Difference between Reading Frame, Open Reading Frame and Coding Sequence

What is a Reading Frame?

Reading frame is the division of a nucleotide in DNA or RNA molecule into a set of consecutive, non-overlapping triplets that codes for amino acid or stop codons. 

• Both strands are read in the 5′→3′ direction. Each strand has three reading frames, depending on which nucleotide is selected as the starting position.
• Typically, only one reading frame is used in translating a gene (in eukaryotes), and this is often the longest open reading frame. 

You can watch our video here for concept clarity.

What is Open reading frames (ORFs)?

ORF is defined as spans or stretches of DNA sequence between start and stop codons. An ORF is a reading frame that has the potential to be transcribed into RNA and translated into protein. It requires a continuous sequence of DNA from a start codon, multiple of 3 nucleotides, to a stop codon in the same reading frame. 

In the above example, for finding out ORF, 

• First find the start codon, Here in DNA +1 frame, it is ATG, then the nearest stop codon TAA, TAG, or TGA in DNA (UAA, UAG and UGA in RNA).
• In -1, -2 and -3 frames, there is no ORF

Why is ORF called “Open”?

The term “open” refers to the fact that the road is open to keep reading, and the ribosome will be able to keep reading the RNA code and add another amino acid one after another, till meeting the stop codon.

Difference between Reading frame and Open reading frames 

The key difference between a reading frame and an open reading frame is that while both involve the division of nucleotide sequences into triplets or codons, an open reading frame specifically refers to a sequence that begins with a start codon and ends with a stop codon, and has the potential to be transcribed and translated into a protein. All reading frames may not contain ORF and may not form a protein.

What is Coding sequence or CDS?

In eukaryotes, the genes consist of both introns and exons, therefore ORF of a DNA sequence or an unprocessed mRNA sequence contains both coding and non-coding sequences. To annotate the coding sequence, we use conserved splice sites to find out the coding sequence.

Original image credit: https://www.genome.gov/

But the processed mRNA has only coding sequence or with only exons, this is called the coding sequence. A CoDing Sequence (CDS) is a region of DNA or RNA whose sequence determines the sequence of amino acids in a protein.

Difference between Open reading frames and Coding sequence

Open Reading Frame (ORF), is a continuous stretch of DNA codons that begins with a start codon and ends at a STOP codon but may contain non coding sequences that are not coding amino acids.

A CoDing Sequence (CDS) is a region of DNA or RNA whose sequence determines the sequence of amino acids in a protein and does not contain introns or non-coding sequence.