VCE Biology (Year 12) · Nucleic Acids and Proteins · 12m read
DNA Structure and Replication
Ever wondered how you inherited your parent's eye colour or why identical twins share the same DNA? DNA is a twisted ladder-shaped molecule that holds all the instructions for building and running your body—and it copies itself perfectly before every cell divides, a process called semi-conservative replication where each new DNA molecule keeps one original strand and builds a new partner strand. Understanding how this copying works is absolutely crucial because it explains how traits pass between generations and why mutations can happen, making it the foundation for everything else you'll study in biology this year.
Learning objective: Describe the structure of DNA as a double helix and explain the process of semi-conservative replication.
By the end of this lesson you can:
- I can identify the four nucleotide bases and their complementary pairings.
- I describe the 5'–3' directionality of a DNA strand.
- I explain why DNA replication is called "semi-conservative".
DNA replication is unzipping and copying a zipper. Helicase unzips the double helix; DNA polymerase builds a new matching half on each side — semi-conservative copying.
The double helix and base pairing
DNA is a polymer of nucleotides. Each nucleotide has three parts: a deoxyribose sugar, a phosphate group, and one of four nitrogenous bases — adenine (A), thymine (T), cytosine (C), guanine (G). The sugar–phosphate backbone forms the "rails" of the helix; the bases project inward and pair across the two strands.
Base pairing rules: A pairs with T via two hydrogen bonds; C pairs with G via three. The strands run antiparallel: one strand is 5'→3' and the other 3'→5'. The 5' and 3' labels refer to carbon atoms on the sugar.
Replication happens during S phase. Helicase unwinds the helix, DNA polymerase reads each template 3'→5' and builds a new strand 5'→3'. Each daughter DNA contains one old and one new strand — hence "semi-conservative".
DNA and replication rules
- Base pairs: A–T (2 H-bonds), C–G (3 H-bonds).
- Antiparallel: One strand 5'→3', the other 3'→5'.
- Helicase: Unwinds the double helix.
- DNA polymerase: Reads template 3'→5', builds new strand 5'→3'.
- Semi-conservative: Each daughter has one parent strand + one new.
Quick reference
| Enzyme | Role |
|---|---|
| Helicase | Unwinds DNA |
| DNA polymerase | Adds nucleotides 5′→3′ |
| Primase | Lays RNA primer |
| Ligase | Joins Okazaki fragments |
| Model | Semi-conservative |
Worked example (Medium)
A template DNA strand reads 3'-TACGGCAT-5'. What is the sequence of the newly synthesised strand, and in which direction?
- Apply base pairing. A–T and C–G.
Template: 3'-TACGGCAT-5' - Pair each base. T→A, A→T, C→G, G→C.
New: 5'-ATGCCGTA-3' - Check direction. New strand is antiparallel to template.
Built 5'→3'
Answer: New strand: 5'-ATGCCGTA-3', built in the 5' to 3' direction.
Worked example (Hard)
A double-stranded DNA molecule contains 30% adenine. Determine the percentage of each base.
- Apply Chargaff's rules. A pairs with T, so A = T = 30%.
A = T = 30% - Remaining bases. Total = 100%. C + G = 100% − 60% = 40%.
C = G = 20% - Verify. 30 + 30 + 20 + 20 = 100% ✓.
Answer: A = T = 30%, C = G = 20%. Chargaff's rules guarantee equal complementary base percentages.
Common mistakes
- Forgetting DNA strands are antiparallel One strand runs 5'→3' and the other 3'→5' — this is essential for replication and transcription. Fix: Always label both ends of any strand you draw or write. The template strand is read 3'→5'; new strand is built 5'→3'.
- Saying DNA polymerase "reads" 5'→3' DNA polymerase reads the template 3'→5' and synthesises the new strand 5'→3'. Fix: Remember: the enzyme BUILDS 5'→3' but READS the template in the opposite direction (3'→5').
- Confusing "semi-conservative" with "conservative" or "dispersive" Semi-conservative means each daughter molecule has one old and one new strand — proven by Meselson-Stahl. Fix: Semi = half. Each daughter DNA retains (conserves) one parent strand and gains one newly synthesised strand.
Meselson and Stahl — the most beautiful experiment in biology
In 1958, Matthew Meselson and Franklin Stahl performed what is often called the "most beautiful experiment in biology" to prove DNA replication is semi-conservative.
Method: They grew *E. coli* in heavy nitrogen (¹⁵N) medium until all DNA was "heavy." They then transferred bacteria to light nitrogen (¹⁴N) medium and extracted DNA after each generation. Using caesium chloride density gradient centrifugation, they separated DNA by density.
Results: After one generation, ALL DNA was intermediate density — ruling out conservative replication (which would produce one heavy and one light band). After two generations, DNA was 50% intermediate and 50% light — ruling out dispersive replication and confirming semi-conservative.
Why it matters for VCE: VCAA frequently tests your understanding of the experimental logic. You must be able to explain what each replication model would predict and how the results distinguish between them. The experiment also demonstrates the scientific method — hypothesis, prediction, experiment, conclusion.
Connection to modern biology: Semi-conservative replication means mutations on one strand can be detected by mismatch repair enzymes comparing the two strands. This is the basis of DNA proofreading and repair — essential for maintaining genome integrity across billions of cell divisions.
Practice questions
Which two bases are linked by three hydrogen bonds?
- C and G
- A and T
- A and G
- C and T
(2 marks)
DNA replication is described as "semi-conservative" because:
- Each daughter DNA has one parental and one new strand
- Only half the DNA is copied each time
- The new strand is a conserved copy of the original
- DNA polymerase works on alternate bases
(3 marks)
Explain why DNA polymerase can only synthesise in the 5'→3' direction.
(3 marks)
Summary
- Identify the four nucleotide bases and their complementary pairings.
- Describe the 5'–3' directionality of a DNA strand.
- Explain why DNA replication is called "semi-conservative".