a small DNA sequence called a probe that has a fluorescent molecule attached to it.
FISH helps scientist to visualize the location of particular gene to check for a variety of chromosomal abnormalities.
FISH procedure
Summary of steps
1. Cells cultured, harvested, prepared on microscopic slides and are denatured (now DNA is single stranded for probe attachment)
Cells on metaphase stage of division is selected (as maximum condensation on metaphase stage)
2. Fluorescently labeled hybridization probe is added
(The hybridization probe is a short fragment of DNA that has a fluorescent dye attached that enable scientist to visualize the site of probe attachment. A typical FISH probe would be 10 - 100 kb long)
3. If the DNA corresponding to the probe is present in the sample, then the fluorescently labeled probe will attach to the DNA and will be visible under a fluorescent microscope.
4. This allows deletions (no fluorescent spot at the expected position) and rearrangements (spot present, but in an unexpected chromosomal location) to be detected. Thus helps in diagnosis of genetic diseases.
FISH example
Figure. An example of FISH-treated metaphase chromosomes: Here, chromosomes 1, 2, and 4 were labeled yellow with FISH and the other chromosomes were stained red. Translocations between yellow and red chromosomes are detected. The left picture represents a normal cell (the numbers in the figure indicate chromosome numbers) and the right picture is an example of reciprocal translocation with two bi-color chromosomes (indicated by two arrows). http://www.rerf.jp/dept/genetics/fish_e.html
Applications:
  • To detect chromosomal aberrations or abnormalities in humans
  • To study somaclonal variation* in plants
  • In chromosome mapping
Genome in situ hybridization (GISH) is an in situ hybridization method that uses whole genomic DNA as a probe to study the relationships, divergence and evolution of the genome between different species.
clip_image006

Summary of Steps
  • Extraction of total genomic DNA of one of the species of interest (to be used as probe)
  • Chromosome preparations of the species 2 being studied
  • Repeated sequences in both species anneal quickly than the unique sequences of the genome
  • Thus helps in assessing genome relationship between species
Applications
  • GISH allows characterization of the genome and chromosome of hybrid plats and recombinant breeding lines
  • Helps in assessing phylogenetic relationship between different species of plants
Reference
· Devi, J., Ko, J. M., & Seo, B. B. (2005). FISH and GISH: Modern cytogenetic techniques. Indian Journal of Biotechnology, 4(3), 307.
· Younis, A., Ramzan, F., Hwang, Y. J., & Lim, K. B. (2015). FISH and GISH: molecular cytogenetic tools and their applications in ornamental plants. Plant Cell Reports, 34(9), 1477-1488.
· http://www.geneticseducation.nhs.uk/laboratory-process-and-testing-techniques/fish
· Snowdon, R. J., Köhler, A., Köhler, W., & Friedt, W. (1999). FISHing for new rapeseed lines: the application of molecular cytogenetic techniques to Brassica breeding. New horizons for an old crop. Proc 10th Int Rapeseed Congr. The Regional Institute, Gosford, NSW, Australia. (GISH image credit)
*Somaclonal variation: genetic variation in plants raised by tissue culture.
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Difference between Allopatric and Sympatric Speciation

Allopatric vs Sympatric speciation
The process of origin of new species is called speciation. The formation of new species from existing species can occur in two ways; sympatrically or allopatrically.

Allopatric speciation (‘Allo’: different, ‘patris’: country)
  • Speciation occurs when the population becomes separated by geographical barriers like mountains, rivers etc.
  • Thus the chance of interbreeding between these populations is greatly reduced.
  • Each population acquires mutations by natural selection to adapt to the new environment. After a long time, reproductive isolation sets in separating two populations into two species.
  • Geographical isolation leads to reproductive isolation and speciation.
  • This is the most common form of speciation.

clip_image002
Sympatric speciation (‘Sym’: same or together, ‘patris’: country)
  • It is the formation of two or more species from a single ancestral species all occupying the same geographical area.
  • In sympatric speciation, the populations are not geographically separated.
  • Sympatric speciation often occurs through polyploidy. A diploid individual cannot interbreed with tetraploid individual leading to reproductive isolation.
  • This type of speciation is rare and occurs often in plants as self fertilization and polyploidy is common in plants compared to animals.
Allopatric vs Sympatric speciation
The process of origin of new species is called speciation. The formation of new species from existing species can occur in two ways; sympatrically or allopatrically.

Allopatric speciation (‘Allo’: different, ‘patris’: country)
  • Speciation occurs when the population becomes separated by geographical barriers like mountains, rivers etc.
  • Thus the chance of interbreeding between these populations is greatly reduced.
  • Each population acquires mutations by natural selection to adapt to the new environment. After a long time, reproductive isolation sets in separating two populations into two species.
  • Geographical isolation leads to reproductive isolation and speciation.
  • This is the most common form of speciation.

clip_image002
Sympatric speciation (‘Sym’: same or together, ‘patris’: country)
  • It is the formation of two or more species from a single ancestral species all occupying the same geographical area.
  • In sympatric speciation, the populations are not geographically separated.
  • Sympatric speciation often occurs through polyploidy. A diploid individual cannot interbreed with tetraploid individual leading to reproductive isolation.
  • This type of speciation is rare and occurs often in plants as self fertilization and polyploidy is common in plants compared to animals.
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Difference between Microevolution, Macroevolution and Megaevolution with examples

Evolution is a slow, step by step irreversible change or transformation, from simple to more complex or advanced, occurring in time and space.
The process of evolution can be broadly classified into two
1. Inorganic evolution: Which refers to the evolution of the living world.
2. Organic evolution: Which refers to the evolution of the non-living world.
examples of Micro maco and mega evolution
Microevolution
1. It is the evolutionary change happening below the level of species.
2. Microevolution produces differences between different populations of a species (within species).
3. Microevolution over time may result in the formation of sub-species or geographical races.
4. The most common driving force of microevolution is natural selection in response to different environments.
Examples: Variety of beak shape among the finches of Galapagos island.
                Colour change in peppered moth in England during Industrial Revolution.
                Evolution of DDT resistant Insect populations.
                Evolution of antibiotic resistant bacteria*.
All examples are changes occurring within a population in response to changing environment.
Macroevolution
1. It is the evolutionary change happening at or above the level of species.
2. Macroevolution occurs between species.
3. Macroevolution over time may result in the formation of new species, genera, families, orders etc..
4. The evolutionary changes occurring in macroevolution is massive, involving macromutation, genetic divergence and adaptive radiation, producing major changes above species level.
Examples: Evolution of modern horse (Equus equus) from the genus Eohippus, which was less than 11 inches in height.
                  Evolution of birds from dinosaurs**
                  Evolution of tetrapods (animals with four legs) from fish**
Megaevolution
1. It is the large scale evolutionary change happening at the level of class and phyla or division.
2. Megaevolution is a rare phenomenon, occurred only very few times in the entire evolutionary history of life.
3. Megaevolution may result in the formation of new biological organizational system, as new classes or divisions. 
4. The actual process leading to megaevolution is not known as the phenomenon is extremely rare.
Examples: Origin of plants, animals and microorganisms
                  All these divisions are unique with distinctive biological organization.
                  Bacterial cells (Prokaryotes), leading to Eukaryotic cells with true nucleus and organelles
                  Evolution of Fungi, plants and animals from unicellular organisms
        Evolution of amphibians from fishes.***
                  Evolution of reptiles from amphibians.***
                 Evolution of birds and mammals from reptiles.***
All the above groups belongs to different classes as Pisces, Amphibia, Reptilia, Aves and Mammalia respectively.
Remember, all the above terms are abstract and citing a clear cut example is often impossible. Evolutionary biologists greatly differ in their opinions.
Reference
*Russell, P., Hertz, P., & McMillan, B. (2013). Biology: the dynamic science. Cengage Learning.
**Starr, C., Evers, C., & Starr, L. (2010). Biology: Concepts and Applications Without Physiology. Cengage Learning.
***Yadev, B. N., & Kumar, D. (2000). Vertebrate zoology and evolution. Daya Books.
Image credits:
Evolution is a slow, step by step irreversible change or transformation, from simple to more complex or advanced, occurring in time and space.
The process of evolution can be broadly classified into two
1. Inorganic evolution: Which refers to the evolution of the living world.
2. Organic evolution: Which refers to the evolution of the non-living world.
examples of Micro maco and mega evolution
Microevolution
1. It is the evolutionary change happening below the level of species.
2. Microevolution produces differences between different populations of a species (within species).
3. Microevolution over time may result in the formation of sub-species or geographical races.
4. The most common driving force of microevolution is natural selection in response to different environments.
Examples: Variety of beak shape among the finches of Galapagos island.
                Colour change in peppered moth in England during Industrial Revolution.
                Evolution of DDT resistant Insect populations.
                Evolution of antibiotic resistant bacteria*.
All examples are changes occurring within a population in response to changing environment.
Macroevolution
1. It is the evolutionary change happening at or above the level of species.
2. Macroevolution occurs between species.
3. Macroevolution over time may result in the formation of new species, genera, families, orders etc..
4. The evolutionary changes occurring in macroevolution is massive, involving macromutation, genetic divergence and adaptive radiation, producing major changes above species level.
Examples: Evolution of modern horse (Equus equus) from the genus Eohippus, which was less than 11 inches in height.
                  Evolution of birds from dinosaurs**
                  Evolution of tetrapods (animals with four legs) from fish**
Megaevolution
1. It is the large scale evolutionary change happening at the level of class and phyla or division.
2. Megaevolution is a rare phenomenon, occurred only very few times in the entire evolutionary history of life.
3. Megaevolution may result in the formation of new biological organizational system, as new classes or divisions. 
4. The actual process leading to megaevolution is not known as the phenomenon is extremely rare.
Examples: Origin of plants, animals and microorganisms
                  All these divisions are unique with distinctive biological organization.
                  Bacterial cells (Prokaryotes), leading to Eukaryotic cells with true nucleus and organelles
                  Evolution of Fungi, plants and animals from unicellular organisms
        Evolution of amphibians from fishes.***
                  Evolution of reptiles from amphibians.***
                 Evolution of birds and mammals from reptiles.***
All the above groups belongs to different classes as Pisces, Amphibia, Reptilia, Aves and Mammalia respectively.
Remember, all the above terms are abstract and citing a clear cut example is often impossible. Evolutionary biologists greatly differ in their opinions.
Reference
*Russell, P., Hertz, P., & McMillan, B. (2013). Biology: the dynamic science. Cengage Learning.
**Starr, C., Evers, C., & Starr, L. (2010). Biology: Concepts and Applications Without Physiology. Cengage Learning.
***Yadev, B. N., & Kumar, D. (2000). Vertebrate zoology and evolution. Daya Books.
Image credits:
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Difference between Mitochondrial DNA and Chloroplast DNA

Both mitochondrion and chloroplast are semi autonomous organelles with DNA. Majority of proteins required for mitochondrion and chloroplast are coded by genes of the nucleus. Only some proteins are coded by the DNA of mitochondrion and chloroplast. That is why; these organelles are called as semi-autonomous organelles.
Difference between mitochondrial DNA and Chloroplast DNA

Mitochondrial DNA
1. Mitochondrial DNA is called as mt DNA which is double stranded, circular and not covered by a membrane.
2. Size varies between species
· Human mt DNA 16 Kb (16569 bp)
· Plant cell mt DNA 200 kb-2500kb
3. Typically dozens of copies of DNA in each mitochondria
4. Animal mitochondrial genome contains 37 genes that encode 13 proteins, 22 tRNAs, and 2 rRNAs
The 13 mitochondrial genes codes for protein subunits of the enzyme complexes of the oxidative phosphorylation system, which helps mitochondria to act as the powerhouses of our cells.
5. Genes are transcribed in a polycistronic manner- a large mRNA molecule with instruction to make many proteins simultaneously
6. AT (adenine-thymine) rich (65% approx) and lack introns.
· Kinetoplast: In Trypanosomes, each mitochondria contains 1000 of copies of mt DNA arranged into a complex interlinked structure  called Kinetoplast.
Chloroplast DNA
1. Chloroplast DNA is called as ct DNA, cP DNA or plastome
2. Genome size 120-170 kb of DNA
3. Exist in multiple copies
    Each chloroplast with several nucleoid regions containing 8-10 rings of DNA molecules
    The number of DNA copies in mature chloroplast is 15-20.
4. Chloroplast genome include ~100 genes, 46-90 protein coding genes,
    Code for 4 ribosomal RNAs, 21 ribosomal proteins, 4 RNA polymerase subunits and over 30 tRNA genes
    Highly conserved throughput plant species
5.  Majority of genes are transcribed as polycistronic operons
6. Generally, AT rich and lack introns
Similarities between mitochondrial and chloroplast genome
· Both mt DNA and cp DNA are circular, double stranded and not enveloped by a membrane.
· Both mt DNA and cp DNA are devoid of histone proteins
· Both lack introns
· Both are generally AT rich genomes
See the Difference between Mitochondrial DNA and Nuclear DNA
Both mitochondrion and chloroplast are semi autonomous organelles with DNA. Majority of proteins required for mitochondrion and chloroplast are coded by genes of the nucleus. Only some proteins are coded by the DNA of mitochondrion and chloroplast. That is why; these organelles are called as semi-autonomous organelles.
Difference between mitochondrial DNA and Chloroplast DNA

Mitochondrial DNA
1. Mitochondrial DNA is called as mt DNA which is double stranded, circular and not covered by a membrane.
2. Size varies between species
· Human mt DNA 16 Kb (16569 bp)
· Plant cell mt DNA 200 kb-2500kb
3. Typically dozens of copies of DNA in each mitochondria
4. Animal mitochondrial genome contains 37 genes that encode 13 proteins, 22 tRNAs, and 2 rRNAs
The 13 mitochondrial genes codes for protein subunits of the enzyme complexes of the oxidative phosphorylation system, which helps mitochondria to act as the powerhouses of our cells.
5. Genes are transcribed in a polycistronic manner- a large mRNA molecule with instruction to make many proteins simultaneously
6. AT (adenine-thymine) rich (65% approx) and lack introns.
· Kinetoplast: In Trypanosomes, each mitochondria contains 1000 of copies of mt DNA arranged into a complex interlinked structure  called Kinetoplast.
Chloroplast DNA
1. Chloroplast DNA is called as ct DNA, cP DNA or plastome
2. Genome size 120-170 kb of DNA
3. Exist in multiple copies
    Each chloroplast with several nucleoid regions containing 8-10 rings of DNA molecules
    The number of DNA copies in mature chloroplast is 15-20.
4. Chloroplast genome include ~100 genes, 46-90 protein coding genes,
    Code for 4 ribosomal RNAs, 21 ribosomal proteins, 4 RNA polymerase subunits and over 30 tRNA genes
    Highly conserved throughput plant species
5.  Majority of genes are transcribed as polycistronic operons
6. Generally, AT rich and lack introns
Similarities between mitochondrial and chloroplast genome
· Both mt DNA and cp DNA are circular, double stranded and not enveloped by a membrane.
· Both mt DNA and cp DNA are devoid of histone proteins
· Both lack introns
· Both are generally AT rich genomes
See the Difference between Mitochondrial DNA and Nuclear DNA
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Difference between Mitochondrial DNA and Nuclear DNA

Difference between Mitochondrial and Nuclear genome
1. Mitochondrial DNA or mt DNA is double stranded and circular.
Nuclear DNA is double stranded and linear.
Difference between Mitochondrial and Nuclear DNA
2. Mitochondrial DNA is not enveloped by a membrane.
Nuclear DNA or genome is enveloped by nuclear membrane.
3. In Humans, the mitochondrial genome size is 16,569 DNA base pairs.
Nuclear genome is made of 3.3 billion DNA base pairs.
4. Each mitochondrion contains dozens of copies of mt DNA. Several mitochondria in a cell accounts for 1000 of copies of mt DNA per cell.
Only two copies of nuclear genome per somatic cell.
5. The mitochondrial genome contains 37 genes that encode 13 proteins, 22 tRNAs, and 2 rRNAs.
Nuclear genome has 20,000-25,000 genes including mitochondrial genes.
6. Mitochondrion is a semi-autonomous organelle as majority of proteins required are coded by nuclear genes.
Nuclear genes codes for all proteins required for its function.
7. Generally introns or non coding sequences are absent in mitochondrial DNA.
Nuclear genome has introns or non-coding DNA and accounts for 93% of total DNA.
8. Some mitochondrial coding sequences (triplet codons) do not follow the universal codon
pattern, when they are translated into proteins. Examples: In mitochondrion, AUA codes for methionine (not Isoleucine) and UGA codes for tryptophan (not a stop codon as in mammalian genome).
9. Transcription of mitochondrial genes is polycistronic, an mRNA formed with sequences coding for many proteins.
Transcription of nuclear genes is monocistronic, an mRNA with sequence coding for a single protein.
10. The mitochondrial mode of inheritance is maternal. Therefore mitochondrial associated diseases are inherited maternally.
The nuclear genome is inherited equally from both parents.
See the Difference between mitochondrial DNA and chloroplast DNA 
Difference between Mitochondrial and Nuclear genome
1. Mitochondrial DNA or mt DNA is double stranded and circular.
Nuclear DNA is double stranded and linear.
Difference between Mitochondrial and Nuclear DNA
2. Mitochondrial DNA is not enveloped by a membrane.
Nuclear DNA or genome is enveloped by nuclear membrane.
3. In Humans, the mitochondrial genome size is 16,569 DNA base pairs.
Nuclear genome is made of 3.3 billion DNA base pairs.
4. Each mitochondrion contains dozens of copies of mt DNA. Several mitochondria in a cell accounts for 1000 of copies of mt DNA per cell.
Only two copies of nuclear genome per somatic cell.
5. The mitochondrial genome contains 37 genes that encode 13 proteins, 22 tRNAs, and 2 rRNAs.
Nuclear genome has 20,000-25,000 genes including mitochondrial genes.
6. Mitochondrion is a semi-autonomous organelle as majority of proteins required are coded by nuclear genes.
Nuclear genes codes for all proteins required for its function.
7. Generally introns or non coding sequences are absent in mitochondrial DNA.
Nuclear genome has introns or non-coding DNA and accounts for 93% of total DNA.
8. Some mitochondrial coding sequences (triplet codons) do not follow the universal codon
pattern, when they are translated into proteins. Examples: In mitochondrion, AUA codes for methionine (not Isoleucine) and UGA codes for tryptophan (not a stop codon as in mammalian genome).
9. Transcription of mitochondrial genes is polycistronic, an mRNA formed with sequences coding for many proteins.
Transcription of nuclear genes is monocistronic, an mRNA with sequence coding for a single protein.
10. The mitochondrial mode of inheritance is maternal. Therefore mitochondrial associated diseases are inherited maternally.
The nuclear genome is inherited equally from both parents.
See the Difference between mitochondrial DNA and chloroplast DNA 
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Difference between Compression Wood and Tension Wood

Environmental factors like wind, gravity may induce production of wood with special features, properties or growth patterns. The wood thus formed on leaning trunks or branches by the environmental stress is called as reaction wood.
Reaction wood are of 2 types
1. Compression wood
2. Tension wood
Compression wood and tension wood
1. Compression wood:
1. It is the reaction wood of conifers which develops on the lower side of leaning trunk or branch
2. It is formed by the increases cambial activity on the lower side of the branch
3. It is 15-40% heavier than the normal wood and is rich in lignin
4. Formed in conifers like Pinus

2. Tension wood:
1. It is the reaction wood of dicots which develops on the upper side of leaning trunk or branch
2. It is formed by the increases cambial activity on the upper side of the branch
3. It is rich in cellulose and gelatinous fibres
4. Formed in dicots like Mango tree, Acacia etc
Environmental factors like wind, gravity may induce production of wood with special features, properties or growth patterns. The wood thus formed on leaning trunks or branches by the environmental stress is called as reaction wood.
Reaction wood are of 2 types
1. Compression wood
2. Tension wood
Compression wood and tension wood
1. Compression wood:
1. It is the reaction wood of conifers which develops on the lower side of leaning trunk or branch
2. It is formed by the increases cambial activity on the lower side of the branch
3. It is 15-40% heavier than the normal wood and is rich in lignin
4. Formed in conifers like Pinus

2. Tension wood:
1. It is the reaction wood of dicots which develops on the upper side of leaning trunk or branch
2. It is formed by the increases cambial activity on the upper side of the branch
3. It is rich in cellulose and gelatinous fibres
4. Formed in dicots like Mango tree, Acacia etc
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Difference between Sense and Antisense strands of DNA

Sense strand vs antisense strand of DNA
A DNA molecule has a double stranded structure. It consists of two strands. Based on the strand that serves as template for mRNA formation or transcription, one strand is called the sense strand and the other is called the antisense strand.
Difference between sense strand and antisense strand of DNA

Sense strand
1. This strand is also called as coding strand, plus strand or non-template strand.
2. Coding strand is same as mRNA except that thymine in DNA is replaced by Uracil in RNA.
3. The coding strand contains codons.
8
Antisense strand
1. This strand is also called as non-coding strand, minus strand or template strand.
2. This strand acts as template for the synthesis of mRNA. Therefore antisense strand is complementary to the sense strand and mRNA (U in RNA in place of T).
3. The non-coding strand contains anti-codons.
Sense strand vs antisense strand of DNA
A DNA molecule has a double stranded structure. It consists of two strands. Based on the strand that serves as template for mRNA formation or transcription, one strand is called the sense strand and the other is called the antisense strand.
Difference between sense strand and antisense strand of DNA

Sense strand
1. This strand is also called as coding strand, plus strand or non-template strand.
2. Coding strand is same as mRNA except that thymine in DNA is replaced by Uracil in RNA.
3. The coding strand contains codons.
8
Antisense strand
1. This strand is also called as non-coding strand, minus strand or template strand.
2. This strand acts as template for the synthesis of mRNA. Therefore antisense strand is complementary to the sense strand and mRNA (U in RNA in place of T).
3. The non-coding strand contains anti-codons.
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Difference between Pili and Fimbriae

Pili and fimbriae are cell surface appendages present in bacteria other than flagella. These structures are not for locomotion.
Pili and fimbriae are surface appendages for attachment.
Pili and fimbriae Pili vs Fimbriae
Pili
Fimbriae
Long, thicker, tubular structures made up of protein pilin Thin and shorter than the pili
Pili are found only in Gram negative bacteria Fimbriae are found both in Gram negative and Gram positive bacteria
The number of pili are less (3-5 per cell) The number of fimbriae is 300-400 per cell
Formation of pili is governed by plasmid genes Formation of fimbriae is governed by bacterial genes in the nucleoid region
Pili involved in cell to cell attachment during bacterial conjugation. Therefore called as sex-pili Fimbriae is involved in cell to surface attachment of bacteria
Pili and fimbriae are cell surface appendages present in bacteria other than flagella. These structures are not for locomotion.
Pili and fimbriae are surface appendages for attachment.
Pili and fimbriae Pili vs Fimbriae
Pili
Fimbriae
Long, thicker, tubular structures made up of protein pilin Thin and shorter than the pili
Pili are found only in Gram negative bacteria Fimbriae are found both in Gram negative and Gram positive bacteria
The number of pili are less (3-5 per cell) The number of fimbriae is 300-400 per cell
Formation of pili is governed by plasmid genes Formation of fimbriae is governed by bacterial genes in the nucleoid region
Pili involved in cell to cell attachment during bacterial conjugation. Therefore called as sex-pili Fimbriae is involved in cell to surface attachment of bacteria
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Difference between Archaebacterial and Eubacterial flagella

Flagella are organs for movement in bacteria. Electron microscopic studies revealed that flagella consist of three components: a long filament made up of protein flagellin, a hook that connects the filament to the basal body and basal body embedded in the cell wall and plasma membrane from where the flagellum arises.
Flagella, pili and fimbriae

 Archaebacterial flagella vs Eubacterial flagella
1. Eubacterial flagella is thicker than archae bacterial flagella.
2. Eubacterial flagella is powered by flow of H+ ions.
    Archaebacterial flagella is ATP driven.
3. Eubacteria possess many flagellar filaments that rotates independently.
    Archae flagella are often bundled composed of many filaments that rotate as a single unit.
4. During growth of flagella, flagellin subunits are added at the tip in Eubacterial flagella where as in archeal flagella, flagellin subunits are added at the base.
Flagella are organs for movement in bacteria. Electron microscopic studies revealed that flagella consist of three components: a long filament made up of protein flagellin, a hook that connects the filament to the basal body and basal body embedded in the cell wall and plasma membrane from where the flagellum arises.
Flagella, pili and fimbriae

 Archaebacterial flagella vs Eubacterial flagella
1. Eubacterial flagella is thicker than archae bacterial flagella.
2. Eubacterial flagella is powered by flow of H+ ions.
    Archaebacterial flagella is ATP driven.
3. Eubacteria possess many flagellar filaments that rotates independently.
    Archae flagella are often bundled composed of many filaments that rotate as a single unit.
4. During growth of flagella, flagellin subunits are added at the tip in Eubacterial flagella where as in archeal flagella, flagellin subunits are added at the base.
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Difference between Stomata and Hydathodes (Water stomata)

Stomata are small apertures in the epidermal surface of a plant leaf, stem or flower through which gaseous exchange and water transpiration occur. Stomata are minute apertures in the epidermis of aerial parts of the plants bounded by two kidney shaped guard cells. Stomata are absent in roots.
stomata structure
It may occur three different positions in relation to epidermal cells:
 i) They may be at the same level as the adjoining epidermal cells as in most mesophytic plants. 
ii) In xerophytic plants, the stomata are sunken as they are located in a cup shaped depression.            iii) Sometimes the stomata are slightly raised above the surface of epidermis.  
Hydathodes are specialized pores along the margins and apex of the leaf through which the secretion of water (guttation) takes place. Hydathodes or water stomata consists of vein endings, epithem, chamber and pores. Pore is surrounded by guard cells but it remains open permanently.
hydathodes
Stomata vs Hydathodes (Water Stomata)
Stomata
1.  Stomata present only on the lower epidermis of dorsiventral leaf and on both epidermis of isobilateral leaf. (In the case of lotus stomata present on the upper epidermis.)
2. Guard cells always present and due to their flaccidity the stomatal pore may be closed.
3. Epithem does not present.
4. Only water comes out in the form of vapour through the stomata. No mineral salts liberated with water.
Hydathodes (Water stomata)
1. Water stomata present near the margin of leaf, like tomato
2. Guard cells always absent. The pore always remains open, and never closed.
3. Thin walled cells with intercellular spaces called epithem is present.
4. Water along with mineral (inorganic) salts are liberated thorough the pores in the form of liquid droplets.
Stomata are small apertures in the epidermal surface of a plant leaf, stem or flower through which gaseous exchange and water transpiration occur. Stomata are minute apertures in the epidermis of aerial parts of the plants bounded by two kidney shaped guard cells. Stomata are absent in roots.
stomata structure
It may occur three different positions in relation to epidermal cells:
 i) They may be at the same level as the adjoining epidermal cells as in most mesophytic plants. 
ii) In xerophytic plants, the stomata are sunken as they are located in a cup shaped depression.            iii) Sometimes the stomata are slightly raised above the surface of epidermis.  
Hydathodes are specialized pores along the margins and apex of the leaf through which the secretion of water (guttation) takes place. Hydathodes or water stomata consists of vein endings, epithem, chamber and pores. Pore is surrounded by guard cells but it remains open permanently.
hydathodes
Stomata vs Hydathodes (Water Stomata)
Stomata
1.  Stomata present only on the lower epidermis of dorsiventral leaf and on both epidermis of isobilateral leaf. (In the case of lotus stomata present on the upper epidermis.)
2. Guard cells always present and due to their flaccidity the stomatal pore may be closed.
3. Epithem does not present.
4. Only water comes out in the form of vapour through the stomata. No mineral salts liberated with water.
Hydathodes (Water stomata)
1. Water stomata present near the margin of leaf, like tomato
2. Guard cells always absent. The pore always remains open, and never closed.
3. Thin walled cells with intercellular spaces called epithem is present.
4. Water along with mineral (inorganic) salts are liberated thorough the pores in the form of liquid droplets.
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Difference between Angiosperms and Pteridophytes

Pteridophytes p(Gk. Pteron=feather; phyta=plants) are called as seedless vascular cryptogams as they represent the first group of land plants with vasculature, xylem and phloem. Plant body is sporophytic differentiated into true roots, stem and leaves. Pteridophytes are commonly called as the “botanical snakes” as they evolved after bryophytes (the amphibians of the plant kingdom).
Example: Pteris, Adiantum

Angiosperms (Gk. Angios=closed; spermae=seeds)
Angiosperms are flowering, seed bearing vascular plants that form the largest and the most evolved group of plant kingdom. They are true flowering plants where the seeds are completely enclosed inside a fruit wall. The ovules are protected inside the ovary which later develops into fruit.
Example: coconut, mango, rose
Difference between angiosperms and pteridophytes
1. Angiosperms are adapted to wide variety of environments (may be mesophytes, hydrophytes, xerophytes, epiphytes etc).
    Pteridophytes are mostly terrestrial moisture or shade loving plants.

2. Flowers, seeds and fruits are present. Therefore angiosperms are seed bearing plants.
    Flowers, seeds and fruits are absent. Therefore pteridophytes are spore bearing plants.

3. Angiosperms vary greatly in size and shape; may be herb, shrub or tree.
    Pteridophytes are generally herbaceous.

4. Xylem consists of well developed vessels and tracheids in angiosperms.
    Xylem lacks true vessels in Pteridophytes.

5. In phloem, companion cells and sieve tubes are present in angiosperms.
  In phloem, sieve cells are present, companion cells and sieve tubes are absent in pteridophytes.

6. In angiosperms, secondary growth is present except monocots.
    Secondary growth is absent in pteridophytes.

7. Angiosperms are heterosporous forming male and female spores which is critical for seed habit.
 Pteridophytes are mostly homosporous. Some pteridophytes like Selaginella is heterosporous but there is no seed formation.

8. In angiosperms, stamens and carpels are the male and female reproductive structures which are organized to form the flower.
    In pteridophytes, antheridium and archegonium are the male and the female sex organs.

9. In angiosperms, water is not essential for fertilization as pollination is siphonogamous (via pollen     tube) and gametes are without flagella.
    In pteridophytes, water is essential for fertilization and male gametes are ciliated.

10. In angiosperms, megasporangium is usually large.
      In pteridophytes, megasporangium is usually small.

11. In angiosperms, megasporangium is covered by one or more integuments that offers protection.
      In pteridophytes, integuments are absent.

12. In angiosperms, usually a single megaspore is functional in the entire megasporangium (ovule) and is retained within the magasporangium.
      In pteridophytes, many megaspores are functional and generally not retained in the                             megasporangium.

13. Tapetum is absent and endosperm is the nutritive tissue for the developing embryo in angiosperms.
      Tapetum is present, but endosperm is absent in pteridophytes.
Pteridophytes p(Gk. Pteron=feather; phyta=plants) are called as seedless vascular cryptogams as they represent the first group of land plants with vasculature, xylem and phloem. Plant body is sporophytic differentiated into true roots, stem and leaves. Pteridophytes are commonly called as the “botanical snakes” as they evolved after bryophytes (the amphibians of the plant kingdom).
Example: Pteris, Adiantum

Angiosperms (Gk. Angios=closed; spermae=seeds)
Angiosperms are flowering, seed bearing vascular plants that form the largest and the most evolved group of plant kingdom. They are true flowering plants where the seeds are completely enclosed inside a fruit wall. The ovules are protected inside the ovary which later develops into fruit.
Example: coconut, mango, rose
Difference between angiosperms and pteridophytes
1. Angiosperms are adapted to wide variety of environments (may be mesophytes, hydrophytes, xerophytes, epiphytes etc).
    Pteridophytes are mostly terrestrial moisture or shade loving plants.

2. Flowers, seeds and fruits are present. Therefore angiosperms are seed bearing plants.
    Flowers, seeds and fruits are absent. Therefore pteridophytes are spore bearing plants.

3. Angiosperms vary greatly in size and shape; may be herb, shrub or tree.
    Pteridophytes are generally herbaceous.

4. Xylem consists of well developed vessels and tracheids in angiosperms.
    Xylem lacks true vessels in Pteridophytes.

5. In phloem, companion cells and sieve tubes are present in angiosperms.
  In phloem, sieve cells are present, companion cells and sieve tubes are absent in pteridophytes.

6. In angiosperms, secondary growth is present except monocots.
    Secondary growth is absent in pteridophytes.

7. Angiosperms are heterosporous forming male and female spores which is critical for seed habit.
 Pteridophytes are mostly homosporous. Some pteridophytes like Selaginella is heterosporous but there is no seed formation.

8. In angiosperms, stamens and carpels are the male and female reproductive structures which are organized to form the flower.
    In pteridophytes, antheridium and archegonium are the male and the female sex organs.

9. In angiosperms, water is not essential for fertilization as pollination is siphonogamous (via pollen     tube) and gametes are without flagella.
    In pteridophytes, water is essential for fertilization and male gametes are ciliated.

10. In angiosperms, megasporangium is usually large.
      In pteridophytes, megasporangium is usually small.

11. In angiosperms, megasporangium is covered by one or more integuments that offers protection.
      In pteridophytes, integuments are absent.

12. In angiosperms, usually a single megaspore is functional in the entire megasporangium (ovule) and is retained within the magasporangium.
      In pteridophytes, many megaspores are functional and generally not retained in the                             megasporangium.

13. Tapetum is absent and endosperm is the nutritive tissue for the developing embryo in angiosperms.
      Tapetum is present, but endosperm is absent in pteridophytes.
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Difference between Viroids and Prions

Viroids are small, naked infectious RNA molecules. The term viroid was coined by T.O. Diener (1971) to describe the causal agent of the potato spindle tuber disease.

Prions are infectious protein particles causing neurological degenerative diseases in humans and animals. Stanley B. Prusiner discovered prions.
Viroids vs Prions 
viroids vs Prions
          Viroids:
1. It is an infectious RNA particle
2. Viroid is formed of only small single stranded circular RNA
3. A protein coat is absent. 
4. Viroids are inactivated by ribonuclease digestion but resistant to proteinase K and trypsin digestion
5. Viroid has a smaller size than viruses
6. Viroid infects only higher plants (Exception: hepatitis D virus in humans is similar to viroid)
Common plant diseases include Potato Spindle tuber disease, Chrysanthemum stunt disease. 
Prions: 
1. It is an infectious protein particle 
2. It is formed of only proteins 
3. RNA or DNA is absent 
4. Prions are inactivated by proteinase K and trypsin digestion but resistant to ribonuclease treatment 
5. Mostly, smaller than viroid 
6. Prions infects animals causing neurological degenerative diseases 
‘Mad cow disease’ (Bovine spongiform encephalopathy) in cow and Scrapie disease in sheep and goat. 
Creutzfeldt-Jakob Disease (CJD), Kuru and Gerstmann-Strausler-Sheinker syndrome in humans.
Viroids are small, naked infectious RNA molecules. The term viroid was coined by T.O. Diener (1971) to describe the causal agent of the potato spindle tuber disease.

Prions are infectious protein particles causing neurological degenerative diseases in humans and animals. Stanley B. Prusiner discovered prions.
Viroids vs Prions 
viroids vs Prions
          Viroids:
1. It is an infectious RNA particle
2. Viroid is formed of only small single stranded circular RNA
3. A protein coat is absent. 
4. Viroids are inactivated by ribonuclease digestion but resistant to proteinase K and trypsin digestion
5. Viroid has a smaller size than viruses
6. Viroid infects only higher plants (Exception: hepatitis D virus in humans is similar to viroid)
Common plant diseases include Potato Spindle tuber disease, Chrysanthemum stunt disease. 
Prions: 
1. It is an infectious protein particle 
2. It is formed of only proteins 
3. RNA or DNA is absent 
4. Prions are inactivated by proteinase K and trypsin digestion but resistant to ribonuclease treatment 
5. Mostly, smaller than viroid 
6. Prions infects animals causing neurological degenerative diseases 
‘Mad cow disease’ (Bovine spongiform encephalopathy) in cow and Scrapie disease in sheep and goat. 
Creutzfeldt-Jakob Disease (CJD), Kuru and Gerstmann-Strausler-Sheinker syndrome in humans.
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Difference between Mesophyll Chloroplast and Bundle Sheath Chloroplast

Photosynthesis is the anabolic reaction carried out by green plants responsible for life on this planet. It is the process by which suns light energy is converted to chemical energy which is the source of energy for all other living organisms.

Photosynthesis involves 2 reactions: Light reaction and Dark reaction
  • Light reaction: takes place in the grana of the chloroplast where light energy from the sun is converted to chemical energy in the form of ATP and NADPH
  • Dark reaction: takes place in the stroma of the chloroplast. The ATP and NADPH produced in the light reaction are used to fix CO2 to carbohydrates.
Certain plants such as sugarcane, sorghum, Amaranthus etc possess an alternate pathway of CO2 fixation in addition to C3 cycle. The first stable compound produced in this pathway is a 4-carbon compound, oxaloacetic acid, hence called as C4 cycle. In C4 cycle, chloroplast in both mesophyll and bundle sheath cells are involved in CO2 fixation. In C4 plants, initial carbon fixation takes place in the mesophyll cells and Calvin Benson cycle or C3 cycle occurs in the bundle sheath cells. This alteration helps C4 plants to prevent photorespiration and also providing optimum condition for the enzyme RuBP for CO2 fixation.
Mesophyll chloroplast vs Bundle sheath chloroplast in C4 Plants
Mesophyll chloroplast
1. Large well developed numerous grana are present.
2. The enzyme Ribulose bi phosphate carboxylase (RuBP carboxylase) is absent. Therefore no CO2 fixation or C3 cycle occurs. In C3 plants, RuBP carboxylase is abundantly present in mesophyll chloroplast where CO2 fixation occurs
3. The activity of photosystem II is high making plenty of ATP, NADPH2 and O2
4.  Starch grains are absent in mesophyll cells
5. The key enzymes of starch synthesis are absent in mesophyll cells.

Mesophyll Chloroplast and Bundle Sheath Chloroplast - Kranz anatomy

Bundle sheath chloroplast
1. Grana few or absent, if present very small and poorly developed.
2.High concentration of RuBP carboxylase. Therefore CO2 fixation or C3 cycle occurs.
3. Low activity of photosystem II , hence few ATP, NADPH2 and O2 generated
4. Abundant large starch grains are present in bundle sheath chloroplast
5. The key enzymes of starch synthesis (ADP-glucose pyrophosphorylase, starch synthase, and branching enzyme) are located primarily in the bundle sheath cells

Reference: Roles of the bundle sheath cells in leaves of C3plants Richard C. Leegood* J. Exp. Bot. (2008) 59 (7): 1663-1673.
Photosynthesis is the anabolic reaction carried out by green plants responsible for life on this planet. It is the process by which suns light energy is converted to chemical energy which is the source of energy for all other living organisms.

Photosynthesis involves 2 reactions: Light reaction and Dark reaction
  • Light reaction: takes place in the grana of the chloroplast where light energy from the sun is converted to chemical energy in the form of ATP and NADPH
  • Dark reaction: takes place in the stroma of the chloroplast. The ATP and NADPH produced in the light reaction are used to fix CO2 to carbohydrates.
Certain plants such as sugarcane, sorghum, Amaranthus etc possess an alternate pathway of CO2 fixation in addition to C3 cycle. The first stable compound produced in this pathway is a 4-carbon compound, oxaloacetic acid, hence called as C4 cycle. In C4 cycle, chloroplast in both mesophyll and bundle sheath cells are involved in CO2 fixation. In C4 plants, initial carbon fixation takes place in the mesophyll cells and Calvin Benson cycle or C3 cycle occurs in the bundle sheath cells. This alteration helps C4 plants to prevent photorespiration and also providing optimum condition for the enzyme RuBP for CO2 fixation.
Mesophyll chloroplast vs Bundle sheath chloroplast in C4 Plants
Mesophyll chloroplast
1. Large well developed numerous grana are present.
2. The enzyme Ribulose bi phosphate carboxylase (RuBP carboxylase) is absent. Therefore no CO2 fixation or C3 cycle occurs. In C3 plants, RuBP carboxylase is abundantly present in mesophyll chloroplast where CO2 fixation occurs
3. The activity of photosystem II is high making plenty of ATP, NADPH2 and O2
4.  Starch grains are absent in mesophyll cells
5. The key enzymes of starch synthesis are absent in mesophyll cells.

Mesophyll Chloroplast and Bundle Sheath Chloroplast - Kranz anatomy

Bundle sheath chloroplast
1. Grana few or absent, if present very small and poorly developed.
2.High concentration of RuBP carboxylase. Therefore CO2 fixation or C3 cycle occurs.
3. Low activity of photosystem II , hence few ATP, NADPH2 and O2 generated
4. Abundant large starch grains are present in bundle sheath chloroplast
5. The key enzymes of starch synthesis (ADP-glucose pyrophosphorylase, starch synthase, and branching enzyme) are located primarily in the bundle sheath cells

Reference: Roles of the bundle sheath cells in leaves of C3plants Richard C. Leegood* J. Exp. Bot. (2008) 59 (7): 1663-1673.
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Difference between sigma and pi bond

The process of mixing of atomic orbitals of nearly same energy to produce a set of entirely new orbitals of equivalent energy (hybrid orbitals) is known as hybridization. Atomic orbitals of almost the same energy belonging to the same atom or ion can take part in hybridization.. The number of hybrid orbitals formed is always equal to the number of atomic orbitals taking part in hybridization. The hybrid orbitals repel each other and tend to be farthest apart. Hybrid orbitals forms only sigma bonds) σ and pi bonds( π bonds )are formed by unhybridised orbitals.

Sigma bond: This type of covalent bond is formed by the axial or end to end overlapping of half filled atomic orbitals of the atoms participating in bonding. The electron cloud formed as a result of axial overlap is cylindrically symmetrical about inter nuclear axis.

Pi bond: This type of bond is formed by lateral or side wise overlap of the half filled atomic orbitals of the atoms participating in bonding. The pi bond consists of two charged clods above and below the plane of the atoms involved in bond formation.
Sigma bond vs Pi bond
Sigma bond (σ bonds)
1. The covalent bond formed by the overlap of atomic orbitals along the internuclear axis is called sigma bond.

2. The overlapping orbitals are oriented along the internuclear axis.

3. The bond is rotationally symmetrical around the internuclear axis

4. A as well as p orbitals can form this type of bonds.

5. It is stronger than a pi bond

Pi  bond (π bonds)
1. The covalent bond formed by the lateral overlap of two p orbitals which are mutually parallel but oriented perpendicular to the internuclear axis is called a pi bond.

2. The overlapping orbitals are oriented perpendicular to the inter nuclear axis.

3. The bond is not rotationally symmetrical around the internuclear axis.

4. Only p orbitals can form this bond.

5. It is weaker than a sigma bond.
The process of mixing of atomic orbitals of nearly same energy to produce a set of entirely new orbitals of equivalent energy (hybrid orbitals) is known as hybridization. Atomic orbitals of almost the same energy belonging to the same atom or ion can take part in hybridization.. The number of hybrid orbitals formed is always equal to the number of atomic orbitals taking part in hybridization. The hybrid orbitals repel each other and tend to be farthest apart. Hybrid orbitals forms only sigma bonds) σ and pi bonds( π bonds )are formed by unhybridised orbitals.

Sigma bond: This type of covalent bond is formed by the axial or end to end overlapping of half filled atomic orbitals of the atoms participating in bonding. The electron cloud formed as a result of axial overlap is cylindrically symmetrical about inter nuclear axis.

Pi bond: This type of bond is formed by lateral or side wise overlap of the half filled atomic orbitals of the atoms participating in bonding. The pi bond consists of two charged clods above and below the plane of the atoms involved in bond formation.
Sigma bond vs Pi bond
Sigma bond (σ bonds)
1. The covalent bond formed by the overlap of atomic orbitals along the internuclear axis is called sigma bond.

2. The overlapping orbitals are oriented along the internuclear axis.

3. The bond is rotationally symmetrical around the internuclear axis

4. A as well as p orbitals can form this type of bonds.

5. It is stronger than a pi bond

Pi  bond (π bonds)
1. The covalent bond formed by the lateral overlap of two p orbitals which are mutually parallel but oriented perpendicular to the internuclear axis is called a pi bond.

2. The overlapping orbitals are oriented perpendicular to the inter nuclear axis.

3. The bond is not rotationally symmetrical around the internuclear axis.

4. Only p orbitals can form this bond.

5. It is weaker than a sigma bond.
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