DNA is the inherited material that tells organisms what they are and what each cell should do. Four nucleotides arrange themselves in paired sequences in a predetermined order specific to the species' and individual's genome. At first glance, this creates all of the genetic diversity within any given species, as well as between species.
Upon closer examination, though, it appears that there is much more to DNA.
For example, simple organisms tend to have just as many or more genes as the human genome. Considering the complexity of the human body compared to a fruit fly or even simpler organisms, this is difficult to understand. The answer lies in how complex organisms, including humans, make use of their genes in more intricate ways.
The Function of Exon and Intron DNA Sequences
The different sections of a gene can be broadly divided into two categories:
- Coding regions
- Non-coding regions
The non-coding regions are called introns. They provide organization or a kind of scaffolding to the coding regions of the gene. The coding regions are called exons. When you think of "genes," you are probably thinking specifically about exons.
Often, the region of a gene that is going to be coding switches with other regions, depending on the needs of the organism. Therefore, any part of the gene can operate as an intron non-coding sequence or as an exon coding sequence.
There are typically a number of exon regions on a gene, interrupted sporadically by introns. Some organisms tend to have more introns than others. Human genes consist of approximately 25 percent introns. The length of exon regions can vary from a small handful of nucleotide bases to thousands of bases.
The Central Dogma and Messenger RNA
Exons are the regions of a gene that undergo the process of transcription and translation. The process is complex, but the simplified version is commonly referred to as the "central dogma," and looks like this:
DNA ⇒ RNA ⇒ Protein
RNA is nearly identical to DNA and is used to copy, or transcribe the DNA and move it out of the nucleus to the ribosome. The ribosome translates the copy in order to follow instructions for building new proteins.
In this process, the DNA double helix unzips, leaving one half of each nucleotide base pair exposed, and RNA makes a copy. The copy is called messenger RNA, or mRNA. The ribosome reads the amino acids in the mRNA, which are in triplet sets called codons. There are twenty amino acids.
As the ribosome reads the mRNA, one codon at a time, transfer RNA (tRNA) bring the correct amino acids to the ribosome that can bind with each amino acid as it is read. A chain of amino acid forms, until a protein molecule is made. Without living things adhering to the central dogma, life would end very quickly.
It turns out that exons and introns play a significant role in this function and others.
The Importance of Exons in Evolution
Until recently, biologists were uncertain why DNA replication included all of the gene sequences, even the non-coding regions. These were the introns.
The introns are spliced out and the exons connected, but the splicing can be done selectively and in different combinations. The process creates a different kind of mRNA, lacking all introns and containing only exons, called mature mRNA.
The different mature messenger RNA molecules, depending on the splicing process, create the possibility for different proteins to be translated from the same gene.
The variability made possible by exons and RNA splicing or alternative splicing allows for quicker leaps in evolution. Alternative splicing also creates the possibility for greater genetic diversity in populations, differentiation of cells and more complex organisms with smaller amounts of DNA.
- University of Utah: Anatomy of a Gene
- Baylor College of Medicine Thomas Cooper Lab: Alternative Splicing
- University of Chicago Medical Center: RNA splicing mutations play major role in genetic variation and disease
- Cold Spring Harbor Perspectives in Biology: Origin of Spliceosomal Introns and Alternative Splicing
- University of Massachusetts Medical School RNA Therapeutics Institute: What is RNA?