One Gene Can Code for More Than One Protein
- Gene expression can be regulated after an mRNA transcript has been produced
- In eukaryotes, transcription and translation occur in separate parts of the cell, allowing for significant post-transcriptional modification to occur
- Post-transcriptional modification mechanisms include
- Splicing
- Alternative splicing
Splicing
- Polypeptides are made during the process of protein synthesis, during which the DNA base code is transcribed and translated
- The DNA code within eukaryotic cells contains many non-coding sections
- Non-coding DNA can be found within genes; these sections are called introns, whilst sections of coding DNA are called exons
- During transcription, eukaryotic cells transcribe both introns and exons to produce pre-mRNA molecules
- Before the pre-mRNA exits the nucleus, a process called splicing occurs
- The non-coding intron sections are removed
- The coding exon sections are joined together
- The resulting mRNA molecule contains only the coding sequences of the gene
- Since these modifications are made after transcription occurred, they are called post-transcriptional modifications
Pre-mRNA is spliced before it exits the nucleus
Alternative splicing
- The exons (coding regions) of genes can be spliced in many different ways to produce different mature mRNA molecules through alternative splicing
- A particular exon may or may not be incorporated into the final mature mRNA
- Polypeptides translated from alternatively spliced mRNAs may differ in their amino acid sequence, structure and function
- This means that a single eukaryotic gene can code for multiple proteins
- This is part of the reason why the proteome is much bigger than the genome
Alternative splicing of a gene can produce more than one type of protein
Examiner Tip
It is important you learn the terms pre-mRNA and mRNA, their location and whether they include introns as well as exons. A handy way to distinguish between introns and exons is to remember that EXons are EXpressed.