Why is PCR exponential?

Each cycle product becomes the template of the next cycle, so the amount doubles every round.

How does recognition length affect specificity?

A longer recognition site appears less often by chance, so cutting is more specific.

S6 Biology: Biotechnology Paper 1 (PCR, vectors and GM crops)

2026-11-03

S · 6

Biotechnology rests on copying, cutting, inserting and screening DNA; number items need only doubling and ratio reasoning.

Quick answer　1,048,576 times; C is 20%; 4,096 bp; 5,200 bp; 800 bp; 25%; twofold; about 0.0029 expected hits; 400 mL; 2 to the power 30 is about 1.07 billion.

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)

FoundationPCR doubles the DNA amount every cycle. After 20 cycles, how many times the original amount is present?
FoundationIn a double-stranded DNA molecule adenine is 30% of all bases. What percentage is cytosine?
FoundationEcoRI recognises a six base pair sequence. With four bases equally likely, how many bp apart is a site on average?
FoundationA plasmid of 4,000 bp takes a 1,200 bp insert. How long is the recombinant plasmid?
IntermediateA pre-mRNA is 1,100 bp long and carries two introns totalling 300 bp. How long is the mature mRNA after splicing?
IntermediateA GM maize raises yield from 8 tonnes to 10 tonnes per hectare. What is the percentage increase?
IntermediateIn gel electrophoresis a 500 bp fragment moves 6 cm while a 1,000 bp fragment moves 3 cm. How many times further does the short fragment travel?
ChallengeA guide RNA is 20 nt long and the human genome is about 3.2 x 10^9 bp. How many random matches are expected, and is that specific enough?
ChallengeA viral vector contains 5 x 10^8 particles per millilitre and a treatment needs 2 x 10^11 particles. How many millilitres are required?
Error redoError repair: a student claims 30 PCR cycles give 30 times the original DNA. What is the correct figure?

03　Answers and one-line notes

#	Answer	One-line note
1	1,048,576 times	Twenty doublings mean 2 to the power 20, which equals 1,048,576.
2	20%	A and T take 60% together, so G and C share the remaining 40% and C is 20%.
3	every 4,096 bp	Each position has four possibilities, so six positions give 4 to the power 6, which is 4,096.
4	5,200 bp	The vector length plus the insert gives 4,000 plus 1,200, which is 5,200 bp.
5	800 bp	Splicing removes the introns, so 1,100 minus 300 leaves 800 bp.
6	25%	The gain of 2 tonnes divided by the original 8 tonnes, times 100, gives 25%.
7	twice as far	Six divided by three is 2, because shorter fragments travel faster through the gel.
8	about 0.0029 expected matches, under one, so usually a single target site	Four to the power 20 is about 1.1 x 10^12, and 3.2 x 10^9 divided by it gives 0.0029, so the site is nearly unique.
9	400 mL	Two times 10 to the 11 divided by five times 10 to the 8 gives 400.
10	about 1.07 billion times (2^30 = 1,073,741,824)	PCR doubles every round, so growth is exponential rather than additive; 30 rounds give 2 to the power 30.

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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