The process of semi-conservative replication of DNA in terms of: unwinding…

The idea

DNA helicase moves along the molecule and breaks the hydrogen bonds between complementary bases, so the double helix unwinds and the two strands separate, each then acting as a template. Free DNA nucleotides present in the nucleoplasm are attracted to the exposed complementary bases, adenine to thymine and cytosine to guanine, and hydrogen bonds form between them. DNA polymerase then catalyses condensation reactions between adjacent nucleotides, joining the phosphate of one to the deoxyribose of the next to form phosphodiester bonds and building the new sugar-phosphate backbone, with water released each time. DNA polymerase is itself an enzyme with a specific active site, complementary to only one end of the developing strand, so nucleotides can be added in one direction only, from five prime to three prime.

What earns the marks

• DNA helicase breaking the hydrogen bonds between complementary bases so the strands separate.
• Each separated strand acting as a template, with free DNA nucleotides attracted to the exposed complementary bases.
• Specific base pairing, adenine with thymine and cytosine with guanine.
• DNA polymerase catalysing the condensation reaction that joins adjacent nucleotides by phosphodiester bonds.
• Explaining one-directional synthesis by the specificity of DNA polymerase, whose active site is complementary to only one end of the developing strand.

Common mistakes and how to put them right

• Naming RNA polymerase, or RNA nucleotides, which describes transcription and not replication.

• Saying only one strand acts as a template, which is true of transcription but not of replication.

• Explaining the five prime to three prime rule by hydrogen bonding or base pairing, which examiners reject, instead of by the specific active site of DNA polymerase.

• Writing that helicase 'unzips the DNA' without saying that hydrogen bonds between bases are broken.

• Saying DNA polymerase forms the hydrogen bonds between bases, when it catalyses the condensation reaction that forms phosphodiester bonds.

Exam tips

• Name both enzymes and give each one a different job; a six-mark description that mentions only 'enzymes' cannot reach full marks.
• Use 'attracted to exposed bases' and 'complementary base pairing' as separate points, because they are credited separately.
• If the question gives you a diagram of the two strands, use it: say which end each new strand grows from before explaining why.