1. Pair A
Which base pairs with A?
Help beside the question
Read or point first. Use A–T, C–G.
Answer guidance
T.
DNA structure: pair, twist, critique
Learning objective
Describe DNA as a double-stranded polymer with complementary bases and evaluate a model of its structure.
40 minutes · one route · start from what you show today. Point, speak, sign, draw, write or direct an adult. An adult may read or scribe your exact words. Choose one route; do not complete all three.
LAUNCH · GCSE BIOLOGY FOUNDATION · W12 · LESSON 2 · 4 MINUTES
Four clues to start
Supported · 4 questions
2. Pair C
Which base pairs with C?
Help beside the question
Read or point first. Use A–T, C–G.
Answer guidance
G.
3. Strands
How many strands form the DNA double helix?
Help beside the question
Read or point first. Double means…
Answer guidance
Two.
4. Unit
Name one of the three parts of a nucleotide.
Help beside the question
Read or point first. Sugar, phosphate or base.
Answer guidance
A sugar, a phosphate or a base.
Standard · 4 questions
1. Pair A
Which base pairs with A?
Help beside the question
Use A–T, C–G.
Answer guidance
T.
2. Pair C
Which base pairs with C?
Help beside the question
Use A–T, C–G.
Answer guidance
G.
3. Strands
How many strands form the DNA double helix?
Help beside the question
Double means…
Answer guidance
Two.
4. Unit
Name one of the three parts of a nucleotide.
Help beside the question
Sugar, phosphate or base.
Answer guidance
A sugar, a phosphate or a base.
Stretch · 4 questions
1. Pair A
Which base pairs with A?
Help beside the question
Use A–T, C–G.
Answer guidance
T.
2. Pair C
Which base pairs with C?
Help beside the question
Use A–T, C–G.
Answer guidance
G.
3. Strands
How many strands form the DNA double helix?
Help beside the question
Double means…
Answer guidance
Two.
4. Unit
Name one of the three parts of a nucleotide. Give a reason.
Help beside the question
Sugar, phosphate or base.
Answer guidance
A sugar, a phosphate or a base. Accept a scientifically correct reason that uses the relevant model or evidence.
LAUNCH · GCSE BIOLOGY FOUNDATION · W12 · LESSON 2 · 4 MINUTES
Choose a first idea
Which rung is correctly paired?
Think or discuss first. No answer chosen yet.
LAUNCH · GCSE BIOLOGY FOUNDATION · W12 · LESSON 2 · 5 MINUTES
Watch the model
A typical body cell has a nucleus containing chromosomes. These pictures are not drawn to scale.
Worked explanation · reveal after thinking
A cell-to-gene zoom is a schematic, not a literal scale model. Not every cell has a nucleus and some DNA is outside the nucleus. For this Foundation model use a typical nucleated body cell. The ladder is two-dimensional; real DNA is three-dimensional.
32-second silent captioned explainer · optional
No sound or autoplay. Use instead of part of the modelling time, not as an extra activity.
Full transcript
Locate the chromosome. A typical body cell has a nucleus containing chromosomes. These pictures are not drawn to scale.
Unpack the DNA. Each chromosome contains a long DNA molecule. A gene is a section of that DNA.
Pair the bases. A pairs with T and C pairs with G. Each nucleotide contains a sugar, phosphate and base.
Twist the ladder. Two DNA strands form a double helix. Weak hydrogen bonds hold complementary bases together.
LAUNCH · GCSE BIOLOGY FOUNDATION · W12 · LESSON 2 · 5 MINUTES
Sort using the evidence
Choose a card, then choose a group. Explain one placement using its clue.
Drag, or use Tab and Enter to select a card and its group. Pointing or adult-directed placement works too.
Fits the DNA model
Needs repair
Choose a card first.
Read the card clues
A pairs with T: These are complementary bases.
C pairs with G: These are complementary bases.
Two strands twist into a double helix: The strands form the characteristic shape.
A pairs with C: A pairs with T, not C.
A nucleotide contains sugar, phosphate and base: These are its three components.
Each rung is an entire chromosome: The rung models a base pair, not a chromosome.
Would a small change help? · optional
Ask the lesson teacher for clearer labels, fewer cards, reading help, an adult-directed model or more thinking time. Your teacher will explain what can change, try it and check its effect. Ask privately or pass; no extra form is needed.
Group explanations · reveal after sorting
A pairs with T → Fits the DNA model. These are complementary bases.
C pairs with G → Fits the DNA model. These are complementary bases.
Two strands twist into a double helix → Fits the DNA model. The strands form the characteristic shape.
A pairs with C → Needs repair. A pairs with T, not C.
A nucleotide contains sugar, phosphate and base → Fits the DNA model. These are its three components.
Each rung is an entire chromosome → Needs repair. The rung models a base pair, not a chromosome.
LAUNCH · GCSE BIOLOGY FOUNDATION · W12 · LESSON 2 · 4 MINUTES
Work through one example
Worked example
Complete the complementary strand for A G T C.
Step 1
Read the first base A: pair it with T.
Step 2
Read G: pair it with C.
Step 3
Read T and C: pair them with A and G.
Step 4
The complementary sequence is T C A G.
LAUNCH · GCSE BIOLOGY FOUNDATION · W12 · LESSON 2 · 4 MINUTES
Build and check six base pairs
Complete the second strand. Label one nucleotide, the sugar–phosphate backbone and the bonds between bases. Twist paper only if comfortable; a flat model is valid.
| Position | 1 | 2 | 3 | 4 | 5 | 6 |
|---|---|---|---|---|---|---|
| Strand 1 | A | C | G | T | T | A |
| Strand 2 | ___ | ___ | ___ | ___ | ___ | ___ |
Worked explanation · reveal after thinking
Complementary strand: T, G, C, A, A, T. A nucleotide is sugar + phosphate + one base. Weak hydrogen bonds connect paired bases; the sugar–phosphate backbone runs along each strand.
LAUNCH · GCSE BIOLOGY FOUNDATION · W12 · LESSON 2 · 10 MINUTES
Show your understanding
Supported · choose one route
1. Pair
Complete the six pairs and check each with the rule.
Point to one base at a time. Use A–T and C–G, writing one partner underneath each base.
2. Label
Label the three components of one nucleotide.
Use four labels: sugar / phosphate / base / nucleotide. Group the first three into one nucleotide.
3. Critique
Give one strength and one limit of your model.
Use: the flat ladder shows ___ correctly, but real DNA is shaped as a ___.
Standard · choose one route
1. Pair
Complete the six pairs and check each with the rule.
A–T and C–G.
2. Label
Label the three components of one nucleotide.
Do not label a whole rung as one nucleotide.
3. Critique
Give one strength and one limit of your model.
What does it show accurately, and what is simplified?
Stretch · choose one route
1. Pair
Complete the six pairs and check each with the rule.
A–T and C–G.
2. Label
Label the three components of one nucleotide.
Do not label a whole rung as one nucleotide.
3. Explain the rule
One strand changes from A C G T to A T G T. What complementary sequence is now needed?
Apply the pairing rule position by position.
LAUNCH · GCSE BIOLOGY FOUNDATION · W12 · LESSON 2 · 4 MINUTES
Two small checks
Supported · 2 questions
1. Complete
Write the complementary sequence to A C T G.
Help beside the question
Point or arrange first. Use: first ___, then ___, finally ___. Pair each base separately.
Answer guidance
T G A C.
2. Describe
State one feature a paper ladder shows and one it leaves out.
Help beside the question
Use: my answer is ___ because ___. Point to a model or card first. Think base pairing and three-dimensional shape.
Answer guidance
It can show complementary base pairing; a flat ladder does not show the full three-dimensional double helix or scale.
Standard · 2 questions
1. Complete
Write the complementary sequence to A C T G.
Help beside the question
Pair each base separately.
Answer guidance
T G A C.
2. Describe
State one feature a paper ladder shows and one it leaves out.
Help beside the question
Think base pairing and three-dimensional shape.
Answer guidance
It can show complementary base pairing; a flat ladder does not show the full three-dimensional double helix or scale.
Stretch · 2 questions
1. Complete
Write the complementary sequence to A C T G.
Help beside the question
Pair each base separately.
Answer guidance
T G A C.
2. Explain the rule
One strand changes from A C G T to A T G T. What complementary sequence is now needed?
Help beside the question
Apply the pairing rule position by position.
Answer guidance
T A C A. The second position changes because T pairs with A, not G.
Keep one useful science explanation. No additional participation record is required.