Biology • Year 12 • Module 5 • Lesson 6
DNA Structure and DNA Replication
Lock in the Watson and Crick model, the complementary base-pairing rules, and what semiconservative replication actually means.
1. Label the DNA double-helix diagram
The diagram below shows a short section of DNA from the Watson and Crick model. Write the missing labels into boxes A–H. Each label is drawn from this lesson's Key Terms or from Cards 1–3. 8 marks
- A — building block (one whole _______________________)
- B — sugar component (name: _______________________)
- C — phosphate group _______________________
- D — nitrogenous base _______________________
- E — base pair labelled (A pairs with _______________________)
- F — base pair labelled (C pairs with _______________________)
- G — type of bond holding paired bases together _______________________
- H — overall shape of the molecule (___ helix) _______________________
| Box | Your label |
|---|---|
| A | |
| B | |
| C | |
| D | |
| E | |
| F | |
| G | |
| H |
2. Term–definition match
The ten definitions below are shuffled. In the right-hand column write the matching term from this list: nucleotide, double helix, complementary base pairing, hydrogen bonds, semiconservative replication, template strand, adenine, thymine, cytosine, guanine. 10 marks
| # | Definition (shuffled) | Matching term |
|---|---|---|
| 2.1 | The building block of DNA, consisting of a sugar, a phosphate group and a nitrogenous base. | |
| 2.2 | The twisted "ladder" shape formed by two DNA strands wound around each other. | |
| 2.3 | The specific pairing rule by which adenine pairs with thymine and cytosine pairs with guanine. | |
| 2.4 | Weak bonds between complementary bases that help hold the two DNA strands together. | |
| 2.5 | The form of DNA replication in which each new DNA molecule contains one original strand and one newly synthesised strand. | |
| 2.6 | An original DNA strand used as a guide for building a new complementary strand during replication. | |
| 2.7 | The DNA base that pairs with thymine. | |
| 2.8 | The DNA base that pairs with adenine (it is replaced by uracil in RNA). | |
| 2.9 | The DNA base that pairs with guanine. | |
| 2.10 | The DNA base that pairs with cytosine. |
3. True or false — with correction
For each statement, circle T or F. If the statement is false, write the corrected version. 10 marks (1 for T/F, 1 for the correction where needed)
3.1 A DNA nucleotide is made of a sugar, an amino acid and a phosphate group. T / F
3.2 In DNA, adenine pairs with cytosine and thymine pairs with guanine. T / F
3.3 The two strands of a DNA molecule are held together by hydrogen bonds between complementary bases. T / F
3.4 Semiconservative replication means that each new DNA molecule is built entirely from new nucleotides, with both original strands discarded. T / F
3.5 The two strands of a DNA molecule are identical to each other. T / F
4. Cloze — Watson, Crick and semiconservative replication
Use each word from the word bank below once to fill the blanks. 10 marks (1 per blank)
Word bank: nucleotides · double helix · complementary · hydrogen · template · semiconservative · thymine · guanine · sequence · separate
The Watson and Crick model describes DNA as a (4.1) ____________________ made of two strands of (4.2) ____________________. Each nucleotide contains a sugar, a phosphate group and a nitrogenous base, and the (4.3) ____________________ of these bases stores hereditary information.
The two strands are held together by (4.4) ____________________ bonds between paired bases. Adenine pairs with (4.5) ____________________, while cytosine pairs with (4.6) ____________________ — a relationship called (4.7) ____________________ base pairing.
During DNA replication, the two original strands (4.8) ____________________. Each old strand then acts as a (4.9) ____________________ for building a new strand. The outcome is that every new DNA molecule contains one original strand and one new strand, which is why replication is described as (4.10) ____________________.
5. Function recall
Answer each in 1–2 sentences using precise terms from the lesson. 8 marks (2 each)
5.1 What is the function of the sugar–phosphate backbone of a DNA strand?
5.2 What is the function of complementary base pairing during DNA replication?
5.3 What is the function of an original (template) strand when a new DNA molecule is being built?
5.4 What is the function of accurate DNA replication for daughter cells produced by cell division?
Q1 — Labelled DNA diagram (8 marks)
A: nucleotide. B: deoxyribose (sugar). C: phosphate group. D: nitrogenous base (A, T, C or G). E: thymine (A pairs with T). F: guanine (C pairs with G). G: hydrogen bonds. H: double helix. Mark each box as 1 mark for the correct biological label.
Q2 — Term–definition matches (10 marks)
2.1 nucleotide • 2.2 double helix • 2.3 complementary base pairing • 2.4 hydrogen bonds • 2.5 semiconservative replication • 2.6 template strand • 2.7 adenine • 2.8 thymine • 2.9 cytosine • 2.10 guanine.
Q3 — True / false with correction (10 marks)
3.1 False. Correction: a DNA nucleotide is made of a sugar, a phosphate group and a nitrogenous base (not an amino acid — amino acids are protein building blocks).
3.2 False. Correction: in DNA, adenine pairs with thymine and cytosine pairs with guanine.
3.3 True.
3.4 False. Correction: semiconservative replication means each new DNA molecule contains one original strand and one newly synthesised strand — the originals are not discarded.
3.5 False. Correction: the two strands of a DNA molecule are complementary (and antiparallel), not identical — each is the template for producing the other.
Q4 — Cloze answers (10 marks)
4.1 double helix • 4.2 nucleotides • 4.3 sequence • 4.4 hydrogen • 4.5 thymine • 4.6 guanine • 4.7 complementary • 4.8 separate • 4.9 template • 4.10 semiconservative.
Q5.1 — Function of the sugar–phosphate backbone
The sugar–phosphate backbone provides the structural "rails" of each DNA strand. The repeating sugar and phosphate units form a stable, continuous chain along which the bases project inwards, so the molecule keeps its shape and the bases are positioned to pair with the complementary strand.
Q5.2 — Function of complementary base pairing in replication
Because each base has only one complementary partner (A with T, C with G), the existing strand specifies exactly which base must be added at each position on the new strand. This means the original sequence guides the new sequence, allowing the molecule to be copied accurately rather than randomly.
Q5.3 — Function of the original (template) strand
An original strand acts as a template for the new strand: free nucleotides are matched to it according to the complementary base-pairing rules. Without the original strand the cell would have no guide for which base to add next, so accurate copying would be impossible.
Q5.4 — Function of accurate replication for daughter cells
Accurate replication ensures that both daughter cells receive a complete and correct copy of the hereditary information. This preserves the instructions for proteins and cell function, so the daughter cells behave as cells of the same organism — and, ultimately, supports continuity of the species through reliable inheritance.