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Processing of mRNA
DNA contains some regions pre-mRNA
that do not code for proteins.
These are known as introns.
exon
To produce functional proteins,
these introns need to be spliced
out of the mRNA, leaving only
the regions that code for
proteins, called exons.
intron
exon
spliceosome
A molecule called a spliceosome
removes the introns, producing
mature mRNA that contains only
exons. Before splicing, mRNA is
known as pre-mRNA.
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intron
mature
mRNA
exons
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Translation
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Cracking the code
In 1961, Nirenberg and
Heinrich deciphered which
amino acids are coded for by
which codons.
They used synthetic mRNA
made from only one base type,
e.g. uracil. They radioactively
labelled an amino acid and
assessed the radioactivity of
each protein produced.
They discovered that mRNA
made from just uracil codes for
phenylalanine (codon UUU).
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Post-translational modifications
Once translated, a polypeptide chain may undergo changes.
These post-translational modifications include:

removal of methionine – the start codon for each gene
codes for a methionine; in many cases this will be removed

addition of functional groups –
e.g. phosphate or acetate

structural changes – e.g. the
addition of disulfide bridges or
cleavage of a part of the chain
Modifications of the primary structure give the protein its
specific secondary structure that allows it to perform its function.
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Post-translational modification of insulin
Insulin is synthesised in the beta
cells of the pancreas.
The polypeptide chain produced during
translation is preproinsulin. It undergoes
several post-translational changes before
it becomes the active form of insulin.
preproinsulin
proinsulin
In the endoplasmic reticulum, a part of
the chain is removed, forming proinsulin.
The chain is then cleaved at two
points, resulting in the active form
of insulin being produced.
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insulin
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Protein synthesis
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The stages of protein synthesis
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