Split Genes

Most eukaryotic genes are split into segments. In decoding the open reading frame of a gene for a known protein, one usually encounters periodic stretches of DNA calling for amino acids that do not occur in the actual protein product of that gene. Such stretches of DNA, which get transcribed into RNA but not translated into protein, are called introns. Those stretches of DNA that do code for amino acids in the protein are called exons. Examples:

• The gene for one type of collagen found in chickens is split into 52 separate exons.

• The gene for dystrophin, which is mutated in boys with muscular dystrophy, has 79 exons.

• even the genes for rRNA and tRNA are split.

In general, introns tend to be much longer than exons. An average eukaryotic exon is only 140 nts long, but one human intron stretches for 480,000 nucleotides!

The cutting and splicing of mRNA must be done with great precision. If even one nucleotide is left over from an intron or one is removed from an exon, the reading frame from that point on will be shifted, producing new codons specifying a totally different sequence of amino acids from that point to the end of the molecule (which often ends prematurely anyway when the shifted reading frame generates a STOPcodon).

The removal of introns and splicing of exons is done with the spliceosome. This is a complex of several snRNA molecules and some 145 different proteins.

The introns in most pre-mRNAs begin with a GU and end with an AG. Presumably these short sequences assist in guiding the spliceosome.

Translational Control

• There are regions on the beginning of mRNA which do not code for proteins. These are the leaders.

  • Proteins and other molecules can bind to the leader which can enhance or restrict ribosome binding (and thus translation)

• mRNA molecules in cytoplasm may be degraded and recycled to make more RNA.

• This varies the amount of gene product that is produced (as a mRNA that's degraded quickly won't express much protein).

Post-Translational Control

• Protein Cleavage and/or Splicing - the newly formed protein is rarely functional as is. They typically need to be modified (i.e. insulin)

 

 

• Chemical modification. Protein function can be modified by addition of methyl, phosphoryl, or glycosyl groups.  Signal sequences direct packaging and secretion. Some proteins have "signal sequences" which direct their packaging in the Golgi and movement through the endoplasmic reticulum (ER) to be secreted. The signal sequences usually end up cleaved off.