Difference Between CDS and ORF

The key difference between CDS and ORF is that CDS is that actual nucleotide sequence of a gene which translates into a protein while ORF is a stretch of DNA sequence that begins with translation initiation site (start codon) and ends with a translation termination site (stop codon).

A gene has a coding sequence (CDS). It consists of total exons of the gene and a start codon and a stop codon. It is the actual part of the gene that translates and produces the protein. Open reading frame or ORF is a nucleotide sequence located between a start codon and a stop codon. There is no stop codon inside an ORF interrupting the genetic code which translates into a protein. In prokaryotes, CDS and ORF of a gene are the same.

1. Overview and Key Difference
2. What is CDS
3. What is ORF
4. Similarities Between CDS and ORF
5. Side by Side Comparison – CDS vs ORF in Tabular Form
6. Summary

What is CDS?

CDS or coding sequence is the part of the gene that actually translates into a protein. It comprises of exons and two codons known as AUG codon and stop codon. CDS does not contain two UnTranslated regions: 5’ UTR and 3” UTR. Moreover, introns are not included in CDS.

Figure 01: Coding Sequence

When compared with the whole genome of an individual, coding sequences are a small fraction. Coding sequence consists of the necessary nucleotide sequence to make the amino acid sequence of the protein. Therefore, CDS are concentrated exons which can be divided into nucleotide triplets or codons. Codons give rise to amino acids.

What is an ORF?

Open reading frame or ORF is the continuous stretch of a nucleotide sequence that begins with a start codon and ends with a stop codon. In simple words, ORF refers to the region of the nucleotide sequence located between start and stop codons. In between, there is no stop codon interrupting the ORF. The nucleotide sequence between start and stop codon encodes for amino acids. Generally, start codon is ATG while stop codons are TAG, TAA, and TGA. ORF gives a functional protein when transcribed and translated. Hence, ORF includes a start codon, several codons in middle region and a stop codon. Interestingly, ORF has a length which can be divided by three.

Figure 02: Open Reading Frame

In prokaryotes, since there are no introns, ORF is the coding sequence of a gene which transcribes directly into mRNA. Therefore, CDS and PRF are same in prokaryotes. When searching for genes in prokaryotes, it is easy to detect an ORF and find a gene in prokaryotes. In eukaryotes, since there are introns, ORF is the codon sequence that forms after processing or RNA splicing. ORF is a piece of evidence that assists gene prediction as long ORF is likely to be a part of a gene.

What are the Similarities Between CDS and ORF?

In prokaryotes, CDS and ORF are the same.
Both have start codon and stop codon.
They have a number of nucleotides which can be divided by three.
Once they translate, they produce amino acid sequences.

What is the Difference Between CDS and ORF?

CDS is the actual part of the gene which translates into a protein while ORF is the stretch of DNA between a start codon and a stop codon. So, this is the key difference between CDS and ORF. Furthermore, CDS does not contain introns, but ORF may contain introns. CDS transcribes fully into a complete mRNA sequence while ORF can be a part of the mRNA sequence. Thus, this is another difference between CDS and ORF.

Below infographic tabulates side by side the differences between CDS and ORF.

Summary – CDS vs ORF

CDS and ORF are two important parts of a gene. CDS refers to the actual region of DNA that translates into a protein. ORF is a sequence of DNA that starts with start codon “ATG” and ends with any of the three termination codons (TAA, TAG or TGA). ORF may be a part of the complete mRNA of a gene. However, the coding sequence of a gene transcribes to complete mRNA sequence. All CDSs are ORFs. But not all ORFs are CDSs. Thus, this summarizes the difference between CDS and ORF.

Reference:

1. “Open Reading Frame.” Wikipedia, Wikimedia Foundation, 27 Nov. 2020, Available here.

Image Courtesy:

1. “Coding Region in DNA” By Pragpats – Own work (CC BY-SA 4.0) via Commons Wikimedia
2. “Sampleorf” By Luongdl – Own work (Public Domain) via Commons Wikimedia