3.19 Post-Transcriptional Changes to mRNA

• 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