The structure of DNA — answer key

1. NQ1 · Practice

Model answer. A DNA molecule is made of two strands coiled together to form a double helix. Each strand contains chemicals called bases. Bonds between pairs of bases hold the two strands together.

A full answer includes

• g1: strands.
• g2: helix.
• g3: bases.
• g4: bases (base pairs).

Also accept

• strands; helix; bases; bases

Watch for

• “DNA is a single chain, like starch.” — DNA has two strands, coiled round each other.

2. NQ2 · Practice

Options as numbered on the pupil sheet: 1 — A; 2 — T; 3 — C; 4 — G.

Model answer. A always pairs with T, and C always pairs with G. So T pairs with A, and G pairs with C.

A full answer includes

• A: T.
• T: A.
• C: G.
• G: C.

Also accept

• A–T, T–A, C–G, G–C

Watch for

• “Any base can pair with any other base.” — Each base has exactly one partner. Start with A–T, then C–G; T and G are the same two pairs read from the other side.

3. NQ3 · Practice

Model answer. a. Bonds between the two bases of each base pair hold the strands together. A DNA molecule has a great many base pairs, so together the bonds hold the strands firmly.

A full answer includes

• I chose a.

Also accept

• a

Watch for

• “The strands are joined end to end.” — The strands lie side by side, joined across by the base pairs.

4. NQ4 · Practice

Model answer. Each base on the other strand is the partner of the base opposite it: A pairs with T, C with G, G with C, T with A, T with A and A with T. So the other strand reads T G C A A T.

A full answer includes

• T G C A A T.

Also accept

• TGCAAT

Watch for

• “The other strand is the same as the first.” — Each base faces its partner, so the other strand has the partner bases.
• “Any base can pair with any other base.” — Write the partner under each letter in turn: A gets T, C gets G, G gets C, T gets A. Check that every column is an A–T or C–G pair.

5. NQ5 · Practice

Model answer. a and c. In DNA, A pairs only with T, and C pairs only with G (so G–C is the same pair seen from the other strand). A–C and T–G never form.

A full answer includes

• I chose a and c.

Also accept

• a, c

Watch for

• “Any base can pair with any other base.” — A–C and T–G never form: check each option against the two pairs A–T and C–G.

6. NQ6 · Practice

Model answer. a, c and d. DNA is two strands coiled into a double helix; each strand contains bases; bonds between pairs of bases hold the strands together. It is not a single strand (b), and the bases always pair A with T and C with G (e).

A full answer includes

• I chose a, c and d.

Also accept

• a, c, d

Watch for

• “Any base can pair with any other base.” — Statement e is wrong: the pairing rule allows only A with T and C with G. Leave e unticked and check the other four against the double-helix diagram.

7. NT1 · Apply

Model answer. Every A is paired with a T, so T = 30 %. Together A and T make 60 % of the bases, so C and G make 100 − 60 = 40 %. Every C is paired with a G, so they are equal: C = 20 % and G = 20 %.

Checked automatically

• T: 30 % (within ±0.1 %)
  • Common wrong answer 70 : T is the partner of A, so it has the same percentage as A.
• C: 20 % (within ±0.1 %)
  • Common wrong answer 30 : C pairs with G, not with A. C and G share what is left after A and T.
  • Common wrong answer 40 : C and G together make 40 %; they are equal, so each is half of that.
  • Common wrong answer 70 : Take away both A and T: 100 − 30 − 30 = 40, then halve it.
• G: 20 % (within ±0.1 %)
  • Common wrong answer 30 : G pairs with C, not with A. C and G share what is left after A and T.
  • Common wrong answer 40 : C and G together make 40 %; G is half of that.

A full answer includes

• T = 30 % (it pairs with A).
• A + T = 60 %, so C + G = 40 %.
• C = 20 % and G = 20 %.

Also accept

• T 30 %, C 20 %, G 20 %

Watch for

• “All four bases come in equal numbers.” — Only partners are equal: A = T and C = G.
• “C = 70 %, because the rest of the bases are C.” — The other 40 % (after A and T) is shared equally between C and G.

8. NT2 · Apply

Model answer. The student is not necessarily right. Each G pairs with a C, so opposite the 4 G bases the other strand has 4 C bases. The G bases on the other strand sit opposite C bases on the first strand, and we are not told how many C bases the first strand has. In the whole molecule, the number of G equals the number of C, but that is counted across both strands, not strand by strand.

A full answer includes

• Not necessarily: opposite each of the 4 G bases is a C, so the other strand has (at least) 4 C bases.
• The number of G bases on the other strand equals the number of C bases on the first strand, which we are not told.

Also accept

• opposite each G is a C; so 4 C on the other strand; G on the other strand matches C on the first, unknown

Watch for

• “Both strands of DNA are the same.” — The strands are partners, not copies: each base faces its partner.