Functions, Structure, and Evolution of Introns in Molecular Biology

Koutarou Nishimura^*
*Correspondence: Koutarou Nishimura, Department of Hematology-Oncology, University of Biomedical Research and Innovation, Kobe, Japan, Email:

Introduction

In molecular biology, an intron is a non-coding segment of DNA or RNA that interrupts the coding sequence of a gene. In other words, it is a region of DNA or RNA that does not code for a protein, and instead acts as a spacer between the exons. In the process of gene expression, introns are transcribed into RNA but are removed before the final mRNA is produced. This process is called splicing, and it allows the production of multiple different proteins from the same gene. In this article, we will delve deeper into the world of introns and explore their functions, structure, and evolution.

Description

Structure of introns

Introns are generally found in eukaryotic genes, which are composed of both exons and introns. Exons contain the coding sequence of a gene and are translated into proteins, while introns are the intervening sequences between the exons. The size of introns varies greatly between species, with some being as small as a few dozen nucleotides, while others can be thousands of nucleotides in length.

Introns are flanked by two exons, and the splice sites at the boundaries of the intron and exon are highly conserved in both sequence and position. The consensus sequence at the 5’ end of the intron is typically GT, while the consensus sequence at the 3’ end is AG. The splicing of introns is mediated by a complex of small nuclear RNAs (snRNAs) and proteins, collectively known as the spliceosome. The spliceosome recognizes the splice sites at the boundaries of the intron and exon and catalyzes the removal of the intron and the ligation of the two exons.

Functions of introns

The functions of introns are still not fully understood, but several hypotheses have been proposed. One of the main hypotheses is that introns allow for alternative splicing, which is the process by which different exons within a gene can be combined to produce multiple different mRNA isoforms. Alternative splicing allows for the production of multiple different proteins from the same gene, and it is estimated that over 90% of human genes undergo alternative splicing.

Introns have also been proposed to play a role in the regulation of gene expression. For example, some introns contain regulatory elements such as enhancers and silencers, which can affect the transcription of the adjacent exons. Introns have also been shown to affect the stability and localization of mRNA molecules.

Introns may also play a role in genome evolution. It has been proposed that introns facilitate the evolution of new genes by allowing for the shuffling and recombination of exons. This process, known as exon shuffling, can create new proteins with novel functions. In addition, introns may also act as a buffer against harmful mutations, as they can accumulate mutations without affecting the coding sequence of the gene.

Evolution of introns

The origin and evolution of introns is still the subject of much debate. One hypothesis is that introns originated early in the evolution of eukaryotes, as they are found in all eukaryotic lineages, but are absent from prokaryotes. The most widely accepted hypothesis for the origin of introns is the intron-early hypothesis, which proposes that introns were present in the common ancestor of all eukaryotes, but have been lost in some lineages.

Conclusion

Another hypothesis is the intron-late hypothesis, which proposes that introns originated independently in different lineages of eukaryotes. This hypothesis is based on the observation that introns are more abundant and larger in some eukaryotic lineages than in others.