Inquiry Question 2: How important is it for genetic material to be replicated exactly?
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
A focused answer to the HSC Biology Module 5 dot point on DNA replication. The semi-conservative model, the enzymes involved (helicase, primase, DNA polymerase, ligase), the leading and lagging strands, and the standard worked exam example.
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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.
Common traps
Forgetting the antiparallel orientation. DNA polymerase only works in the 5' to 3' direction. This is why the lagging strand needs Okazaki fragments.
Confusing primase and ligase. Primase makes the primer; ligase joins fragments. Markers test this regularly.
Wrong base pairs. A pairs with T (DNA) or U (RNA). G pairs with C. Get this wrong and you lose 1-2 marks immediately.
In one sentence
DNA replication is the semi-conservative copying of the double helix in which helicase unwinds, primase primes, DNA polymerase extends in the 5' to 3' direction (continuously on the leading strand, in Okazaki fragments on the lagging strand), and ligase joins the fragments.
Past exam questions, worked
Real questions from past NESA papers on this dot point, with our answer explainer.
2021 HSC4 marksDescribe the process of DNA replication in eukaryotic cells.Show worked answer →
A high-band answer for 4 marks needs four substantive points, in order.
Unwinding. The DNA double helix is unwound by the enzyme helicase, which breaks the hydrogen bonds between complementary base pairs to expose two single strands at the replication fork.
Priming. Primase synthesises a short RNA primer on each template strand to provide a free 3' end for DNA polymerase to extend from.
Elongation. DNA polymerase adds free nucleotides to the growing strand in the 5' to 3' direction, following the base-pairing rules (A with T, G with C). The leading strand is synthesised continuously; the lagging strand is synthesised in short Okazaki fragments.
Ligation. DNA ligase joins the Okazaki fragments together, producing two identical daughter DNA molecules. The process is described as semi-conservative because each daughter molecule contains one original strand and one newly synthesised strand.
Markers reward naming the four enzymes explicitly and using the term "semi-conservative."
2019 HSC3 marksExplain why DNA replication is described as semi-conservative.Show worked answer →
"Semi-conservative" means that each of the two daughter DNA molecules produced by replication contains one original (parental) strand and one newly synthesised strand.
The original double helix unwinds into two single strands. Each single strand acts as a template for the synthesis of a new complementary strand. The result is two double helices, each made of one old strand and one new strand. Neither daughter molecule is entirely new, and neither is entirely original.
The semi-conservative model was confirmed by the Meselson-Stahl experiment (1958) using nitrogen isotopes. Their results ruled out the conservative model (which would have produced one entirely old and one entirely new helix) and the dispersive model.
Related dot points
- Construct appropriate representations to model and compare the processes of transcription and translation, including but not limited to: the roles of mRNA, tRNA, rRNA and ribosomes in polypeptide synthesis
A focused answer to the HSC Biology Module 5 dot point on protein synthesis. Transcription in the nucleus (DNA to mRNA), translation at the ribosome (mRNA to polypeptide), the roles of mRNA, tRNA, rRNA, the codon-anticodon match, and the standard worked exam example.
- Model the processes involved in cell replication, including but not limited to: mitosis and meiosis, the role of meiosis and gamete formation in maintaining the chromosome number across generations
A focused answer to the HSC Biology Module 5 dot point on meiosis. The two divisions, crossing over and independent assortment as sources of genetic variation, comparison with mitosis, and how gamete formation maintains chromosome number across generations.