What rule governs base pairing?

A always pairs with T and G with C, one purine to one pyrimidine, which keeps the helix diameter even.

Why is replication called semi-conservative?

Each daughter keeps one parental strand and one new strand, so half of the original is conserved.

S5 Biology: DNA and Protein Synthesis (Paper 1)

2026-11-01

The first paper builds the picture of the double helix from base pairing, then turns base counts and lengths into quantitative answers.

Direct answer　DNA is two antiparallel strands held by hydrogen bonds: two between A and T, three between G and C. Replication is semi-conservative, so each daughter keeps one parental strand.

01　Paper details (check answers afterwards)

Item	Detail
Questions	10
Suggested time	25
Difficulty mix	Foundation 4 · Intermediate 3 · Challenge 2 · Error redo 1
Chapter	Chapter: S5 Biology: DNA and Protein Synthesis
Level	Four basic, three advanced, two challenge and one error-repair item

02　Questions (10 items)

FoundationHow many polynucleotide strands make up the DNA double helix?
FoundationWhich four bases are found in DNA?
FoundationHow many hydrogen bonds join adenine to thymine, and guanine to cytosine?
FoundationA DNA molecule has 800 bases, 120 of them adenine. How many thymine, guanine and cytosine bases are there?
IntermediateWhich mode of replication does DNA follow?
IntermediateWhat does DNA polymerase do during replication?
IntermediateOne DNA molecule replicates twice. How many DNA molecules result?
ChallengeA DNA segment has 900 base pairs and guanine makes up 30 per cent of all bases. How many adenine bases are there?
ChallengeA DNA molecule has 5 000 base pairs of 0.34 nm each. What is its length in nm and in micrometres?
Error redoA student says adenine pairs with guanine. What is wrong?

03　Answers and one-line notes

#	Answer	One-line note
1	Two, running in opposite directions	Two strands run in opposite directions and are held together by hydrogen bonds between paired bases.
2	Adenine (A), thymine (T), guanine (G) and cytosine (C)	The sequence of these four bases carries the genetic information.
3	Two for A with T, three for G with C	The extra hydrogen bond between G and C makes GC-rich DNA harder to separate.
4	Thymine 120, guanine 280, cytosine 280	A pairs with T so both number 120, leaving 560 bases shared equally by G and C at 280 each.
5	Semi-conservative replication	Each daughter molecule keeps one parental strand and gains one newly built strand.
6	It adds complementary nucleotides using the parent strand as a template	Base pairing fixes the order on the new strand, so both daughters match the parent.
7	Four molecules	Each round doubles the number, so two rounds give two squared, which is four molecules.
8	360 adenine bases	Nine hundred pairs give 1 800 bases; G and C take 540 each, leaving 720 shared by A and T at 360 each.
9	1 700 nm, or 1.7 micrometres	Five thousand times 0.34 nm gives 1 700 nm, which is 1.7 micrometres after dividing by a thousand.
10	Adenine pairs with thymine and guanine with cytosine	Two purines are both bulky, so pairing is always one purine with one pyrimidine to keep the helix uniform.

04　What to do after marking

After marking, redo only the wrong items and write down which step went wrong (formula, unit, or misread number).
Repeat the same item two days later; move to the next paper only when both attempts are correct.
Parents only need the answer and note columns; there is no need to explain every item.

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Questions and answers are written originally by the TutorZone team and are not taken from any school, publisher or examination authority paper. Every value is checked by computer.