OCR GCSE Biology A: Sexual and asexual reproduction, haploid and diploid cells and meiosis (J247) – Revision Notes

These notes condense statements B5.1f, B5.1g and B5.1h of the OCR GCSE (9-1) Biology A Gateway Science specification (J247), Version 4.0, for first assessment in 2018. None of the three is printed in bold, so none is Higher Tier only. For full explanations and worked examples read the study guide, then try the practice questions. Course links: the OCR GCSE Biology hub, the printable checklist and the free diagnostics. Mitosis is in Transport across membranes, mitosis and stem cells.

Key definitions

Term	Meaning
Asexual reproduction	One parent. Offspring made by mitosis. Genetically identical to the parent (clones). No gametes.
Sexual reproduction	Two gametes fuse at fertilisation. Offspring genetically different from each parent and from each other.
Zygote	The cell formed when two gametes fuse. Diploid.
Diploid (2n)	Cell with two sets of chromosomes, in pairs.
Haploid (n)	Cell with one set of chromosomes, one of each pair.
Meiosis	Cell division in the reproductive organs that makes haploid gametes from a diploid cell.

B5.1f Asexual versus sexual reproduction

	Asexual	Sexual
Parents	One	Two (two gametes)
Offspring	Identical to the parent	Varied
Speed	Usually quick, no mate to find	Usually slower, needs a mate
Numbers	Can be very many in a short time	Often fewer at a time
Strength	Good in a stable environment that suits the parent	Good in a changing environment
Weakness	One new disease or change can affect all	Costs time and energy, fewer offspring

Examples to learn

• Bacteria: binary fission. Some species can divide about every 20 minutes in good conditions.
• Strawberry: runners (asexual) and flowers (sexual).
• Potato: tubers (asexual) and flowers (sexual).
• Humans and mice: sexual only. Humans usually have one baby per birth, mice have a litter.
• Fungi: spores (asexual), and some also reproduce sexually.

Method: “evaluate the advantages of …” in steps

1. Name the type of reproduction and the organism.
2. State a feature: one parent or two, number of offspring, speed.
3. Link it to a consequence: numbers rise quickly, or the population varies.
4. Link variation to environmental pressure: disease, predators, competition, climate change.
5. Give the balancing point: what the other method does better.

Linking points to environmental pressures

• Identical individuals have identical weaknesses.
• Variation means some individuals are likely to survive a new pressure.
• Variation is what natural selection acts on.
• A well-adapted organism in a stable habitat can keep its successful features by cloning.

B5.1g Haploid and diploid

Organism	2n	n
Human	46	23
Pea	14	7
Fruit fly	8	4
Dog	78	39

• Body cells: diploid. Gametes: haploid. Zygote: diploid.
• 2n is always even, because chromosomes are in pairs.
• A gamete has one chromosome from each pair, not half of every chromosome.
• One chromosome of each pair came from the mother and one from the father.

Method: chromosome-number questions

1. Decide whether the cell is a body cell, a gamete or a zygote.
2. Body cell or zygote: diploid, 2n. Gamete: haploid, n.
3. To go from 2n to n, halve it. To go from n to 2n, double it.
4. Check the answer. Is a diploid number even? Is the gamete exactly half?

B5.1h Meiosis

Why it matters

• Gametes must be haploid so that fertilisation restores the diploid number.
• Without it the number would double every generation (human: 46, then 92, then 184).
• It is a source of genetic variation.

Steps

1. DNA is copied. Each chromosome now has two identical copies.
2. First division: pairs separate. Two haploid cells. This halves the number.
3. Second division: copies separate. Four cells.
4. Four haploid cells, all genetically different.

Mitosis versus meiosis

	Mitosis	Meiosis
Where	Body cells	Reproductive organs
Divisions	1	2
Cells made	2	4
Chromosomes	Stay at 2n	Halved to n
Cells	Identical	Different
Purpose	Growth, repair, asexual reproduction	Gametes

Where the variation comes from

• Each pair separates independently of the others.
• Which chromosome of a pair ends up in a gamete is random.
• Any sperm can fuse with any egg, adding more variation.

Must-know distinctions

• Clone versus sibling. Asexual offspring are clones of one parent. Sexual offspring are siblings with different mixtures of genetic information.
• Gamete versus zygote. A gamete is haploid and a zygote is diploid.
• Haploid versus diploid. One set of chromosomes versus two sets. The diploid number is twice the haploid number.
• Mitosis versus meiosis. Mitosis keeps the number, makes identical cells and needs one division. Meiosis halves the number, makes different cells and needs two divisions.
• Fertilisation versus meiosis. Meiosis halves the number to make gametes. Fertilisation doubles it again by fusing two gametes.
• Variation among offspring versus variation caused by the environment. Meiosis and fertilisation produce genetic variation. Clones can still differ in size if one gets more light or water.

Model explanations

Why is the potato crop at risk when grown from tubers? Tubers grow into plants with the same genetic information as the parent, so all the plants are genetically identical. They share the same weaknesses. If a pathogen can infect one plant it can infect them all, and there are no resistant plants to survive and reproduce.

Why is sexual reproduction an advantage when the environment changes? Sexual reproduction produces offspring that vary. If the environment changes, for example a new disease appears or the climate becomes warmer, some individuals are likely to have features that help them survive. They reproduce and pass those features on.

Why must gametes be haploid? Fertilisation joins two gametes. If each had 46 chromosomes, the zygote would have 92. Haploid gametes with 23 chromosomes each give a zygote with 46, so the diploid number is maintained in every generation.

Why are the four cells from meiosis different? Each pair of chromosomes separates independently, and which chromosome of each pair goes to which cell is random, so every cell ends up with its own combination.

Maths reminders

Skill	Rule	Example
Doubling	Number after d divisions = 2 to the power d	6 divisions: 2⁶ = 64
Combinations of chromosomes	2 to the power (number of pairs)	23 pairs: 2²³ = 8 388 608
Probability	Multiply independent chances	One specific parent’s chromosome in all of 3 pairs: (1/2)³ = 0.125
Ratios	Write as simple whole numbers	8 : 4 = 2 : 1 for diploid : haploid

Worked reminders

Chromosome number. A cat has 38 chromosomes in a skin cell. A skin cell is diploid, so 2n = 38, and a sperm cell has n = 38 ÷ 2 = 19. A kitten’s cells have 19 + 19 = 38.

Doubling. A bacterium divides every 20 minutes. In 2 hours there are 120 ÷ 20 = 6 divisions, so 2⁶ = 64 cells, assuming none die.

Probability. A mouse gamete receives one chromosome from each of 20 pairs. For each pair the chance of the maternal copy is 1/2. The chance of getting the maternal copy of two named pairs is 1/2 × 1/2 = 1/4 = 0.25.

Quick self-test

1. State the number of chromosomes in a human sperm cell.
2. A horse has 64 chromosomes in a body cell. How many are in an egg cell?
3. Give two advantages of asexual reproduction.
4. Give two disadvantages of sexual reproduction.
5. How many cells does one cell make by meiosis, and how many by mitosis?
6. Why are gametes haploid?
7. A bacterium divides every 30 minutes. How many bacteria after 3 hours from one cell?
8. Explain why a crop grown from clones is at risk from a new disease.
9. Where does meiosis happen in a flowering plant?
10. State how meiosis causes variation.

Answers

3. Any two: one parent only, no need for a mate, fast, many offspring quickly, offspring keep the successful features of the parent.
4. Any two: needs two parents or two gametes, takes time and energy to find a mate, usually slower, fewer offspring at a time.
5. Meiosis four, mitosis two.
6. So that fertilisation restores the diploid number and the number does not double each generation.
7. 3 hours = 180 minutes, 180 ÷ 30 = 6 divisions, 2⁶ = 64.
8. The plants are genetically identical, so any weakness is shared. A disease that can infect one can infect them all.
9. In the anthers and the ovules.
10. The pairs of chromosomes separate independently, so every gamete ends up with its own combination of chromosomes from the two parents.

Where marks are usually lost

• Writing “identical twins” or “clones” for sexual offspring.
• Describing asexual offspring as “the same” without saying genetic information.
• Saying meiosis makes “two” cells.
• Stating that gametes have 46 chromosomes, or 23 pairs.
• Explaining the advantage of sexual reproduction without linking variation to an environmental change or disease.
• Giving “it is quicker” as the only point on sexual reproduction, when asexual is usually the quicker one.
• Saying haploid cells are “half a cell”.
• Not stating that fertilisation restores the diploid number.
• Showing no working in a doubling question, so a wrong power of 2 cannot be traced to a slip in counting the divisions.

Official syllabus

OCR GCSE (9-1) Biology A (Gateway Science) specification J247, Version 4.0 (August 2026), for first assessment in 2018, published by OCR: statements B5.1f, B5.1g and B5.1h.