The shape of the lesson
By the end of the lesson, learners can
- Chromosomes, DNA and genes: Use the zoom chips in the sim to go from a whole cell to its nucleus, one chromosome, the DNA double helix and finally two alleles of one gene.
- Chromosome sets and sex: Compare a female body cell, a male body cell, an egg and a sperm, then press 20 children (XX x XY) a few times and watch the numbers of girls and boys.
- From gene to protein: Extended.
- Same genes, different jobs: Extended.
- Chromosome = DNA; gene = length of DNA coding for a protein; allele = alternative form of a gene.
- Sex: XX female, XY male; eggs all X, sperm half X and half Y, so 1 : 1.
- Extended: gene stays in nucleus → mRNA copy → cytoplasm → ribosome joins amino acids in the order set by the bases.
- Extended: diploid = two sets (23 pairs in humans); haploid = one set (23).
One picture every idea returns to
Chromosomes are made of DNA, which contains genetic information in the form of genes. A gene is a length of DNA that codes for a protein, and an allele is an alternative form of a gene. Females are XX and males are XY. Extended: the sequence of bases in a gene sets the sequence of amino acids in a protein, and that sequence sets the protein's shape.
Forty-five minutes, phase by phase
| Time | Phase | What happens in the room | Grouping |
|---|---|---|---|
| 0 to 5 min | Hook: predict | Open with the question in the title, The code in every nucleus.. Chromosomes are made of DNA, and DNA carries genes. Put the figure on the board: it is the one picture every idea in the lesson returns to. | Whole class, then pairs |
| 5 to 11 min | Teach and check: Chromosomes, DNA and genes | Use the zoom chips in the sim to go from a whole cell to its nucleus, one chromosome, the DNA double helix and finally two alleles of one gene. Project tab 1 of the lab (Zoom from a cell to a gene) and let a student drive it while the class predicts. | Whole class, then pairs |
| 11 to 17 min | Teach and check: Chromosome sets and sex | Compare a female body cell, a male body cell, an egg and a sperm, then press 20 children (XX x XY) a few times and watch the numbers of girls and boys. Project tab 2 of the lab (Chromosome sets and the inheritance of sex) and let a student drive it while the class predicts. | Pairs |
| 17 to 23 min | Teach and check: From gene to protein | Extended. Project tab 3 of the lab (How a protein is made) and let a student drive it while the class predicts. | Pairs |
| 23 to 30 min | Teach and check: Same genes, different jobs | Extended. Project tab 4 of the lab (Gene expression in different cells) and let a student drive it while the class predicts. | Connect-Extend-Challenge |
| 30 to 36 min | Check | The four Stage 1 questions from the student lesson, answered on mini whiteboards or by show of fingers. Re-teach the one most get wrong. | Individual, then whole class |
| 36 to 43 min | Plenary | The exam-style questions from Stage 2, one at a time: thirty seconds alone, then a random call. The feedback lines in the key give the examiner's wording. | Individual, random call |
| 43 to 45 min | Exit and homework | Set the worksheet below (6 questions, 21 marks). Learners self-assess against the objectives before they leave. | Individual |
Protect the parts that carry the learning
The sequence carries about fifty minutes of material, so the lab is driven by one student while the class predicts rather than explored freely. Times are a guide.
Protected: the check at 30 minutes. It is the only individual read on whether the key idea landed, and it is not cut.
Cut first if the build overruns: the last check in Same genes, different jobs, never the check or the plenary.
In a 60 minute block: run worksheet questions 1 to 3 in class as a Rally Coach set and model both the hardest worked example and its check live.
Connect-Extend-Challenge
After the new idea, each student writes three lines: how it connects to what they knew, how it extends their thinking, and what still challenges them. Pairs compare, then the challenges become the plenary.
The full structure, with its variations and the other twelve, is on the cooperative-learning page.
What to head off, and how
| Trap learners fall into | Teaching move that pre-empts it |
|---|---|
| Do not mix up gene and allele. | The gene is the instruction for a feature such as flower colour; the alleles are the different versions of that instruction, such as purple and white. |
| The mother cannot decide the sex of a child: all her eggs carry an X. | Whether the child is XX or XY depends on which sperm fertilises the egg. |
| The DNA itself never leaves the nucleus. | It is the mRNA copy that travels to the ribosome. |
Chromosomes, genes and proteins: worksheet
6 questions, 21 marks. Written in the style of the papers; no past paper question is reproduced.
Define (a) gene and (b) allele, and (c) state what chromosomes are made of.
Draw a genetic diagram to show how sex is inherited in humans, and state the expected ratio of boys to girls.
Explain why it is the father, not the mother, who determines the sex of a child.
Extended. Describe how a cell uses a gene to make a protein.
Extended. Explain how DNA controls the function of a cell. Refer to the shape of protein molecules in your answer.
Extended. The table shows the number of chromosomes in some cells.
| Cell | Number of chromosomes |
|---|---|
| human skin cell | 46 |
| human sperm | 23 |
| dog liver cell | 78 |
| dog egg cell | X |
(a) State which human cell is haploid. (b) Give the value of X. (c) State how many pairs of chromosomes are in a human skin cell. (d) Explain why a skin cell and a muscle cell from the same person make different proteins.
Answer key and marking notes
- (a) A length of DNA that codes for a protein.
- (b) An alternative form of a gene.
- (c) DNA (which contains genetic information in the form of genes).
- Parents: female XX x male XY.
- Gametes: eggs all X; sperm X or Y (in equal numbers).
- Offspring: XX, XX, XY, XY, so 2 female : 2 male.
- Ratio 1 boy : 1 girl (a 50% chance of each).
- All the mother's eggs carry an X chromosome.
- Half the father's sperm carry X and half carry Y, so the sex depends on whether an X or a Y sperm fertilises the egg.
- The gene (DNA) stays in the nucleus.
- A copy of the gene is made as messenger RNA (mRNA) in the nucleus.
- The mRNA moves out of the nucleus into the cytoplasm.
- The mRNA passes through a ribosome, which assembles amino acids into a protein.
- The sequence of bases in the mRNA determines the sequence of amino acids in the protein.
- DNA controls which proteins the cell makes, such as enzymes, membrane carriers and receptors for neurotransmitters.
- The sequence of bases in a gene sets the sequence of amino acids, and different sequences give proteins different shapes.
- The shape of each protein (for example an enzyme's active site or a carrier's binding site) determines what it does, so the proteins made decide what the cell can do.
- (a) The sperm (a single set of chromosomes).
- (b) 78 ÷ 2 = 39.
- (c) 23 pairs.
- (d) Both contain the same genes, but each cell only expresses the genes for the proteins it needs; the other genes are not expressed.
8 multiple-choice questions on this lesson
From the topic question bank. Assign them in the app for a marked score in your gradebook, or run the topic as a class game.
Which list is in order of size, from largest to smallest?
- Anucleus, chromosome, gene
- Bchromosome, nucleus, gene
- Cgene, chromosome, nucleus
- Dnucleus, gene, chromosome
The gene for eye colour in fruit flies has a red-eye form and a white-eye form. What are these two forms called?
- Aalleles
- Bchromosomes
- Cgenotypes
- Dphenotypes
A couple have four daughters. What is the probability that their fifth child will be a boy?
- A0.5
- B0.2
- C0.8
- D1.0
Which statement about human sex chromosomes is correct?
- AAll eggs carry an X chromosome; half of all sperm carry a Y
- BAll sperm carry an X chromosome; half of all eggs carry a Y
- CFemales are XY and males are XX
- DEvery egg and every sperm carries both an X and a Y
Use the table. Which cells are haploid?
- AQ and S
- BP and R
- CP and Q
- DR and S
Where in a cell is a protein molecule assembled from amino acids?
- Aat a ribosome in the cytoplasm
- Bon the gene inside the nucleus
- Cinside the mRNA as it is copied
- Dat the cell membrane, as it is absorbed
A change in one base of the gene for an enzyme stops the enzyme from working. What is the best explanation?
- AA different amino acid changes the shape of the active site
- BThe enzyme now has fewer genes, so it cannot be made
- CThe change stops mRNA leaving the nucleus at all
- DThe enzyme is now made of DNA instead of amino acids
A cell in the salivary gland makes amylase but not haemoglobin. A red blood cell precursor in bone marrow makes haemoglobin but not amylase. Which explains this?
- ABoth cells have both genes, but each expresses only the genes it needs
- BEach cell contains only the genes for its own proteins
- CSalivary gland cells are haploid and bone marrow cells are diploid
- DAmylase and haemoglobin are coded for by the same gene
Quiz key
The nucleus contains the chromosomes, and each chromosome is a long DNA molecule carrying many genes, so the order is nucleus, chromosome, gene. A chromosome sits inside the nucleus, so it cannot be larger. A gene is a short length of the DNA in a chromosome, so it is the smallest, not the largest, and it cannot be larger than the chromosome that carries it.
An allele is an alternative form of a gene, so red-eye and white-eye are two alleles of the eye-colour gene. A chromosome is a whole DNA molecule carrying many genes. A genotype is the combination of alleles an organism has, such as two red-eye alleles. A phenotype is the observable feature itself, such as red eyes, not the form of the gene.
Each fertilisation is independent: half the sperm carry X and half carry Y, so every child has a 0.5 chance of being a boy. 0.2 treats the child as one of five equally likely events. 0.8 and 1.0 assume a boy is now "due", but previous births do not change the chance.
Females are XX, so every egg carries an X; males are XY, so half the sperm carry X and half carry Y. Eggs never carry a Y, because females have no Y to give. Females are XX and males XY, not the other way round. Gametes are haploid, so each carries only one sex chromosome, not both.
Q (32) has half the number in horse body cells (64) and S (24) has half the number in potato body cells (48), so both are gametes or gamete nuclei with a single set: haploid. P and R are body cells with two sets, so they are diploid. Each wrong pair contains at least one of these diploid body cells. Comparing numbers across different species (48 and 32) does not tell you which is haploid.
mRNA made in the nucleus moves to the cytoplasm and passes through a ribosome, which assembles the amino acids into a protein. The gene stays in the nucleus and acts only as the template for the mRNA; no protein is built there. The mRNA carries the base sequence but does not join amino acids itself. Proteins are made inside the cell, not assembled as they are absorbed at the membrane.
The base sequence sets the amino acid sequence, and a different amino acid can make the protein fold into a different shape, so the active site no longer fits the substrate. An enzyme is a protein; it does not contain genes. A single base change does not normally stop mRNA from leaving the nucleus. The enzyme is still a chain of amino acids; DNA is only the code.
Most body cells contain the same genes, but many are not expressed because each cell makes only the specific proteins it needs. Cells do not lose the genes they do not use. Body cells are diploid; only gametes are haploid. Amylase and haemoglobin are different proteins with different amino acid sequences, so they are coded for by different genes.
What to project
The student lesson embeds these tabs. Open them full screen on the projector; every slider has a drag handle on the stage.
Slides for this lesson
A projectable deck for every pack is on the way. Until then, present the student lesson full screen: its sections, figure and lab tabs follow the same order as the plan above.