Cambridge International A Level Biology Revision Guide

Chapter 6: Nucleic acids and protein synthesis
valine, lysine, leucine and glycine’. The complete set of
triplet codes is shown in Appendix 2.
3΄
G T T A A A C T T G G G
C A A T T T G A A C C C
5΄
this is the strand
which is ‘read’
the code is read
in this direction
Figure 6.12 A length of DNA coding for four amino acids.
QUESTION
6.3 There are 20 different amino acids which cells use for
making proteins.
a How many different amino acids could be coded
for by the triplet code? (Remember that there are
four possible bases, and that the code is always
read in just one direction on the DNA strand.)
b Suggest how the ‘spare’ triplets might be used.
c Explain why the code could not be a two-letter
code.
An example of mutation: sickle cell
anaemia
One mutation that has a significant effect is the one involved
in the inherited blood disorder sickle cell anaemia.
Haemoglobin is the red pigment in red blood cells
which carries oxygen around the body. A haemoglobin
molecule is made up of four polypeptide chains, each with
one iron- containing haem group in the centre. Two of
these polypeptide chains are called α chains, and the other
two β chains. (The structure of haemoglobin is described
on pages 43 – 44.)
The gene which codes for the amino acid sequence in
the β polypeptides is not the same in everyone. In most
people, the β polypeptides begin with the amino acid
sequence:
Val-His-Leu-Thr-Pro-Glu-Glu-Lys-
This is coded from the Hb A (normal) allele of the gene.
But in some people, the base sequence CTT is replaced
by CAT, and the amino acid sequence becomes:
Val-His-Leu-Thr-Pro-Val-Glu-Lys-
This is coded from the Hb S (sickle cell) allele of the gene.
This type of mutation is called a substitution. In this
case, the small difference in the amino acid sequence
results in the genetic disease sickle cell anaemia in
individuals with two copies of the Hb S allele. You can read
more about sickle cell anaemia on pages 407–408.
5΄
3΄
Protein synthesis
The code on the DNA molecule is used to determine the
sequence of amino acids in the polypeptide. Figure 6.13 on
pages 120–121 describes the process in detail, but briefly
the process is as follows.
The first stage is called transcription. In the nucleus,
a complementary copy of the code from a gene is made
by building a molecule of a different type of nucleic acid,
called messenger RNA (mRNA), using one strand (the
sense strand) as a template. Three RNA nucleotides are
joined together by the enzyme RNA polymerase. This
process copies the DNA code onto an mRNA molecule.
Transcription of a gene begins when RNA polymerase
binds to a control region of the DNA called a promoter
and ends when the enzyme has reached a terminator
sequence. At this point, the enzyme stops adding
nucleotides to the growing mRNA. The hydrogen bonds
holding the DNA and RNA together are broken and
double-stranded DNA reforms. The last triplet transcribed
onto mRNA is one of the DNA triplets coding for ‘stop’
(ATT, ATC or ACT – see Appendix 2).
The next stage of protein synthesis is called translation
because this is when the DNA code is translated into an
amino acid sequence. The mRNA leaves the nucleus and
attaches to a ribosome in the cytoplasm (page 15).
In the cytoplasm, there are molecules of transfer RNA
(tRNA). These have a triplet of bases at one end and a region
where an amino acid can attach at the other. There are
at least 20 different sorts of tRNA molecules, each with a
particular triplet of bases at one end and able to attach to a
specific amino acid at the other (Figure 6.14 on page 122).
The tRNA molecules pick up their specific amino acids
from the cytoplasm and bring them to the mRNA on
the ribosome. The triplet of bases (an anticodon) of each
tRNA links up with a complementary triplet (a codon) on
the mRNA molecule. Two tRNA molecules fit onto the
ribosome at any one time. This brings two amino acids
side by side, and a peptide bond is formed between them
(page 39). Usually, several ribosomes work on the same
mRNA strand at the same time. They are visible, using an
electron microscope, as polyribosomes (Figure 6.15 on
page 122).
So the base sequence on the DNA molecule determines
the base sequence on the mRNA, which determines
which tRNA molecules can link up with them. Since
each type of tRNA molecule is specific for just one amino
acid, this determines the sequence in which the amino
acids are linked together as the polypeptide molecule
is made.
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