Eukaryotic Transcription (AQA AS Biology): Revision Note

Eukaryotic transcription

• 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

Splicing

• 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 (after transcription) contains only exons and exits the nucleus before joining a ribosome for translation

    • This is called mature mRNA

Alternative splicing

• The exons (coding regions) of genes can be spliced in many different ways to produce different mature mRNA molecules through alternative splicing

  • Different combinations of exons are joined together from the same pre-mRNA transcript

• 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

Prokaryotic transcription

• The transcription process is simpler and more direct in prokaryotic cells (such as bacteria) than in eukaryotic cells:

  • There is no pre-mRNA stage

    • In prokaryotes, transcription produces mRNA directly from the DNA template

    • This is because prokaryotic genes do not contain introns, so there is no need for splicing

  • Transcription and translation are coupled

    • In prokaryotes, translation can begin while transcription is still in progress, because both processes occur in the cytoplasm (prokaryotes do not have a nucleus)

    • This allows for rapid protein synthesis