Model the processes involved in cell replication, including but not limited to: mitosis and meiosis, DNA replication using the Watson and Crick DNA model, including nucleotide composition, pairing and bonding

What this dot point is asking

NESA wants you to model the process of DNA replication using the Watson and Crick double-helix structure, name the enzymes involved, and explain the base-pairing rules that make accurate replication possible. This is one of the most heavily examined dot points in Module 5 and appears at least once per cycle.

The answer

DNA replication is the process by which a cell copies its entire genome before dividing. It is semi-conservative, meaning each daughter molecule contains one original strand and one newly synthesised strand.

The Watson and Crick model

DNA is a double helix of two antiparallel strands held together by hydrogen bonds between complementary base pairs. Adenine pairs with thymine (A-T) by two hydrogen bonds. Guanine pairs with cytosine (G-C) by three hydrogen bonds. Each strand has a 5' end and a 3' end; the two strands run in opposite directions (antiparallel).

The four steps

1. Unwinding

The enzyme helicase breaks the hydrogen bonds between base pairs, separating the double helix into two single strands at the replication fork.

2. Priming

Primase synthesises a short RNA primer on each single strand, giving DNA polymerase a free 3'-OH group to extend from.

3. Elongation

DNA polymerase reads each template strand in the 3' to 5' direction and adds complementary free nucleotides to the growing daughter strand in the 5' to 3' direction.

• The leading strand runs continuously toward the replication fork.
• The lagging strand runs away from the fork and is synthesised in short fragments called Okazaki fragments.

4. Ligation. DNA ligase joins the Okazaki fragments into one continuous strand.

The result: two identical daughter DNA molecules, each containing one parental and one new strand.

Why semi-conservative matters

The semi-conservative model was proposed by Watson and Crick in 1953 and confirmed by Meselson and Stahl in 1958 using nitrogen isotopes (^15N and ^14N). Their experiment ruled out two alternative models (conservative and dispersive) and is the standard cited example in HSC responses.

Examples in context

Example 1. Replication speed in a dividing skin keratinocyte. A basal keratinocyte in human skin completes the S phase of its cell cycle in about six hours, copying the entire 3.2 billion base pair genome before mitosis. To achieve this in such a short window, replication initiates simultaneously from roughly 50,000 origins of replication along the chromosomes. At each origin, two replication forks travel outward in opposite directions, with DNA polymerase adding around 50 nucleotides per second on the leading strand. The lagging strand at each fork is built from thousands of Okazaki fragments, each 100-200 nucleotides long, joined by ligase. If a single replication fork stalls, repair proteins flag it for resolution before mitosis begins.

Example 2. PCR exploits DNA replication in the lab. Polymerase chain reaction (PCR), now central to NSW Health pathology labs running COVID-19 and pertussis diagnostic tests, copies a target DNA region by repeating three temperature steps: 95 degrees C to denature (mimicking helicase), 55 degrees C to anneal short DNA primers to the template (mimicking primase), and 72 degrees C for Taq polymerase to elongate. Each cycle doubles the target DNA, so 30 cycles produce roughly a billion copies. Taq polymerase, isolated from a Yellowstone hot-spring bacterium, replaced human DNA polymerase in PCR because it tolerates the 95 degree C denaturation step that would inactivate other polymerases.

Try this

Q1. A short DNA template reads 3'-TACGGCATTAGC-5'. Write the sequence of the newly synthesised daughter strand, indicating the 5' and 3' ends. [2 marks]

• Cue. Apply complementary base pairing (A-T, G-C) and note the daughter strand runs antiparallel, so its 5' end aligns with the template's 3' end.

Q2. Predict the result of the Meselson-Stahl experiment after two rounds of replication in normal $^{14}N$ medium, given the parental DNA was fully labelled with $^{15}N$. [3 marks]

• Cue. After round 1, all DNA is hybrid (intermediate density). After round 2, half is hybrid and half is light - this rules out conservative and dispersive models.

Q3. A drug candidate inhibits DNA ligase activity in dividing cells. (a) Describe how this drug would affect DNA replication at the molecular level. (b) Explain why this drug would have a greater effect on rapidly dividing cells such as bone marrow than on neurons. [2+2 marks]

• Cue. (a) Okazaki fragments accumulate unjoined; the lagging strand stays fragmented and unable to support stable chromosome structure. (b) Neurons are post-mitotic and rarely replicate DNA; bone marrow turns over millions of cells per day and depends on continuous replication.