Difference Between Introns and Exons

The key difference between introns and exons is that introns are non-coding sequences of a gene while exons are coding sequences. Hence, introns do not appear in mature mRNA molecules while exons collectively make the final RNA molecule.

Introns and exons are frequently used terms in the field of molecular biology, but when someone starts to familiarize with these terms, confusion would arise as both these are nucleotide sequences of genes.

CONTENTS

1. Overview and Key Difference
2. What are Introns 
3. What are Exons
4. Similarities Between Introns and Exons
5. Side by Side Comparison – Introns vs Exons in Tabular Form
6. Summary

What are Introns

Introns are sequences of nucleotides present in the genes between exons. These nucleotide sequences do not code for proteins, and that means introns are not immediately important for the process of protein synthesis. When a messenger RNA (mRNA) strand is created through transcription of DNA at a gene, the nucleotide sequence of introns is excluded. Moreover, the exclusion of the intron sequence from the mRNA strand takes place through the process called RNA splicing; it could be through cis-splicing when there is only one intron incorporated with the gene, trans-splicing occurs when there are two or more introns associated with the gene.

A mature mRNA strand, which is ready to code for a protein, is formed after removing the introns from the strand. Since both DNA and RNA contain these non-coding sequences, the term intron could refer to the non-coding nucleotide sequences of DNA and their corresponding sequences in RNA.

It is also important to notice that ribosomal RNA (rRNA) and transfer RNA (tRNA) contain genes with introns, but those are removed when the genes are expressed. In other words, introns go through transcription, but not through translation. Therefore, these are called untranslated sequences of DNA. The immediate function of introns is a little unclear, but it is believed that these are important to constitute for a diversified, yet related proteins from a single gene. In addition, intron-mediated enhancing of gene expression has been accepted as another important function of introns.

What are Exons

Exons are the nucleotide sequences of genes that are expressed and are found at either side of an intron. In simple terms, it could be stated that exons really hit the ground in the expression of genes or in the protein synthesis. After removing non-coding sequences from the pre mRNA, mature mRNA molecule comprises only of exon sequences. Then the nucleotide sequence of mature mRNAs is converted into the amino acid sequence of the 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); two ORFs mark the ends of a gene within those exons are located. However, there are instances where exons are not 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 Introns and Exons?

• Introns and Exons are nucleotide sequences of genes.
• Both sequences transcribe into pre mRNA.
• They are intragenic sequences.
• They are present in DNA and RNA.
• Both are present in eukaryotes.

What is the Difference Between Introns and Exons?

Summary – Introns vs Extrons

A gene has both coding and non-coding sequences. Non-coding sequences are not involved in protein synthesis. They are introns. Coding sequences bear the genetic code of a protein. They are exons. Overall, this is the key difference between introns and extrons.

Reference:

1. “Intron.” Wikipedia, Wikimedia Foundation, 10 May 2018, Available here.

Image Courtesy:

1.”DNA exons introns” By National Human Genome Research Institute – (Public Domain) via Commons Wikimedia