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

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