A Level Biology - How to Solve: Gene Technologies (PCR, Gel Electrophoresis, Restriction Mapping, Recombinant DNA)

How to Solve: Gene Technologies (PCR, Gel Electrophoresis, Restriction Mapping, Recombinant DNA)

Complete Guide For GCSE/A-Level Biology – Ace Your Exam with Confidence

Introduction

"Mastering gene technologies unlocks 10–15% of your GCSE/A-Level Biology exam—including the 6-mark ‘explain’ questions on genetic engineering. This guide gives you the exact steps to solve any PCR, gel electrophoresis, or restriction mapping problem, just like a real lab scientist."

(On camera: Hold up a past paper with a 6-mark question highlighted.) "This question alone could be the difference between a Grade 6 and a Grade 8. Let’s break it down."

WHAT YOU NEED TO KNOW FIRST

Before diving in, you must understand:
1. DNA structure – Double helix, nucleotides (A, T, C, G), complementary base pairing.
2. Enzymes in DNA replication – DNA polymerase, restriction enzymes, ligase.
3. Electrophoresis basics – Charged molecules move in an electric field; smaller fragments travel faster.

(On camera: Quick whiteboard sketch of DNA structure and enzyme roles.) "If any of these feel shaky, pause and review them first—this guide builds on them."

KEY TERMS & FORMULAS

1. PCR (Polymerase Chain Reaction)

Key Terms: - Template DNA – The original DNA you want to copy. - Primers – Short DNA sequences that bind to the template, marking the start/end of the target region. - Taq polymerase – Heat-resistant DNA polymerase that builds new DNA strands. - Thermocycler – Machine that controls temperature cycles for PCR. - Denaturation – Heating to 95°C to separate DNA strands. - Annealing – Cooling to 50–65°C to let primers bind. - Extension – Heating to 72°C for Taq polymerase to build new strands.

Formula (Number of DNA copies after PCR cycles): Number of copies = 2ⁿ - n = number of PCR cycles - MEMORISE THIS – Examiners love asking how many copies are made after X cycles.

(On camera: Write formula on screen, then cover it and ask: "If I run 5 cycles, how many copies do I get?")

2. Gel Electrophoresis

Key Terms: - Agarose gel – Porous gel that DNA fragments move through. - Loading dye – Makes DNA visible and helps it sink into wells. - DNA ladder – Known fragment sizes for comparison. - Bands – Visible DNA fragments after staining. - Anode (+) / Cathode (–) – DNA (negatively charged) moves toward the anode.

Formula (Distance traveled vs. fragment size): Smaller fragments = Faster movement = Further distance - No formula to memorise – Just remember: smaller = faster = further. - Given on exam sheet – If a graph is provided, use it to estimate sizes.

(On camera: Draw a gel with 3 lanes—ladder, sample A, sample B. Label bands.) "The smallest band in the ladder is 100 bp. If a sample band is halfway between 100 bp and 500 bp, what’s its size? (Answer: ~300 bp.)"

3. Restriction Mapping

Key Terms: - Restriction enzyme – Cuts DNA at specific recognition sequences (e.g., EcoRI cuts at GAATTC). - Recognition site – The DNA sequence where an enzyme cuts. - Sticky ends – Overhanging single-stranded DNA after a cut (e.g., EcoRI leaves 5’-AATT overhangs). - Blunt ends – No overhangs (e.g., SmaI cuts straight across). - Restriction map – Diagram showing where enzymes cut along a DNA sequence.

Key Rule: - Number of fragments = Number of cuts + 1 - Example: 2 cuts → 3 fragments. - MEMORISE THIS – Examiners often ask how many fragments are produced.

(On camera: Draw a DNA strand with 2 EcoRI sites. Ask: "How many fragments after cutting?")

4. Recombinant DNA

Key Terms: - Plasmid – Small, circular DNA in bacteria, used as a vector. - Vector – DNA molecule (e.g., plasmid) that carries foreign DNA into a host cell. - Ligase – Enzyme that joins DNA fragments (e.g., sticky ends) together. - Transformation – Process of inserting recombinant DNA into a host cell (e.g., bacteria). - Selectable marker – Gene (e.g., antibiotic resistance) that identifies successful transformation.

Key Steps (MEMORISE ORDER):
1. Cut plasmid and target DNA with the same restriction enzyme (to get matching sticky ends).
2. Mix plasmid and target DNA with ligase to join them.
3. Insert recombinant plasmid into host cells (e.g., bacteria).
4. Grow cells on selective medium (e.g., antibiotic plate) to identify successful transformants.

(On camera: Hold up a diagram of a plasmid with antibiotic resistance gene. Point to each step.) "If you forget the order, think: Cut → Mix → Glue → Insert → Select."

STEP-BY-STEP METHOD

1. PCR Problems

Steps to solve:
1. Identify the target DNA sequence – What region is being amplified?
2. Count the number of cycles – Use the formula 2ⁿ to calculate copies.
3. Check for complications – E.g., primers not binding, wrong temperature.
4. Answer the question – E.g., "How many copies after 4 cycles?" → 2⁴ = 16.

Worked Example: Question: A scientist runs PCR for 6 cycles. How many copies of the target DNA are produced?
1. Target DNA = 1 copy at start.
2. Number of cycles (n) = 6.
3. Use formula: 2⁶ = 64.
4. Answer: 64 copies.

(On camera: Write 2⁶ on screen, then calculate step-by-step.) "What if the question says ‘starting with 2 copies’? Then multiply by 2 at the end: 2 × 2⁶ = 128."

2. Gel Electrophoresis Problems

Steps to solve:
1. Draw the gel – Label anode (+) at the bottom, cathode (–) at the top.
2. Identify the DNA ladder – Note the sizes of known bands.
3. Compare sample bands to ladder – Smaller fragments = further down.
4. Estimate sizes – Use the ladder to interpolate (e.g., halfway between 100 bp and 500 bp ≈ 300 bp).
5. Answer the question – E.g., "Which sample has the largest fragment?"

Worked Example: Question: A gel shows a DNA ladder with bands at 100 bp, 500 bp, and 1000 bp. Sample A has a band halfway between 100 bp and 500 bp. What is its size?
1. Draw gel: Ladder at 100, 500, 1000 bp.
2. Sample A band is halfway between 100 and 500 → (100 + 500) / 2 = 300 bp.
3. Answer: 300 bp.

(On camera: Sketch gel on whiteboard. Point to bands.) "If a band is closer to 500 bp than 100 bp, estimate 400 bp. Always show your working!"

3. Restriction Mapping Problems

Steps to solve:
1. List the enzymes and their recognition sites – E.g., EcoRI = GAATTC.
2. Count the number of cuts – Each recognition site = 1 cut.
3. Calculate fragments – Number of fragments = cuts + 1.
4. Draw the map – Label where each enzyme cuts.
5. Answer the question – E.g., "How many fragments after cutting with EcoRI and BamHI?"

Worked Example: Question: A 5000 bp plasmid has one EcoRI site and one BamHI site. How many fragments are produced if both enzymes are used together?
1. EcoRI = 1 cut, BamHI = 1 cut.
2. Total cuts = 2.
3. Fragments = 2 + 1 = 3 fragments.
4. Answer: 3 fragments.

(On camera: Draw plasmid as a circle. Mark EcoRI and BamHI sites. Cut with scissors animation.) "If the question says ‘linear DNA,’ the rule is the same: cuts + 1. For circular DNA, it’s also cuts + 1."

4. Recombinant DNA Problems

Steps to solve:
1. Identify the plasmid and target DNA – What’s being inserted?
2. Choose the same restriction enzyme – Must cut both plasmid and target DNA.
3. Check for sticky ends – Ensure the enzyme leaves compatible overhangs.
4. Use ligase to join – Glue the fragments together.
5. Select transformants – Use antibiotic resistance to identify successful cells.

Worked Example: Question: A plasmid has a tetracycline resistance gene and one EcoRI site. A human insulin gene is cut with EcoRI. Describe how to create recombinant bacteria that produce insulin.
1. Cut plasmid and insulin gene with EcoRI (same enzyme = matching sticky ends).
2. Mix plasmid and insulin gene with ligase to join them.
3. Insert recombinant plasmid into bacteria (transformation).
4. Grow bacteria on tetracycline plates – Only transformed bacteria survive.
5. Answer: Bacteria with the insulin gene will grow on tetracycline plates and produce insulin.

(On camera: Hold up a plasmid diagram. Point to EcoRI site and insulin gene.) "If the question asks ‘why use the same enzyme?’ say: ‘To create matching sticky ends for ligase to join.’"

WORKED EXAMPLES

Example 1 – Basic PCR

Question: A DNA sample undergoes 3 PCR cycles. How many copies are produced?
1. Start with 1 copy.
2. Number of cycles (n) = 3.
3. 2³ = 8.
4. Answer: 8 copies.

What we did and why: - Used the formula 2ⁿ because each cycle doubles the DNA. - No complications (e.g., primers, temperature) in this question.

Example 2 – Medium Gel Electrophoresis

Question: A gel shows a ladder with bands at 200 bp, 600 bp, and 1000 bp. Sample X has a band at 400 bp. Where would you expect to see this band on the gel?
1. Draw gel: Ladder at 200, 600, 1000 bp.
2. 400 bp is between 200 and 600 bp.
3. Closer to 600 bp than 200 bp → band slightly above the 200 bp mark.
4. Answer: Between the 200 bp and 600 bp ladder bands, closer to 600 bp.

What we did and why: - Compared sample band to ladder to estimate size. - Remembered smaller = faster = further, so 400 bp is between 200 and 600 bp.

Example 3 – Exam-Style Restriction Mapping

Question: A 6000 bp linear DNA molecule is cut with EcoRI (cuts at 1000 bp and 4000 bp) and BamHI (cuts at 2000 bp). How many fragments are produced if both enzymes are used together? What are their sizes?
1. EcoRI cuts at 1000 bp and 4000 bp → 2 cuts.
2. BamHI cuts at 2000 bp → 1 cut.
3. Total cuts = 3 → fragments = 3 + 1 = 4 fragments.
4. Sizes: - 0–1000 bp = 1000 bp - 1000–2000 bp = 1000 bp - 2000–4000 bp = 2000 bp - 4000–6000 bp = 2000 bp
5. Answer: 4 fragments of sizes 1000 bp, 1000 bp, 2000 bp, and 2000 bp.

What we did and why: - Counted cuts and used fragments = cuts + 1. - Mapped cuts on a line to calculate fragment sizes.

COMMON MISTAKES

(On camera: Hold up a "Mistake Alert" sign for each one.) "These mistakes cost marks. Circle them in your notes!"

EXAM TRAPS

(On camera: Point to a past paper question with a trap.) "Examiners love hiding these. Underline key words like ‘same enzyme’ or ‘ladder’ to spot them."

1-MINUTE RECAP

(On camera: Speak directly to the student, fast but clear.)

"Here’s everything you need to remember for gene technologies—right before your exam:

1. PCR: Use 2ⁿ for copies. 3 cycles = 8 copies. 5 cycles = 32.
2. Gel electrophoresis: Smaller fragments = further down. Compare to ladder.
3. Restriction mapping: Fragments = cuts + 1. Draw the DNA to count.
4. Recombinant DNA: Same enzyme → matching sticky ends → ligase → transformation → select with antibiotics.

Common mistakes to avoid: - Forgetting DNA moves to the anode (+). - Mixing up linear vs. circular DNA in restriction mapping. - Using different enzymes for recombinant DNA.

Exam traps: - If they ask ‘why the same enzyme,’ say ‘matching sticky ends.’ - If there’s no ladder, write ‘cannot determine size.’ - For PCR, ignore primers when calculating copies.

You’ve got this. Now go ace that exam!"

(On camera: Hold up a highlighter and mark key points in a past paper.) "Highlight these steps in your notes. Good luck!"

End of Guide (Word count: ~1500 – Ready for classroom or camera!)