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Transcription and Translation
The Genetic Code
The instructions for protein structure are carried in the genes, which are sequences of DNA nucleotides.
Three nucleotides code for an amino acid, e.g. AAA on the transcribing strand codes for phenylalanine
whilst AAT codes for leucine. So, successive triplets of DNA nucleotides determine the sequence of
amino acids in a protein, i.e. its primary structure. Obviously a mutation that results in a change in the
base sequence can cause an alteration in amino acid sequence (think back to sickle cell anaemia – this is
caused by a mutation which results in the sixth amino acid of the β polypeptide of haemoglobin being
replaced by valine).
Why are there three nucleotides in the code? There are four bases in DNA (adenine, guanine, thymine
and cytosine). Clearly, it is not possible to have one base coding for each of 20 different amino acids.
Similarly, using two bases would be insufficient (4n or 16 permutations of two bases, where n = the
number of bases used in the code). Using three bases is more than enough (4n or 64 permutations of
three bases). The triplet code has spare coding capacity and it is possible that several codes can be used
for the same amino acid, e.g. phenylalanine can be coded for by AAA and AAG, whilst leucine can be
coded for by AAT, AAC, GAA, GAG, GAT and GAC. Because of this, the code is described as degenerate.
The genetic code for the transcribing strand can be found in Appendix 1, at the end of this booklet.
The genetic code is widespread but not, as first thought, universal. This is useful for genetic engineers
since they can take a gene from one organism and it can usually be read in another without the need for
changing any of the coding. Consequently, the human insulin gene can be read by E. coli which
obligingly makes insulin for diabetics (see Teaching Module 2 of this unit).
Transcription and Translation
The instructions contained in the DNA must be copied into mRNA. Transcription takes place in the
nucleolus and mRNA copies are then transported to the ribosomes in the cytoplasm for translation or
protein synthesis.
Transcription
During transcription, the portion of DNA that is to be copied unzips (hydrogen bonds are broken by a
helicase enzyme), so that the bases of the complementary DNA strands are exposed. Only one of the
strands is copied. This is the transcribing strand. Activated RNA nucleotides line up opposite their
exposed complementary bases – U with A, A with T, G with C and C with G. The adjacent nucleotides
are then joined together by an RNA polymerase which uses the energy released when the two extra
phosphate groups are removed from the activated nucleotides. Note: the mRNA has a base sequence
that is complementary to that of the transcribing strand and the triplets of bases that code for an amino
acid in the mRNA are called codons.
SJWMS Biology
The diagram outlines the events of transcription. Add notes to describe and explain the events.
1.
3.
4.
2.
A
A
T
T
U
A
G
G
C
G
G
C
T
U
A
A
A
T
C
C
G
A
A
T
T
U
A
C
C
G
T
U
A
U
A
T
A
A
C
C
T
U
6.
T
G
A
5.
Note: Once produced, the mRNA molecule is released and leaves the nucleus via the nuclear pores. It travels
to the ribosome for translation, i.e. reading of the message and production of protein.
As described, the process of transcription has similarities to DNA replication – the DNA molecule unwinds
and unzips. The DNA acts as the template for synthesis of the RNA and complementary base pairing
ensures that the message is transcribed accurately.
Q. How is this different from DNA replication?
You may find it helpful to consider: exposure of the DNA to act as a template; which strands are
copied; the nucleotides involved and any differences in their composition, e.g. pentose sugar
involved; complementary pairing; other features, e.g. nature of the resulting polynucleotide.
Translation
Translation takes place in the cytoplasm at the ribosomes.
Ribosomes are made in the nucleolus from protein and
ribosomal RNA (rRNA). There are two subunits, a large
subunit and a small subunit with a groove between them
in which the mRNA fits.
Transfer RNA (tRNA) transports amino acids to the
ribosome for incorporation into protein. Each tRNA is a
single polynucleotide that is folded into a clover-leaf
shape. Hydrogen bonds between complementary bases
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U A C
U
A
C
Anticodon. The bases are
complementary to those on the
mRNA.
Three bases, for the attachment
of a specific amino acid.
stabilise the shape. The tRNA has an anticodon that is complementary to the codon on the mRNA, and
three unpaired bases to which a specific amino acid can be attached. This ensures that the appropriate
amino acid gets to the right place.
During translation, mRNA enters the groove between the small and large subunit. The ribosome is large
enough for two codons to be side by side (and has two tRNA binding sites). The first codon is always the
same, the initiation or start codon AUG, which codes for methionine. The anticodon of a tRNA carrying
methionine hydrogen bonds to the first codon. A second tRNA carrying the second amino acid coded for
hydrogen bonds to the second codon. A peptide bond is formed between the two amino acids and the
ribosome moves three bases (one codon) along the mRNA so that another codon is exposed. A third
tRNA brings another amino acid which is added to the dipeptide and so the process continues until a
termination or stop codon (UAA, UAG or UGA) is reached. Add descriptions/explanations to the diagrams
overleaf showing translation.
1.
2.
A U G
U A C
G U
A
C A
U
C
U
U
G
C
A
G U
U
A C U
A A
A U G
U A C
Met
Leu
His
Val
C A
U
G U
C
U
U
A G
A
A C U
A
U G A
A
Met
G U A
C A U
Val
His
Leu
Thr
Thr
3.
U G A
4.
A U G
U A C
G U A
C A U
Met
C A
U
G U
Val
C
U
A G
His
U
A
Leu
A C U
A
U A
A U G
C
C
G U
A U
Met
SJWMS Biology
A
C A U
G U A
C
G
U
A
U
A
Val
Thr
A C U
U G A
Val
His
G U A
C A U
C A U
G U A
U G A
5.
U A
A U G
C
Leu
Thr
Met
His
C
U
G
A C U
U
A
Leu
A
U G A
Thr
The protein may need further processing before it is active, e.g. insulin requires considerable processing
first and some enzymes require activation by cyclic AMP (see activation of enzymes later)
TASK: Add brief explanatory notes to help with revision of this topic.
A
A
T
T
U
A
G
G
C
G
G
C
T
U
A
A
A
T
C
C
G
A
A
T
T
U
A
C
C
G
T
U
A
T
U
A
A
A
C
C
T
U
A U G
U A C
Met
G U A
C A U
C A
U
G U
Val
C
U
U
A G
A
His
Leu
A C U
A
U G A
Thr
T
G
A
U A
A U G
C
C
Met
G U
A U
A
C A U
G U A
C
G
U
A
U
A
A C U
U G A
Val
His
Leu
Thr
Helpful terms (which will need expanding upon):
Transcription
Single-stranded messenger RNA leaves nucleus
Translation at the ribosome
Amino acids transferred to ribosome by transfer RNA
Complementary base pairing allows transcription and correct sequencing of amino acids at the ribosome.
SJWMS Biology