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1 A mononucleotide is a pentose sugar (either ribose or deoxyribose) with an attached
phosphate and an attached organic nitrogenous base (either adenine, thymine, guanine
or cytosine in DNA, or adenine, uracil, guanine or cytosine in RNA).
2 The complementary base pairs in DNA are adenine and thymine, and guanine and
cytosine. Hydrogen bonds link the bases in a complementary pair that are bonded to
opposite strands. This makes the ‘rungs’ of the DNA helix. As there are many base pairs
in a DNA molecule, and therefore many hydrogen bonds, the two strands are held firmly
together in the helical structure by the hydrogen bonds.
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1 Chargaff’s data showed that the proportions of guanine and cytosine are the same
in DNA, as are the proportions of adenine and thymine. This must be the case if
guanine forms complementary base pairs with cytosine and if adenine pairs with
thymine. As there is no other physical association between bases in DNA, you would
expect no other relationship between the proportions of each base, as Chargaff found.
Pauling’s results showed that hydrogen bonding can maintain a 3D helical structure.
Knowing that the complementary base pairs are held together by hydrogen bonds
helps to explain how the DNA normally forms a helical structure.
2 Development of ideas in science depends on the piecing together of many different
pieces of information. In the case of DNA, before Crick and Watson could decide on
the best model for the structure of DNA, they had to understand the experimental
results from Chargaff, Pauling and Franklin that their model would have to explain. So
communication of ideas between scientists is essential.
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1 Meselson and Stahl showed that after one replication in a medium containing light
nitrogen the density of the DNA produced was half-way between that of DNA
containing all heavy nitrogen and DNA containing all light nitrogen. This could
only be explained by the DNA having one strand containing light nitrogen and one
containing heavy nitrogen, which is what you get after semiconservative replication. If
there had been conservative replication they would have found half the DNA all heavy
and the other half all light.
2 The molecule of DNA is unzipped by DNA helicase which breaks the hydrogen bonds
between complementary base pairs. Free DNA nucleotides form hydrogen bonds with
matching exposed bases on the DNA molecules. Then DNA polymerase and DNA
ligase cause bonds to form between the nucleotides and so build the new DNA strands.
Unit 1 Topic 2: Chapter 2.1
110
Edexcel AS Biology Implementation and Assessment Guide for Teachers and Technicians ©Pearson
Education Limited 2008
Answers to Student Book text questions
Chapter 2.1(cont.)
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1 a The DNA contains the genetic code for the protein.
b The DNA unwinds at the point where the gene for the protein occurs. The coding
(antisense) strand of the gene attracts nucleotides to matching base pairs and these
join to form a strand of messenger RNA. When it is released from the DNA, the
mRNA molecules passes out of the nucleus and attaches to a ribosome.
b Transfer RNA molecules bring matching amino acids to the ribosome and attach
to the messenger RNA so that the amino acids are aligned correctly to form the
protein chain.