DNA replication explained: HSC Biology Module 5

3 marks for a description of DNA replication, 2 for outlining some steps, 1 for some relevant information.

Sample answer (marking guidelines): An enzyme unzips the DNA, creating a replication fork. On each strand, an enzyme attaches to the original DNA nucleotides and uses them as a template. It 'reads' the bases and adds complementary nucleotides. Another enzyme 'glues' the nucleotides together, forming a new, double-stranded section of DNA.

Markers warned against confusing DNA replication with polypeptide (protein) synthesis, and stressed giving adequate detail at each step (unzipping, template reading, complementary base pairing, joining of nucleotides).

Source: NESA 2022 HSC Biology examination and marking guidelines.

1 mark for correctly naming all four enzymes; 1 mark for correctly matching each to its role (markers accept a brief phrase, not a sentence).

• Helicase - unwinds the double helix and breaks the hydrogen bonds between base pairs at the replication fork.
• Primase - lays down a short RNA primer to give DNA polymerase a free 3'-OH to start from.
• DNA polymerase - adds complementary free nucleotides to the template, synthesising the new strand 5' to 3'.
• DNA ligase - joins (seals) the Okazaki fragments on the lagging strand into one continuous strand.

1 mark for the correct complementary sequence; 1 mark for correctly labelling the ends antiparallel to the template; 1 mark for explaining the antiparallel rule.

• Daughter strand: 5'-ATGCCGTAATCG-3' (read A-T, T-A, C-G, G-C against the template, then check Chargaff pairing across the whole string).
• Ends: the daughter's 5' end sits opposite the template's 3' end, because the two strands are antiparallel.
• Why: DNA polymerase can only add nucleotides to a free 3'-OH, so it builds the new strand 5' to 3' while reading the template 3' to 5'. The new strand must therefore run opposite to its template.

1 mark antiparallel strands run in opposite directions; 1 mark DNA polymerase only synthesises 5' to 3'; 1 mark leading vs lagging behaviour at the fork; 1 mark role of Okazaki fragments and ligase.

• The two template strands are antiparallel, yet DNA polymerase can only add nucleotides in the 5' to 3' direction (to a free 3'-OH).
• On the template oriented 3' to 5' toward the fork, the new leading strand is built continuously, following the helicase as it opens the helix.
• On the opposite template, synthesis must run away from the fork, so the new lagging strand can only be made in short pieces each time more template is exposed.
• These short pieces are Okazaki fragments; each needs its own primer, and DNA ligase seals the fragments into one continuous strand.

Up to 2 marks for an accurate model/description of each process; up to 3 marks for a genuine comparison across the three stated criteria (similarities AND differences).

• Mitosis (model): one nuclear division of a body (somatic) cell. DNA is replicated in S phase, then chromosomes line up singly at the metaphase plate and sister chromatids are pulled apart, giving two genetically identical diploid daughter cells.
• Meiosis (model): two successive divisions (meiosis I then meiosis II) of a germ-line cell, after a single round of DNA replication. Homologous chromosomes pair and undergo crossing over in prophase I, then separate (meiosis I); sister chromatids separate in meiosis II, giving four genetically unique haploid gametes.
• Divisions: mitosis = one division; meiosis = two divisions.
• Daughter cells: mitosis = 2, diploid, identical to the parent; meiosis = 4, haploid, genetically varied.
• Role: mitosis enables growth, repair and asexual reproduction; meiosis produces gametes for sexual reproduction and generates genetic variation.
• A top response also notes the shared feature: both are preceded by semi-conservative DNA replication, so accurate replication underpins both.

A Band 6 response makes a clear judgement (the command word is "assess") supported by mechanism and consequence, not just a description of replication.

Make the judgement (1 mark). Accurate DNA replication is essential for the continuity of a species because it ensures genetic information is passed faithfully from one generation of cells - and of organisms - to the next, while still allowing the limited variation evolution requires.

Mechanisms that promote fidelity (2 marks).

• Complementary base pairing (A-T, G-C) means each parental strand templates only one correct sequence, so the semi-conservative mechanism is inherently high-fidelity.
• DNA polymerase proofreading: the enzyme checks each added nucleotide and excises mismatches, and mismatch-repair systems correct errors that slip through, lowering the error rate to roughly one in a billion bases.

Consequences of inaccuracy (2 marks).

• Uncorrected errors become mutations. In somatic cells these can disrupt cell-cycle control and cause cancer; in germ-line cells they are heritable and passed to offspring.
• A high error rate would accumulate harmful mutations faster than selection could remove them, threatening the viability of the population.

Balance the judgement (2 marks). Replication must be accurate but not perfect: rare, un-repaired changes are the ultimate source of the genetic variation on which natural selection acts. Without any variation a species cannot adapt to a changing environment and risks extinction. So continuity of species depends on replication that is overwhelmingly accurate yet leaves a small margin for change - both fidelity and a low rate of variation are needed.

(a) 2 marks - 1 for the immediate molecular effect, 1 for the downstream consequence.

• Without ligase, the Okazaki fragments on the lagging strand cannot be joined, so the newly synthesised strand stays as unconnected pieces with nicks in the sugar-phosphate backbone.
• The lagging strand cannot be completed, leaving fragmented, unstable DNA and stalling production of intact daughter chromosomes.

(b) 2 marks - 1 for the property of each cell type, 1 for linking it to ligase dependence.

• Bone marrow turns over millions of cells per day and so replicates DNA constantly; blocking ligation cripples each S phase and the tissue is hit hard.
• Mature neurons are post-mitotic - they rarely (if ever) replicate DNA - so they need little new lagging-strand synthesis and are largely spared.

(c) 2 marks - 1 for the link to cancer-cell behaviour, 1 for the reasoning/limitation.

• Cancer cells divide rapidly and continuously, so they depend heavily on replication enzymes; a ligase inhibitor should damage them selectively, much like it spares neurons.
• Reasoning: the therapeutic window relies on dividing tumour cells being more vulnerable than most healthy tissue, though fast-dividing healthy tissues (gut lining, bone marrow, hair follicles) would still suffer side effects - the classic trade-off of cytotoxic chemotherapy.