Cambridge International A Level Biology Revision Guide

Cambridge International AS Level Biology
results of centrifuging
DNA from bacteria
grown with only 15 N . . .
. . . and then for
one generation
with 14 N . . .
. . . and then for
a second generation
with 14 N . . .
. . . and then for
a third generation
with 14 N.
caesium
chloride
solution
band of DNA
1
2 3 4
Figure 6.11 Meselson and Stahl’s experimental results.
118
QUESTION
6.2 Look at Figure 6.11.
a Assuming that the DNA has reproduced semiconservatively,
explain why the band of DNA in
tube 2 is higher than that in tube 1.
b What would you expect to see in tube 2 if the
DNA had replicated conservatively?
c What would you expect to see in tube 2 if the
DNA had replicated dispersively?
d Which is the first tube that provides evidence that
the DNA has reproduced semi-conservatively?
Genes and mutations
DNA molecules can be enormous. The bacterium
E. coli has just one DNA molecule, which is four million
base pairs long. There is enough information to code for
several thousand proteins. The total DNA of a human cell is
estimated to be about 3 × 10 9 base pairs long. However, it is
thought that only 3% of this DNA actually codes for protein.
The function of much of the remainder is uncertain.
A part of a DNA molecule, where the nucleotide
sequence codes for just one polypeptide, is called a gene,
and one DNA molecule contains many genes. A change
in the nucleotide sequence of a gene, which may then
result in an altered polypeptide, is called a mutation.
Most genes have several different variants called alleles,
which originally arose by the process of mutation.
DNA, RNA and protein synthesis
DNA controls protein synthesis
How can a single type of molecule like DNA control all the
activities of a cell? The answer is very logical. All chemical
reactions in cells, and therefore all the cells’ activities, are
controlled by enzymes. Enzymes are proteins. DNA is a
code for proteins, controlling which proteins are made.
Thus, DNA controls the cell’s activities.
Protein molecules are made up of strings of amino
acids. The shape and behaviour of a protein molecule
depends on the exact sequence of these amino acids – that
is, its primary structure (page 40). DNA controls protein
structure by determining the exact order in which the
amino acids join together when proteins are made in a cell.
The triplet code
The sequence of nucleotide bases in a DNA molecule is
a code for the sequence of amino acids in a polypeptide.
Figure 6.12 shows a very short length of a DNA molecule,
just enough to code for four amino acids.
The code is a three-letter, or triplet, code. Each
sequence of three bases stands for one amino acid. The
sequence is always read in the same direction and from
only one of the two strands of the DNA molecule (the
so-called sense strand). In this case, assume that this is the
lower strand in the diagram. The complementary strand is
referred to as the anti-sense strand.
Reading from the the left-hand end of the lower strand
in Figure 6.12, the code is:
CAA which codes for the amino acid valine
TTT which codes for the amino acid lysine
GAA which codes for the amino acid leucine
CCC which codes for the amino acid glycine
So this short piece of DNA carries the instruction to
the cell: ‘Make a chain of amino acids in the sequence