By April Cashin-Garbutt, BA Hons (Cantab)
Introns and exons are parts of genes. Exons code for proteins, whereas introns do not. A great way to remember this is by considering introns as intervening sequences and exons as expressed sequences. (1)
According to researchers, there are an average of 8.8 exons and 7.8 introns per human gene. (5)
What are exons?
Exons are parts of DNA that are converted into mature messenger RNA (mRNA). (6) The process by which DNA is used as a template to create mRNA is called transcription.
This mRNA then undergoes a further process called translation where the mRNA is used to synthesize proteins, via another type of molecule called transfer RNA (tRNA). (7)
What are introns?
Introns are parts of genes that do not directly code for proteins. (2)
Introns can range in size from 10’s of base pairs to 1000’s of base pairs. (3)
Where are introns found?
Introns are commonly found in multicellular eukaryotes, such as humans. They are less common in unicellular eukaryotes, such as yeast, and even rarer in bacteria. (1)
It has been suggested that the number of introns an organism’s genes contains is positively related to its complexity. That is the more introns an organism contains, the more complex the organism is. (4)
How are introns removed?
Introns are present in the initial RNA transcript, known as pre-mRNA. They need to be removed in order for the mRNA to be able to direct the production of proteins. Pre-mRNA, therefore, undergoes a process, known as splicing, to create mature mRNA. (6, 8)
It is vital for the introns to be removed precisely, as any left-over intron nucleotides, or deletion of exon nucleotides, may result in a faulty protein being produced. This is because the amino acids that make up proteins are joined together based on codons, which consist of three nucleotides. An imprecise intron removal thus may result in a frameshift, which means that the genetic code would be read incorrectly. (3)
This can be explained by using the following phrase as a metaphor for an exon: “BOB THE BIG TAN CAT”. If the intron before this exon was imprecisely removed, so that the “B” was no longer present, then the sequence would become unreadable: “OBT HEB IGT ANC AT…” (6)
RNA splicing, also known as RNA processing, occurs at special splice sites. These tend to begin with the dinucleotide GU at the 5’ end and AG at the 3’ end. (8, 12)
The process is carried out by small nuclear ribonucleoproteins (snRNPs), which are commonly known as snurps. They bind to both the 5’ and 3’ ends of the intron and cause the intron to form a loop. The intron is then removed from the sequence and the two remaining exons are linked together. (6, 12)
Alternative splicing refers to the way that different combinations of exons can be joined together. This idea was first put forward by Walter Gilbert. He proposed that the different permutations of exons could produce different protein isoforms. These in turn would have different chemical and biological activities. (9, 10)
It is now thought that between 30 and 60% of human genes undergo alternative splicing. Moreover, it is said that many human diseases may be connected to problems with splicing. (11)
One example of a human gene that undergoes alternative splicing is fibronectin. Over 20 different isoforms of fibronectin have been discovered. These have all been produced from different combinations of fibronectin gene exons. (2)
Lodish, Berk et al. (2004) Molecular Cell Biology Fifth Edition. W. H. Freeman and Company