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Protein Synthesis
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Making a protein involves 3 different kinds of
RNA:
– Messenger RNA (mRNA) – carries the genetic
information from the DNA to the ribosomes
– Transfer RNA (tRNA) – binds to amino acids and
lines them up by attaching to the appropriate sets of
three bases along an mRNA molecule
– Ribosomal RNA (rRNA) – is a part of the
ribosomes which ‘read’ the coded message in the
mRNA and use it to link amino acids in the correct
order
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There are three stages to protein synthesis in a
eukaryotic cell:
(1)Transcription:
One of the two strands of the DNA is copied
(transcribed) into mRNA in a similar way to DNA
replication
 The same base pairing rules apply as in DNA
replication except that adenine pairs with uracil
instead of thymine
 The DNA strand involved in base pairing with the
ribonucleotides acts as a template and is called
the template strand
 The RNA strand grows in the 5’-3’ direction and
is controlled by the enzyme RNA polymerase
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(2)RNA Processing
Most genes in eukaryotic cells are interrupted
with section of DNA called introns that do not
code for proteins. The sections of DNA that do
code for proteins and hence are expressed are
called exons.
 Introns are included in the original mRNA copy
but must be removed during this stage to
produce mature mRNA
 After removal of the introns, the mRNA leaves
the nucleus through pores in the nuclear
envelope and goes through to the cytoplasm
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(3)Translation
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This is the stage where the amino acids are linked in the sequence specified by the
base sequence of mRNA.
The mRNA molecule is a copy of a gene and the information it contains is written in
code – the sequence of the bases along it – at this point the 4 letter ‘alphabet’ of
DNA must be translated into the 20 letter ‘alphabet’ of amino acids:
In protein synthesis the bases in an mRNA molecule are read in groups of three
called codons.
A codon stands for a particular amino acid in the polypeptide
It is the function of the tRNA and the ribosomes to decode the mRNA and translate it
into the correct amino acid sequence of a polypeptide.
Transfer RNA (tRNA)
– This is a clover leaf shaped molecule
– It has a 3-base anticodon (complementary to the codon on mRNA) at one end,
and the appropriate amino acid at the other
– It picks up the amino acid specific to each anticodon code and carries it to the
ribosomes to be clicked into the proper order to make a protein
– The cytoplasm stocks all 20 amino acids ready for use by making them or
absorbing them from food.
Ribosomal RNA (rRNA)
– Makes up part of the structure of ribosomes
– Its function is to hold together the mRNA and tRNA so that the peptide bond
between amino acids can be formed (it helps ‘click’ the amino acids together)
– As each new amino acid is joined, the ribosome nudges the mRNA strand along
three ‘notches’ and the next tRNA falls into place.
Protein Synthesis in
Prokaryotes
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This happens in the same way to
eukaryotes except:
– There are no introns in prokaryotes so the
only stages are transcription and translation
– There is no nuclear envelope to separate the
sites of transcription and translation so the
ribosomes don’t have to ‘wait’ for mRNA to be
completed i.e. transcription and translation
can take place at the same time.