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# Where are the instructions?

Year 10 Biology · Genetics

<!-- Speaker note: Students relate DNA, chromosomes, genes and alleles by size and meaning, and explain chromosome pairs (AC9S10U01, node 1, a hinge node). About 20 minutes (Engage and Explore 6 min · Explain 9 min · Evaluate 3 min · Typical adds 2 min, Wider about 1 min). -->

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## Today

**We are learning to** dNA carries the instructions for an organism. It is coiled into chromosomes in the nucleus; a gene is a section of DNA that codes for a protein. Alleles are different versions of the same gene.

- I can put the cell, nucleus, chromosome, DNA and gene in order of size.
- I can define gene and allele.
- I can explain why chromosomes come in pairs.

<!-- Speaker note: Before class: A concept tutorial for introducing or revising the idea; it does not replace a DNA extraction practical (strawberries or kiwifruit). Set the width with `?width=minimum`, `typical` (default) or `wider`. The hinge misconceptions are confusing gene, chromosome and DNA, and thinking there is a separate gene for each version of a characteristic. Board notes: the zoom is not to scale; the nucleus shows only some chromosomes; the base sequence shown is illustrative, not a real gene. -->

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## Why it matters

> Unwound, the DNA in one of your cells would be about two metres long. It fits in a nucleus a hundredth of a millimetre across.

<!-- Speaker note: In Year 9 you learnt gametes carry half the chromosomes. This lesson: what a chromosome is made of, and what a gene is. -->

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<span style="background:#dbeafe; color:#1a1a1a; border:1.5px solid #3b82f6; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">💬 Discussion</span>

## Engage: predict

**Put these in order from biggest to smallest: gene, chromosome, nucleus, DNA molecule.**

- Chromosome, nucleus, gene, DNA
- Nucleus, chromosome, DNA molecule, gene (a section of it)
- Gene, chromosome, DNA, nucleus
- Nucleus, chromosome, gene, DNA

<!-- Speaker note: What happens: Nucleus, then chromosome (one long, coiled DNA molecule), then a gene, which is a short section of that DNA. Take two or three predictions before anyone runs it. -->

<!-- Speaker note: Teacher: see how many put gene and DNA in the wrong order. -->

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## Explore it

{{embed: https://studio.learningwaypoints.com/tutorials/_playground-board/index.html?id=deck%2Fscience%2Fbiology-10%2Fdna-genes%2Fstep-2&code=widget%3A+cell%0Ashow%3A+zoom%0A&fresh=1}}

[Open this step in the tutorial](https://studio.learningwaypoints.com/tutorials/science/biology-10/dna-genes/?full=step-2#step-2)

<!-- Speaker note: Press **Zoom in** and read each level before going on. -->

<!-- Speaker note: Teacher: ask what the red and blue chromosomes mean at the nucleus level. -->

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## What happened?

* Nucleus, then chromosome (one long, coiled DNA molecule), then a gene, which is a short section of that DNA.

<!-- Speaker note: Reveal after running. Ask who predicted it, and what made them think so. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Words

**Match each word to its meaning.**

| | |
|---|---|
| `DNA` | A. One long DNA molecule, tightly coiled |
| `Chromosome` | B. One version of a gene |
| `Gene` | C. A section of DNA that codes for one protein |
| `Allele` | D. A long molecule carrying the coded instructions |

* Answer: DNA → D. A long molecule carrying the coded instructions; Chromosome → A. One long DNA molecule, tightly coiled; Gene → C. A section of DNA that codes for one protein; Allele → B. One version of a gene

<!-- Speaker note:  -->

<!-- Speaker note: Teacher: the earlobe question is the checkpoint (gene versus allele). -->

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## Alleles

**One gene decides your blood group, yet people have blood group A, B, AB or O. How?**

- People have different alleles (versions) of the same gene
- Only some people have the gene
- The environment decides
- There is a separate gene for each blood group

* Answer: People have different alleles (versions) of the same gene

<!-- Speaker note:  -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Pairs

**Your body cells have 46 chromosomes. Why are they described as 23 pairs?**

- One of each pair came from each parent, and the two carry the same genes (perhaps different alleles)
- Boys and girls have different chromosomes
- Pairs are easier to count
- Each chromosome copies itself

* Answer: One of each pair came from each parent, and the two carry the same genes (perhaps different alleles)

<!-- Speaker note:  -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Genotype and phenotype

**Someone has the blood group alleles A and O, and their blood group is A. Which is their genotype and which their phenotype?**

- Genotype: the chromosome the gene is on. Phenotype: the gene
- Both are 'blood group A': the words mean the same thing
- Genotype: the alleles A and O. Phenotype: blood group A, the characteristic that shows
- Genotype: blood group A. Phenotype: the alleles A and O

* Answer: Genotype: the alleles A and O. Phenotype: blood group A, the characteristic that shows

<!-- Speaker note:  -->

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## Elaborate: from code to characteristic

**Put the steps in order.**

Groups: 1st · 2nd · 3rd · 4th

A gene's sequence of bases (A, T, C, G) is read · The cell builds a protein from that code · The protein does a job (a pigment, an enzyme, a structure) · A characteristic appears (such as eye colour)

* Answer: A gene's sequence of bases (A, T, C, G) is read → 1st; The cell builds a protein from that code → 2nd; The protein does a job (a pigment, an enzyme, a structure) → 3rd; A characteristic appears (such as eye colour) → 4th

<!-- Speaker note: What this step is for: Ordering the steps from gene to characteristic answers 'what do genes do?' -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Elaborate: the same code everywhere

**A nerve cell and a skin cell from the same person have the same DNA. Why do they look and work so differently?**

- Nerve cells have extra genes
- Skin cells lose most of their DNA
- Each cell uses (switches on) different genes
- They have different chromosomes

* Answer: Each cell uses (switches on) different genes

<!-- Speaker note: What this step is for: Recognising that all cells hold the same DNA but use different genes links back to specialised cells. -->

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## Evaluate: look back

Write one sentence: "At first I thought… Now I think… because…"

**Revision summary**

- **DNA** carries coded instructions as a sequence of bases (A, T, C, G).
- A **chromosome** is one long DNA molecule, coiled; humans have 46, in 23 pairs (one of each pair from each parent).
- A **gene** is a section of DNA coding for a protein; **alleles** are different versions of one gene.
- **Genotype:** the alleles you have (such as A and O). **Phenotype:** the characteristic that shows (blood group A).

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Exit ticket

**What's the difference between a gene and an allele?**

* A gene is a section of DNA coding for a protein; an allele is one version of that gene.

<!-- Speaker note: Students answer on paper or a mini whiteboard before the reveal. Anyone who cannot answer needs the main idea again next lesson. Where this leads: Next: meiosis (node 3) and Punnett squares (node 4). In class: DNA extraction from strawberries. -->