Cambridge International A Level Biology Revision Guide

Cambridge International AS Level Biology
Answers to self-assessment questions
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Chapter 6
1 a DNA polymerase, which catalyses the linkage
of adjacent nucleotides once they have
correctly base-paired.
(Also ATP, which phosphorylates the
nucleotides, providing energy to drive
the reaction.)
b the nucleus
2 a The DNA in tube 2 is less dense than that
in tube 1. In tube 1, all the nitrogen in the
DNA molecules is nitrogen-15 ( 15 N). In tube
2, each DNA molecule is made up of one
strand containing nitrogen-14 ( 14 N) and one
containing nitrogen-15 ( 15 N).
b One band in the original position (the ‘old’
DNA containing only nitrogen-15, and
another band higher up (the ‘new’ DNA
containing only nitrogen-14).
c Assuming that most strands ended up with a
mix of nitrogen-14 ( 14 N) and nitrogen-15 ( 15 N)
– which you would expect if the bits of each
kind were scattered randomly – then there
would be a single band, like the one shown in
tube 2.
d Tube 3. If the DNA had replicated dispersively,
then instead of two distinct bands there
would be a single wide one, because each
strand of DNA would contain a mix of
nitrogen-14 ( 14 N) and nitrogen-15 ( 15 N).
The band would be higher than that in
tube 2, because there would now be more
nitrogen-14 ( 14 N) and less nitrogen-15 ( 15 N) in
the DNA molecules.
As it is, the top band contains DNA molecules
in which all the nitrogen is nitrogen-14 ( 14 N)
and the lower band contains DNA molecules
in which one strand contains nitrogen-14 ( 14 N)
and one strand contains nitrogen-15 ( 15 N).
c A two-letter code could only code for
16 amino acids.
4 DNA contains the pentose sugar deoxyribose,
while RNA contains ribose.
DNA contains the base thymine, while RNA
has uracil.
DNA is made up of two polynucleotide
strands, whereas RNA has only one.
DNA molecules are much longer than RNA
molecules.
5 There are various possible flow diagrams; for
example, the one below
DNA unwinds and the two strands separate
↓
complementary mRNA molecule built up against
one DNA strand (transcription)
↓
mRNA molecule attaches to ribosome
↓
complementary tRNA, loaded with appropriate
amino acid pairs, with one codon on mRNA
(translation)
↓
peptide bond forms between adjacent
amino acids
3 a 64
b For ‘punctuation marks’ – that is, for starting
or stopping the synthesis of a polypeptide
chain. Also, some amino acids could be coded
for by two or three different base triplets.
Cambridge International AS and A Level Biology © Cambridge University Press 2014