The shape of the lesson
By the end of the lesson, learners can
- From DNA to chromosome: Step up through the four levels of packing in the 3D model: the bare double helix (2 nm), nucleosomes (11 nm), the coiled fibre (30 nm) and the condensed chromosome (about 1400 nm).
- The cell cycle: Press Start and watch the pointer go round.
- Telomeres guard the ends: Choose body cell and press Divide ×10 a few times, watching the telomere length (in kb per end) and the genes intact.
- Stem cells replace and repair: Let the epidermis run, then press Graze the skin and watch the stem cells in the bottom layer speed up.
- When division goes out of control: Start the clock with low carcinogen exposure, then raise it, or press Mutate the centre cell twice.
- Chromosome: DNA + histones; after S phase, two sister chromatids at a centromere.
- Interphase: G1 growth, S DNA replication, G2 growth and checks.
- Mitosis: genetically identical nuclei for growth, replacement, repair, asexual reproduction.
- Telomeres protect genes; stem cells supply new cells; mutation plus uncontrolled mitosis gives a tumour.
One picture every idea returns to
A chromosome is one long DNA molecule wound round histone proteins. In interphase the cell grows (G1), replicates every DNA molecule (S) and grows and checks again (G2); then mitosis divides the nucleus and cytokinesis divides the cytoplasm. After S phase each chromosome is two identical sister chromatids joined at the centromere, so mitosis gives two genetically identical daughter cells, used for growth, replacement, repair and asexual reproduction. Telomeres protect the chromosome ends, stem cells supply new cells, and division that escapes control forms a tumour.
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, Copy it all, then split it fairly.. Two metres of DNA, packed into 46 chromosomes, copied once and shared out exactly. 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: From DNA to chromosome | Step up through the four levels of packing in the 3D model: the bare double helix (2 nm), nucleosomes (11 nm), the coiled fibre (30 nm) and the condensed chromosome (about 1400 nm). Project tab 1 of the lab (Cell cycle lab, DNA to chromosome) and let a student drive it while the class predicts. | Whole class, then pairs |
| 11 to 17 min | Teach and check: The cell cycle | Press Start and watch the pointer go round. Project tab 2 of the lab (Cell cycle lab, the cell cycle) and let a student drive it while the class predicts. | Pairs |
| 17 to 23 min | Teach and check: Telomeres guard the ends | Choose body cell and press Divide ×10 a few times, watching the telomere length (in kb per end) and the genes intact. Project tab 5 of the lab (Cell cycle lab, telomeres) and let a student drive it while the class predicts. | Pairs |
| 23 to 30 min | Teach and check: the remaining sections | Sections 4 to 5 together: Stem cells replace and repair; When division goes out of control. Keep each to one idea and one check. | Mix-Pair-Share |
| 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, 20 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 When division goes out of control, 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.
Mix-Pair-Share
Students mix around the room until you call pair; they pair with the nearest classmate, discuss the prompt, then mix again. Three prompts of rising difficulty, three different partners.
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 |
|---|---|
| A chromosome is not always X-shaped. | Before S phase it is a single chromatid (one DNA molecule); the X shape only appears after replication, once the chromosome has condensed for division. Two sister chromatids are identical copies, not a homologous pair from two parents. |
| Interphase is not a resting stage. | The cell is very active: it makes proteins, replicates its DNA and checks it. And DNA replication happens in S phase of interphase, not during mitosis; mitosis only shares out DNA that is already copied. |
| A tumour forms by mitosis, which is why the tumour cells are genetically identical to the first mutated cell. | Do not say carcinogens make cells divide; they cause mutations in the genes that control division. Usually several mutations in one cell line are needed, which is why risk rises with age. |
Chromosomes and the cell cycle: worksheet
6 questions, 20 marks. Written in the style of the papers; no past paper question is reproduced.
Describe the structure of a chromosome at the start of mitosis. Refer to DNA, histones, chromatids, the centromere and telomeres.
Outline what happens in each of the three phases of interphase.
The mass of DNA in a cell was measured through one cell cycle. It was 6.6 pg for the first 9 hours, rose steadily to 13.2 pg over the next 7 hours, stayed at 13.2 pg for 5 hours, then fell to 6.6 pg. Identify the phases and explain the fall.
Explain the role of telomeres, and suggest why most body cells can only divide a limited number of times.
After a deep cut, the skin heals and the new tissue has the same genotype as the surrounding skin. Explain how this happens.
Explain how exposure to a carcinogen can lead to the formation of a malignant tumour.
Answer key and marking notes
- Each chromatid is one DNA molecule wound round histone proteins (chromatin), tightly coiled.
- There are two genetically identical sister chromatids, formed by replication in S phase.
- They are held together at the centromere.
- Each end of each chromatid has a telomere of repeated, non-coding base sequences.
- G1: the cell grows and makes proteins (including enzymes for replication) and organelles.
- S: every DNA molecule is replicated, so each chromosome becomes two sister chromatids.
- G2: further growth, organelles such as mitochondria divide, and the DNA is checked before mitosis.
- First 9 h at 6.6 pg: G1.
- Steady rise to 13.2 pg over 7 h: S phase, DNA replication; the 5 h at 13.2 pg is G2 (and mitosis).
- The fall to 6.6 pg happens at cytokinesis: the replicated DNA is shared equally between two cells, each with the original mass.
- DNA polymerase cannot replicate the very end of a linear DNA molecule, so the ends get shorter at each replication.
- Telomeres are non-coding repeats, so it is telomere that is lost rather than genes.
- Most body cells lack active telomerase, so once the telomeres are critically short the cell stops dividing, protecting the genes.
- Stem cells near the wound divide repeatedly by mitosis.
- Mitosis produces daughter cells that are genetically identical to the parent cell.
- Some daughter cells differentiate into specialised skin cells, replacing the cells lost and repairing the tissue.
- The carcinogen causes mutations in genes that control the cell cycle, for example a proto-oncogene becomes an oncogene or a tumour suppressor gene is inactivated.
- The cell no longer responds to the signals that stop division, and damaged cells are not destroyed.
- It divides repeatedly by mitosis without control, forming a mass of genetically identical cells: a tumour.
- If tumour cells invade surrounding tissue and spread through the blood or lymph to form secondary tumours, the tumour is malignant.
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 statement correctly describes the histone proteins in a chromosome?
- Apositively charged proteins that the DNA is wound round, forming nucleosomes
- Benzymes that join nucleotides together to replicate the DNA in S phase
- Cshort repeated non-coding sequences that protect the ends of the DNA
- Dproteins that hold the two sister chromatids together at the telomeres
A human body cell has 46 chromosomes. How many DNA molecules does its nucleus contain during G2?
- A92
- B46
- C23
- D184
Which is an example of mitosis being used for asexual reproduction?
- Aa strawberry plant producing new plantlets on its runners
- Ba pollen nucleus fusing with an egg cell in an ovule
- Cnew bone cells forming as a broken leg bone heals
- Dcells in the testes dividing to produce sperm cells
Which event takes place during G1 of interphase?
- Athe cell grows and makes proteins, including enzymes needed for DNA replication
- Bevery DNA molecule is replicated, so each chromosome becomes two chromatids
- Cthe chromosomes condense and become visible under a light microscope
- Dthe cytoplasm is divided in two by a cleavage furrow or a cell plate
People lose about 2 × 1011 red blood cells every day, yet the number in the blood stays constant. Which explanation is correct?
- Astem cells in the bone marrow divide by mitosis and some daughter cells differentiate into red blood cells
- Bmature red blood cells in the blood divide by mitosis to replace the cells that die
- Cstem cells in the bone marrow divide by meiosis to give a variety of different blood cells
- Dstem cells differentiate directly into red blood cells without dividing, until none are left
The graph shows the mass of DNA in a cell during one complete cell cycle of 20 hours. What percentage of the cell cycle is spent replicating DNA?
- A35%
- B65%
- C70%
- D30%
The telomeres of a cell line are 8.0 kb long at each end of every chromosome. Each division removes 0.1 kb from each end, and the cells stop dividing when the telomeres are 3.0 kb long. How many more times can these cells divide?
- A50
- B80
- C25
- D30
The risk of developing most cancers increases with age. Which statement best explains this?
- Aseveral mutations in genes that control the cell cycle are usually needed, and they build up in a cell line over many divisions
- Bcells in older people divide by meiosis more often, which produces cells with an abnormal chromosome number
- Ctelomeres become longer with age, so the body cells of older people can divide an unlimited number of times
- Dcarcinogens build up in the body over time and directly stimulate cells to divide faster by mitosis
Quiz key
Histones are positively charged proteins; the negatively charged phosphate groups of DNA are attracted to them, so the DNA winds round groups of histones to form nucleosomes, packing it compactly. Joining nucleotides during replication is done by enzymes such as DNA polymerase, not histones. Short repeated non-coding sequences at the ends are telomeres, which are DNA, not protein. Sister chromatids are held together at the centromere, not at the telomeres.
In G1 each of the 46 chromosomes is one DNA molecule. In S phase every DNA molecule is replicated, so in G2 each chromosome is two sister chromatids, each one DNA molecule: 46 × 2 = 92. 46 is the number of chromosomes, which counts the centromeres, not the DNA molecules after replication. 23 is the number in a gamete made by meiosis. 184 doubles twice, as if the DNA were replicated again or both chromatids were themselves double.
Plantlets on runners grow from cells of the parent plant dividing by mitosis, so each new plant is a genetically identical clone of its single parent: asexual reproduction. A pollen nucleus fusing with an egg is fertilisation, part of sexual reproduction. Bone healing does use mitosis, but it is repair of a tissue, not the production of a new organism. Sperm are produced by meiosis and are genetically different, used in sexual reproduction.
In G1 the cell grows, makes proteins (including the enzymes for replication) and organelles, and is checked before it can enter S phase. DNA replication, which makes the two sister chromatids, happens in S phase, after G1. Chromosomes condense and become visible in prophase of mitosis; in interphase the chromatin is loosely packed. Division of the cytoplasm is cytokinesis, the last stage of the cycle.
Bone marrow stem cells divide repeatedly by mitosis. Some daughter cells stay as stem cells and others differentiate into red blood cells, so the cells lost are replaced with genetically identical ones. Mature red blood cells have no nucleus, so they cannot divide at all. Meiosis makes gametes with half the chromosome number; it plays no part in replacing body cells. If stem cells only differentiated without dividing, the stem cell population would run out; division by mitosis keeps it going.
The DNA mass rises from 7 pg to 14 pg between 6 h and 13 h, so S phase lasts 13 − 6 = 7 h, and 7 ÷ 20 × 100 = 35%. 65% uses the time at the end of S phase (13 h) as if it were the duration. 70% uses the final DNA mass (14 pg) instead of a time. 30% is the length of the high plateau, 19 − 13 = 6 h, which is G2 and mitosis, after replication has finished.
Telomere that can be lost from each end = 8.0 − 3.0 = 5.0 kb. Each division removes 0.1 kb from that end, so 5.0 ÷ 0.1 = 50 divisions. 80 ignores the 3.0 kb limit and divides the whole telomere by 0.1 kb. 25 counts the loss from both ends (0.2 kb) against the length of only one end; each end has its own 8.0 kb telomere and loses 0.1 kb. 30 divides the critical length, 3.0 kb, instead of the length that can be lost.
A tumour usually needs several mutations in one cell line, for example a proto-oncogene becoming an oncogene and tumour suppressor genes being inactivated. Mutations are passed on to every daughter cell by mitosis, so they accumulate over many years of divisions, and the chance that one cell line collects enough rises with age. Body cells do not divide by meiosis; meiosis only makes gametes. Telomeres shorten, not lengthen, with each division in most body cells. Carcinogens do not make cells divide directly; they cause mutations in the genes that control division.
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.