Eukaryotic Transcription (AQA A Level Biology)

• The genome within eukaryotic cells contains many non-coding sections
• Non-coding DNA can be found:
  • Between genes, as non-coding multiple repeats
  • Within genes, as introns

• During transcription, eukaryotic cells transcribe the whole gene (all introns and exons) to produce pre-mRNA molecules
  • pre-mRNA contains the introns and exons of a certain gene

• Before the pre-mRNA exits the nucleus, splicing occurs:
  • The non-coding sections are removed
  • The coding sections are joined together
  • The resulting mRNA molecule carries only the coding sequences (exons) of the gene
  • mRNA contains only exons and exits the nucleus before joining a ribosome for translation

Image showing the splicing of pre-mRNA 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
• This means that a single eukaryotic gene can code for more than one polypeptide chain
• This is part of the reason why the proteome is much bigger than the genome

Image showing the alternative splicing of a gene to produce two different proteins