Edexcel A (SNAB) A Level Biology:复习笔记6.2.7 Post-transcriptional Modification

Post-transcriptional Modification

  • Antibody molecules consist primarily of protein
  • They contain four separate polypeptide chains, two of which are longer, heavy chains, and two of which are shorter, light chains
  • The structure of the heavy chain determines whether an antibody will be bound to the membrane of a white blood cell, or secreted directly into the blood
  • Some heavy chains contain an extra section which allows the antibody to bind to the surface of a white blood cell
  • White all heavy chains are coded for by the same gene, post-transcriptional modification determines whether or not this extra section of protein is present in the heavy chain of an antibody
  • Post transcriptional modification mechanisms include
    • Splicing
    • Alternative 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, while 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 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
    • E.g. depending on the exons that are removed from the gene coding for the antibody heavy chain, it can produce either a membrane-bound or a directly secreted antibody

Alternative splicing of a gene can produce more than one type of protein