Post-transcriptional Modification (Edexcel A (SNAB) A Level Biology)

• 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

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