Difference Between ORF and Exon

The key difference between ORF and exon is that the ORF or the open reading frame is a stretch of DNA sequence that begins with translation initiation site (start codon) and ends with translation termination site (stop codon) while exon is a nucleotide sequence within a gene that encodes for amino acids.

An open reading frame is a part of a reading frame. Reading frames are read by ribosomes in order to make proteins. ORF is a continuous stretch of codons which gives a fully functional protein. It begins with a start codon and terminates with a stop codon. Inside the ORF, there is no stop codon interrupting the coding sequence. Translation starts at start codon and ends at stop codon. Exon is a nucleotide sequence of a gene. It encodes for amino acids of the protein. Therefore, exons are coding regions of a gene.

CONTENTS

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

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 nucleotide sequence located between the 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. Therefore, ORF includes a start codon, several codons in the middle region and a stop codon. Interestingly, ORF has a length which can be divided by three.

In prokaryotes, since there are no introns, ORF is the coding region of a gene which transcribes directly into mRNA. In eukaryotes, since there are introns, ORF is the codon sequence which results after processing or RNA splicing. ORF is a piece of evidence that assists gene prediction since long ORFs are likely to be a part of a gene.

What is an Exon?

Exons are the coding nucleotide sequences of genes that translate into proteins. They are at either side of an intron. After removing non-coding sequences from the pre mRNA, the mature mRNA molecule only comprises of exon sequences. Then the nucleotide sequence of the final RNA molecule (mature mRNA) converts into an amino acid sequence of a specific protein.

Almost all genes have an initial nucleotide sequence that distinguishes it as a gene from the main DNA or RNA strand, which is known as the Open Reading Frame (ORF. In some genes, two ORFs mark the entire gene and exons are located within the coding sequence. Although it sounds that exons are always expressed in genes, there are instances where intron sequences intervene with the exon to cause mutations, and this process is known as exonization.

What are the Similarities Between ORF and Exon?

• Both ORF and exon are nucleotide sequences.
• Long ORF and exons are parts of a gene.
• Both have coding sequences.

What is the Difference Between ORF and Exon?

ORF and exon are nucleotide sequences. ORF refers to any stretch of DNA sequence located between a start codon and a stop codon. In contrast, the exon is a coding nucleotide sequence of a gene that encodes for amino acids. Thus, this is the key difference between ORF and exon. Exons are parts of a gene while long ORF is likely to be a part of a gene. Moreover, there are introns in both sides of an exon while ORF does not include introns.

Below infographic summarizes the difference between ORF and exon in tabular form.

Summary – ORF vs Exon

The open reading frame (ORF) is a part of the reading frame. It is the continuous stretch of DNA sequence that begins with a start codon and terminates with a stop codon. Exon is a nucleotide sequence of a gene. It encodes for a part of the mRNA sequence. Therefore, exons are parts of the gene sequence that are expressed in the protein. Thus, this summarizes the difference between ORF and exon.

Reference:

1. Greenwood, Michael. “What Are Introns and Exons?” News, 2 Nov. 2018, Available here.
2. “Open Reading Frame.” Wikipedia, Wikimedia Foundation, 1 Sept. 2020, Available here.

Image Courtesy:

1. “RNA splicing diagram en” By LadyofHats – made myself based basically in the information found in Wikipedia (Public Domain) via Commons Wikimedia