Why add the lengths before taking a ratio?

The insert fraction is measured against the recombinant total, not the original vector, so adding first stops the percentage being overstated.

How is the gel mobility ratio used?

Within one gel the shorter fragment travels further, so dividing one distance by the other gives the ratio without needing absolute mobility.

S6 Biology: Biotechnology Paper 3 (multi-step calculations and lab data)

2026-11-03

S · 6

Paper 3 chains two or three steps: find the total first, then convert units or percentages, and finally check against the data.

Quick answer　4,200 bp; G is 35%; 48 copies; every 4,096 bp; about 6.71 x 10^9 copies; four times; 10%; about 0.0000114 matches; 50 g; every 256 bp.

01　Paper details (check answers afterwards)

Item	Detail
Questions	10 questions
Suggested time	25 minutes
Difficulty mix	Foundation 4 · Intermediate 3 · Challenge 2 · Error redo 1
Chapter	Chapter: S6 Biology: Biotechnology
Level	S6

02　Questions (10 items)

FoundationA 2,400 bp plasmid takes a 1,800 bp insert. What is the recombinant length?
FoundationIn a double-stranded DNA adenine is 15% of the bases. What percentage is guanine?
FoundationStarting from three copies, four PCR cycles run. How many copies result?
FoundationAn enzyme recognises six base pairs. With bases equally likely, how far apart is a site on average?
IntermediateTwo hundred copies run through twenty-five cycles. How many copies are expected?
IntermediateIn a gel a 3,000 bp fragment moves 1.8 cm and a 750 bp fragment moves 7.2 cm. How many times further does the short one travel?
IntermediateA 5.4 kb vector takes a 0.6 kb insert. What percentage of the recombinant plasmid is the insert?
ChallengeA guide RNA is 24 nt and the genome is about 3.2 x 10^9 bp. How many random matches are expected?
ChallengeA plant runs five batches of 250 L at 40 mg per litre. What total mass of protein is recovered, in grams?
Error redoError repair: a student claims a four-base cutter cuts every 4 bp, so a 1,000 bp fragment has 250 sites. What is correct?

03　Answers and one-line notes

#	Answer	One-line note
1	4,200 bp	Two thousand four hundred plus one thousand eight hundred is 4,200 bp.
2	35%	A and T take 30% together, so G and C share the remaining 70% and G is 35%.
3	48 copies	Each cycle doubles, so four cycles multiply by 16, and three times sixteen is 48.
4	every 4,096 bp	Four to the power 6 is 4,096, so sites average one every 4,096 bp.
5	about 6.71 x 10^9 copies	Two to the power 25 is 33,554,432, and times two hundred gives about 6.71 x 10^9, fewer in practice as reagents run out.
6	four times as far	Seven point two divided by 1.8 is 4, because shorter fragments migrate further.
7	10%	The total is 6.0 kb and 0.6 divided by 6.0 times one hundred is 10%.
8	about 0.0000114 matches	Four to the power 24 is about 2.81 x 10^14, and 3.2 x 10^9 divided by it is about 0.0000114, far below one.
9	50 g	Five times 250 is 1,250 L, times 40 mg gives 50,000 mg, that is 50 g.
10	one site every 256 bp, so about four sites in 1,000 bp	Each extra recognition base multiplies the average distance by four; four bases give four to the power 4, that is 256 bp, so about four sites in 1,000 bp.

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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