All multicellular organisms start with a single fertilized egg which multiply by cell division.The new cells arise by the division of the pre existing cells (Rudolf Virchow,1885).The mode of cell division is fundamentally similar in all organisms.
There are two types of cell divisions:(a) Mitosis & (b) Meiosis
Mitosis ( Equational Division)
It is type of cell division in which chromosomes of parent cells are duplicated and equally distributed into two daughter cells.Term mitosisis derived from Greek word "Mitos"means thread or fibril. It was first observed by Strasburger in plant cells (1870) and Boveri and Flemming in animal cell (1879).
It is divided into four stages or phases:Prophase, Metaphase, Anaphase and Telophase.
Meiosis (Reduction Division)
It is a process that allows one diploids (2n) cells to divide in a special way to generate haploid (n) cells in eukaryotes.The term meiosis (greek, meio -to lessen) was coined by Farmer and Moore (1905). The division was first coined by Van Benedin (1887), Strasburger (1888), Sutton (1900) and Winiwater (1900). Meiosis consists of two cell divisions which take place one after the other, during which the number of chromosome is halved.
The two divisions are known as the first meiotic division (Meiosis I)and second meiotic division (Meiosis II).
Mitosis vs Meiosis
|Take place in the somatic cells of the body.||Take place in the germ cells.|
|Occurs in both sexually as well as asexually reproducing organisms.||Occurs only in sexually reproducing organisms.|
|The cell divide only once.||There are two cell divisions, the first and the second meiotic divisions.|
|Interphase occurs prior to each division.||Interphase precedes only in meiosis I. It does not occur prior to meiosis II.|
|DNA replication takes place during interphase I.||DNA replication takes place during interphase I but not interphase II|
|The DNA replicates once for one cell division.||The DNA replicates once for two cell divisions|
|The duration of prophase is short, usually of a few hours.||Prophase is comparatively longer and may take days.|
|Prophase is comparatively simple.||Prophase is complicated and divided into leptotene, zygotene, pachytene, diplotene, and diakinesis.|
|The cell divides only once and the chromosomes also divide only once.||There are two cell divisions but the chromosomes divide only once.|
|There is no synapsis. ||Synapsis of homologous chromosomes takesplace during prophase.|
|The two chromatids of a chromosome do not exchange segments during prophase.||Chromatids of two homologous chromosomes exchange segments during crossing over.|
|The arms of the prophase chromatids are close to one another.||The arms of the chromatids are separated widely in prophase II.|
|Chromosomes are already duplicated at the beginning of prophase||When prophase I commences the chromosomes appear single, (although DNA replication has taken place in interphase I).|
|No bouquet stage is recorded. ||Chromosomes of animals and some plants show covergence towards one side during early prophase I. It is known as bouquet stage.|
|A synaptionemal complex is absent. ||Synapsed homologous chromosome develop a synaptonemal complex.|
|Crossing over is absent. ||Crossing over or exchange of similar segments between nonsister chromatids of homologous chromosomes usually take place during pachytene stage.|
|Chiasmata are absent. ||Chiasmata or visible connections between homologous chromosomes of bivalents are observed during diplotene, diakinesis (prophase I) and metaphase I|
|In the metaphase plate all the centromeres line up in same plate.||In metaphase I the centromeres are lined up in two planes which are parallel to one other.|
|The metaphase plate is made up of chromosome pairs.||The metaphase plate is made up of paired chromosome pairs.|
|Two chromatids of a chromosome (Progeny cells) are genetically similar. |
The genetic constitution of the daughter cells is identical to that of the parent cells.
|Two chromatids of a chromosome (Progeny cells) are often genetically different due to crossing over. |
The genetic constitution of the daughter cells differs from that of the parent cell. The chromosomes of daughter cells usually contain a mixture of maternal and paternal genes.
|Division of the centromeres take place during anaphase. ||There is no centromeric division during anaphase I. Centromeres divide only during anaphase II.|
|The chromosomes separates simultaneously during anaphase. ||Short chromosomes separate early, separation of long chromosome is delayed.|
|Anaphase chromosomes are single stranded.||Chromosomes are double stranded in anaphase I, but single stranded in anaphase II.|
|Similar chromosomes move towards the opposite poles in anaphase.||Dissimilar chromosomes move towards the opposite poles both in anaphase I and II.|
|Spindle fibers disappear completely in telophase.||Spindle fibers do not disappear completely in telophase I.|
|Nucleoli reappear at telophase. ||Nucleoli do not reappear at telophase.|
|Cytokinesis follows every mitosis.It produces two new cells.||Cytokinesis often does not occur after the first or reduction division. It is often simultaneous after second division to result in four new cells.|
|The chromosome number remains constant at the end of mitosis.||The chromosome number is reduced from the diploid to the haploid.|
|It helps in multiplication of cells.||Multiplication of cells is not involved.|
|Take part in healing and repair.||Take part in the formation of meispores or gametes and maintenance of chromosome number of the race.|