6 Nucleic acids and protein synthesis
Nucleic acids have roles in the storage and retrieval of genetic information and in the use of this information
to
... [Show More] synthesise polypeptides. DNA is the molecule of heredity and is an extremely stable molecule that cells
replicate with great accuracy. The genetic code explains how the sequence of nucleotides in DNA and
messenger RNA (mRNA) determines the sequence of amino acids that make up a polypeptide. In eukaryotes
this involves the processes of transcription in the nucleus to produce mRNA, followed by translation in the
cytoplasm to produce polypeptides.
6.1 Structure of nucleic acids and
replication of DNA
Learning outcomes
Candidates should be able to:
1 describe the structure of nucleotides, including the
phosphorylated nucleotide ATP (structural formulae are not
expected)
2 state that the bases adenine and guanine are purines with a
double ring structure, and that the bases cytosine, thymine and
uracil are pyrimidines with a single ring structure (structural
formulae for bases are not expected)
3 describe the structure of a DNA molecule as a double helix,
including:
• the importance of complementary base pairing between the
5′ to 3′ strand and the 3′ to 5′ strand (antiparallel strands)
• differences in hydrogen bonding between C–G and A–T base
pairs
• linking of nucleotides by phosphodiester bonds
4 describe the semi-conservative replication of DNA during the
S phase of the cell cycle, including:
• the roles of DNA polymerase and DNA ligase (knowledge
of other enzymes in DNA replication in cells and different
types of DNA polymerase is not expected)
• the differences between leading strand and lagging strand
replication as a consequence of DNA polymerase adding
nucleotides only in a 5′ to 3′ direction
5 describe the structure of an RNA molecule, using the example
of messenger RNA (mRNA)
6.2 Protein synthesis Learning outcomes
Candidates should be able to:
1 state that a polypeptide is coded for by a gene and that a gene
is a sequence of nucleotides that forms part of a DNA molecule
2 describe the principle of the universal genetic code in which
different triplets of DNA bases either code for specific amino
acids or correspond to start and stop codons
6.2 Protein synthesis continued Learning outcomes
Candidates should be able to:
3 describe how the information in DNA is used during
transcription and translation to construct polypeptides,
including the roles of:
• RNA polymerase
• messenger RNA (mRNA)
• codons
• transfer RNA (tRNA)
• anticodons
• ribosomes
4 state that the strand of a DNA molecule that is used in
transcription is called the transcribed or template strand and
that the other strand is called the non-transcribed strand
5 explain that, in eukaryotes, the RNA molecule formed following
transcription (primary transcript) is modified by the removal
of non-coding sequences (introns) and the joining together of
coding sequences (exons) to form mRNA
6 state that a gene mutation is a change in the sequence of
base pairs in a DNA molecule that may result in an altered
polypeptide
7 explain that a gene mutation is a result of substitution or
deletion or insertion of nucleotides in DNA and outline how
each of these types of mutation may affect the polypeptide
produced [Show Less]