DSE Biology | Biotechnology Lesson-Prep Guide (Tutor Edition) | PCR, Gel Electrophoresis and Gene Transfer

2026-11-01

Tutor lesson-prep guides · Secondary Biology

Students call PCR “copying DNA” but cannot say how many copies thirty cycles produce. This lesson turns the abstraction into a number: each cycle doubles, so thirty cycles is about a billion times.

Objectives — Students can: (1) use 2^n to calculate PCR amplification; (2) read fragment size from band position on a gel; (3) describe the restriction enzyme, ligase and transformation steps in order and calculate transformation efficiency.

01 Prerequisites (3-minute check)

If any of these fails, fix it before the lesson starts.

Skill	Quick check	Pass mark
Indices	2 to the power 10	1024
Base pairing	What pairs with A	T
Enzymes	What enzymes do	Lower activation energy, specific

02 Lesson flow (60 minutes)

Time	Stage	Tutor does	Student does
0-8 min	Recall: DNA structure	A-T, G-C	Write the pairs
8-20 min	The three PCR temperatures	Denature, anneal, extend	State the order
20-32 min	Exponential amplification	30 cycles	1.07e+09 fold
32-44 min	Gel electrophoresis	Small fragments travel further	Compare three bands
44-54 min	Enzymes and vectors	Cut, join, insert	2000 colonies per ug
54-60 min	Exit check	Three questions, marked at once	Independent

03 Teaching script (read it out)

Scene 1: PCR doubles every cycle, and thirty cycles runs away with you

One cycle gives two copies, so 30 cycles give 2^30 = 1.074e+09, roughly a billion fold. Students should work out 2 to the power 10 (1024) and then multiply by ten tens rather than memorise the answer.

Board note: PCR doubles every cycle, and thirty cycles runs away with you

Scene 2: Electrophoresis answers one question: how big

The gel is a sieve, DNA is negative and moves to the positive electrode, and smaller fragments pass through the pores faster and travel further. A 2000 bp band stays near the top while 500 bp runs to the bottom, so two bands in one lane are two products of different size.

Board note: Electrophoresis answers one question: how big

Scene 3: Gene transfer has three steps and the order is not optional

A restriction enzyme cuts the vector and the gene to leave sticky ends, ligase seals the join, and the construct goes into bacteria. If 0.1 ug of DNA gives 200 colonies, efficiency is 2000 per ug. Base composition must also balance: A is 30%, so T is 30%, and the remaining 40% splits evenly between G and C at 20% each.

Board note: Gene transfer has three steps and the order is not optional

04 Question types and worked steps

Type	Example	Steps	Answer
Calculation	PCR fold change	2^n	1.07e+09
Interpretation	Gel bands	Smaller travels further	500 bp at the bottom
Calculation	Base composition	A = T, G = C	G 20%
Calculation	Transformation efficiency	Colonies / ug	2000 per ug

Sanity check (teach this): Use a calculator or logs for powers of two; never estimate. bp is a length, not a mass, and the gel separates by length. Efficiency divides by the mass of DNA added in ug, not by the volume of medium.

05 Independent practice (3 questions)

Question	Answer	What to watch
Fold change over 30 cycles	1.07e+09	2^30
G when A is 30%	20%	(100 – 60) / 2
Efficiency for 200 colonies from 0.1 ug	2000 per ug	colonies / ug
Which band travels furthest, 2000 or 500 bp	500 bp	Smaller is faster

06 Common mistakes and what to say

Mistake	Student writes	Say this
Treating PCR as linear	30 cycles means 30 fold	“Each cycle multiplies by two, so it is a billion fold.”
Assuming big fragments travel furthest	Pointing at 2000 bp at the bottom	“The gel is a sieve; bigger fragments get stuck near the top.”
Reversing the order	Ligating before cutting	“Sticky ends must exist before ligase can seal anything.”
Dividing by volume	Using 1 cm3 as the denominator	“Efficiency is per ug of DNA, not per cm3.”

07 Homework (paper, section and question numbers)

DSE Biology practice paper (genetics and biotechnology) — Questions 15-18
DSE Biology practice paper (molecular genetics) — Questions 19-20

08 Exit check (last 5 minutes)

Question	Answer	Decision
2^30	1.07e+09	Wrong: redo indices
Which band is furthest	The smallest fragment	Wrong: revisit the sieve
Efficiency for 200 colonies from 0.1 ug	2000 per ug	Wrong: revisit the denominator

09 Tutor FAQ

Why three temperatures in PCR?

Denaturing breaks hydrogen bonds, annealing lets primers bind, extension lets polymerase copy; the order cannot change.

Why use restriction enzymes rather than random cutting?

They cut only at specific sequences, so the ends match and can be rejoined at the same site.

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A TutorZone lesson-prep guide. Written by the TutorZone team; questions and lesson flow are original and are not taken from any school or publisher material.