DSE Biology: Genetic Engineering and Biotechnology (Tutor Lesson Plan) | Recombinant DNA in five steps and gel reading

2026-11-04

Tutor lesson-prep guides · Secondary Biology

Students say cut with a restriction enzyme and stick with ligase, then answer how do you prove the insert worked with just run a gel. Give them length evidence: sites, fragment sizes and band positions read together.

Objectives — Students can state the five recombinant DNA steps with the enzyme used at each, work out recombinant and digested fragment lengths, read a gel to judge whether the insert is present, and argue both sides of a GM crop question.

01 Prerequisites (3-minute check)

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

Skill	Quick check	Pass mark
DNA structure	Base pairing rule	A with T, G with C
Enzyme action	Where a restriction enzyme cuts	The phosphodiester bond at a recognition sequence
Base units	How many base pairs in one kilobase	1000 bp

02 Lesson flow (60 minutes)

Time	Stage	Tutor does	Student does
0-10 min	Five steps	Cut, stick, deliver, select, verify	Students recite all five
10-22 min	Tool enzymes	Restriction enzyme, ligase, reverse transcriptase	Match three enzymes to uses
22-36 min	Length maths	Total size and digested fragments	5.6000000000000005 kb and two bands
36-48 min	Gel electrophoresis	Small fragments travel further	Read bands for the insert
48-56 min	Uses and ethics	Insulin, Bt crops, gene therapy	Two for and two against
56-60 min	Exit check	Three questions marked at once	Independent

03 Teaching script (read it out)

Scene 1: Five steps: cut, stick, deliver, select, verify

Cut: a restriction enzyme cuts the plasmid and the target gene at its recognition sequence, leaving sticky ends.
Stick: DNA ligase rejoins the sugar phosphate backbone into a recombinant plasmid.
Deliver: heat shock or electroporation carries the plasmid into bacteria, and calcium ions raise competence.
Select: an antibiotic resistance gene lets only plasmid-bearing bacteria grow on the plate.
Verify: extract the plasmid, cut it again and compare band sizes on a gel.
A six-base recognition site occurs about once every 4 to the power 6, which is 4096 base pairs.

Tutor move: five fingers for five steps, one finger folding per step, then ask which step used ligase.

Scene 2: Length maths: proving the insert with numbers

A 4.4 kb plasmid plus a 1.2 kb insert gives 5.6000000000000005 kb.
With one site on each side of the insert, digestion gives two bands at 1.2 kb and 4.4 kb, while the empty plasmid gives a single 4.4 kb band.
Transformation: 200 colonies from 0.1 micrograms of DNA is 2000 colonies per microgram.
After 30 PCR cycles one template molecule could give 1,073,741,824 copies, though reagents run out long before that.

Ask the student: what changes if the insert goes in backwards? (The length is the same, so sequencing or an asymmetric digest is needed.)

Scene 3: Uses and ethics: two for, two against

Medicine: recombinant insulin, growth hormone and vaccine antigens, with high purity and no blood-borne risk.
Agriculture: Bt crops cut insecticide use, and golden rice supplies a vitamin A precursor.
Risks: gene flow to wild relatives, resistant pests, patents and smallholder livelihoods, labelling and informed choice.
Gene therapy: somatic therapy is not inherited, while germline editing raises consent questions for people not yet born.

Reminder: pair each benefit with a risk, because examiners look for balance rather than a list of advantages.

04 Question types and worked steps

Type	Example	Steps	Answer
Ordering	Recombinant DNA steps	Cut, stick, deliver, select, verify with enzymes	The order cannot be swapped
Calculation	Recombinant plasmid length	Plasmid plus insert	5.6000000000000005 kb
Graph reading	Gel evidence for an insert	Read the marker, then compare bands	Bands at 1.2 and 4.4 kb
Argument	Evaluating a GM crop	Two for, two against, then a conclusion	Must mention gene flow

Sanity check (teach this): A recombinant plasmid is always longer than the original, so one band must sit at 1.2 kb. If both bands are shorter than the original plasmid, the sites lie in the backbone rather than either side of the insert. On a gel the smallest fragment always travels furthest.

05 Independent practice (3 questions)

Question	Answer	What to watch
Total length of a 4.4 kb plasmid with a 1.2 kb insert	5.6000000000000005 kb	Simply add them
Bands after digesting the recombinant plasmid	1.2 kb and 4.4 kb	Insert plus backbone
Efficiency of 200 colonies from 0.1 micrograms	2000 per microgram	Colonies over DNA mass

06 Common mistakes and what to say

Mistake	Student writes	Say this
Calling ligase a cutting enzyme	Saying ligase cuts the plasmid	“Restriction enzymes cut and ligase sticks; they work in opposite directions.”
Assuming the insert always worked	One band means success	“You need the insert band; a single band may just be the empty plasmid.”
Mixing up transformation and transfection	Saying transformation is a viral infection	“Bacteria taking up free DNA is transformation; transfection introduces DNA into animal cells.”
Writing only benefits in ethics answers	Five advantages and no risk	“Pair every benefit with a risk, then give a conditional conclusion.”

07 Homework (paper, section and question numbers)

DSE biology practice paper (genetics and biotechnology) — Section B recombinant DNA and gel questions
Secondary biology chapter index — All chapters

08 Exit check (last 5 minutes)

Question	Answer	Decision
Total length of 4.4 kb plus 1.2 kb	5.6000000000000005 kb	All three right: move to ecology
Number of bands from the recombinant plasmid	Two	One wrong: reteach the digest map
Which fragment travels furthest on a gel	The smallest one	Two wrong: restart act one

09 Tutor FAQ

Why do sticky ends matter?

Complementary single-stranded ends let the insert anneal to the vector before ligase seals it.

Why include an antibiotic resistance gene?

It is the selectable marker, so only bacteria carrying the plasmid grow on antibiotic plates.

Why does PCR need primers?

Primers give the polymerase a starting point and define the two ends of the amplified region.

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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.