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 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

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

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

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