Introns and exons are similar because they are both part of the genetic code of a cell but they are different because introns are non-coding while exons code for proteins. This means that when a gene is used for protein production, the introns are discarded while the exons are used to synthesize the protein.
When a cell expresses a particular gene, it copies the DNA coding sequence in the nucleus to messenger RNA, or mRNA. The mRNA exits the nucleus and goes out into the cell. The cell then synthesizes proteins according to the coding sequence. The proteins determine what kind of cell it becomes and what it does.
During this process, the introns and exons making up the gene are both copied. The exon coding parts of the copied DNA are used for producing proteins, but they are separated by noncoding introns. A splicing process removes the introns and the mRNA leaves the nucleus with only exon RNA segments.
Even though the introns have been discarded, both exons and introns play roles in the production of proteins.
Similarities: Introns and Exons Both Contain Genetic Code Based on Nucleic Acids
Exons are at the root of cell DNA coding using nucleic acids. They are found in all living cells and form the basis for the coding sequences that underlie protein production in cells. Introns are noncoding nucleic acid sequences found in eukaryotes, which are organisms made up of cells that have a nucleus.
In general, prokaryotes, which have no nucleus and only exons in their genes, are simpler organisms than eukaryotes, which include both single-cell and multicellular organisms.
In the same way complex cells have introns while simple cells do not, complex animals have more introns than simple organisms. For example, the fruit fly Drosophila has only four pairs of chromosomes and comparatively few introns while humans have 23 pairs and more introns. While it is clear which parts of the human genome are used for coding proteins, large segments are noncoding and include introns.
Differences: Exons Encode Proteins, Introns Do Not
DNA code consists of pairs of the nitrogenous bases adenine, thymine, cytosine and guanine. The bases adenine and thymine form a pair as do the bases cytosine and guanine. The four possible base pairs are named after the first letter of the base that comes first: A, C, T and G.
Three pairs of bases form a codon that encodes a particular amino acid. Since there are four possibilities for each of the three code places, there are 43 or 64 possible codons. These 64 codons encode start and stop codes as well as 21 amino acids, with some redundancy.
During the initial copying of the DNA in a process called transcription, both introns and exons are copied onto pre-mRNA molecules. The introns are removed from the pre-mRNA by splicing the exons together. Each interface between an exon and an intron is a splice site.
RNA splicing takes place with the introns detaching at a splice site and forming a loop. The two neighboring exon segments can then join together.
This process creates mature mRNA molecules that leave the nucleus and control RNA translation to form proteins. The introns are discarded because the transcription process is aimed at synthesizing proteins, and the introns don't contain any relevant codons.
Introns and Exons Are Similar Because They Both Deal With Protein Synthesis
While the role of exons in gene expression, transcription and translation into proteins is clear, introns play a more subtle role. Introns can influence gene expression through their presence at the start of an exon, and they can create different proteins from a single coding sequence through alternative splicing.
Introns can play a key role in splicing the genetic coding sequence in different ways. When introns are discarded from pre-mRNA to allow the formation of mature mRNA, they can leave parts behind to create new coding sequences that result in new proteins.
If the sequence of exon segments is changed, other proteins are formed according to the changed mRNA codon sequences. A more diverse protein collection can help organisms adapt and survive.
Proof of the role of introns in producing an evolutionary advantage is their survival over the different stages of evolution into complex organisms. For example, according to a 2015 article in Genomics and Informatics, introns can be a source of new genes, and through alternative splicing, introns can generate variations of existing proteins.
About the Author
Bert Markgraf is a freelance writer with a strong science and engineering background. He has written for scientific publications such as the HVDC Newsletter and the Energy and Automation Journal. Online he has written extensively on science-related topics in math, physics, chemistry and biology and has been published on sites such as Digital Landing and Reference.com He holds a Bachelor of Science degree from McGill University.