Cambridge International A Level Biology Revision Guide

Chapter 6: Nucleic acids and protein synthesis
Summary
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DNA and RNA are polynucleotides, made up of
long chains of nucleotides. A nucleotide contains a
pentose sugar, a phosphate group and a nitrogencontaining
base. In RNA, the sugar is ribose; in DNA, it is
deoxyribose.
A DNA molecule consists of two polynucleotide chains,
linked by hydrogen bonds between bases. In DNA there
are four bases – adenine always pairs with thymine, and
cytosine always pairs with guanine.
RNA, which comes in several different forms, has only
one polynucleotide chain, although this may be twisted
back on itself, as in tRNA. In RNA, the base thymine is
replaced by uracil.
DNA molecules replicate during interphase by semiconservative
replication. In DNA replication, the
hydrogen bonds between the bases break, allowing
free nucleotides to fall into position opposite their
complementary ones on each strand of the original DNA
molecule. Adjacent nucleotides are then linked, through
their phosphates and sugars, to form new strands. Two
complete new molecules are thus formed from one old
one, each new molecule containing one old strand and
one new strand.
The sequence of nucleotide bases on a DNA molecule
codes for the sequence of amino acids in a polypeptide.
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Each amino acid is coded for by three bases. A length
of DNA coding for just one polypeptide is a gene. A
change in the nucleotide sequence of DNA is a mutation,
producing a new allele of the gene.
The DNA sequences for the Hb A (normal) and Hb S (sickle
cell) alleles of the gene for the β-globin polypeptide
differ by only one base. The triplet CTT in Hb A is replaced
by CAT in Hb S , swapping the amino acid from glutamic
acid to valine. This single difference in the polypeptide
results in sickle cell anaemia in individuals with two Hb S
alleles.
During protein synthesis, a complementary copy of
the base sequence on a gene is made, by building
a molecule of messenger RNA (mRNA) against one
DNA strand. This stage is called transcription. After
transcription, the next stage is called translation. During
translation the mRNA moves to a ribosome in the
cytoplasm.
Transfer RNA (tRNA) molecules with complementary
triplets of bases temporarily pair with base triplets on
the mRNA, bringing appropriate amino acids. When two
amino acids are held side by side, a peptide bond forms
between them. The ribosome moves along the mRNA
molecule, so that appropriate amino acids are gradually
linked together, following the sequence laid down by the
base sequence on the mRNA.
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End-of-chapter questions
1 What can be found in both DNA and messenger RNA (mRNA)?
A double helix structure
B sugar–phosphate chain
C ribose
D thymine [1]
2 Which statement about base pairing in nucleic acids is not correct?
A Adenine can pair with either thymine or uracil.
B Guanine only pairs with cytosine.
C Thymine can pair with either adenine or uracil.
D Uracil only pairs with adenine. [1]
3 How many different arrangements of four bases into triplets can be made?
A 3 + 4
B 3 × 4
C 3 4
D 4 3 [1]