How do sexual and asexual reproduction differ, and how does meiosis make gametes that vary?
Sexual and asexual reproduction, the formation of gametes by meiosis, how meiosis halves the chromosome number and produces genetically varied cells, and the advantages and disadvantages of each type of reproduction.
A focused answer to the OCR Gateway GCSE Biology A topic B5 on reproduction and meiosis, covering sexual and asexual reproduction, how meiosis halves the chromosome number and produces genetically varied gametes, fertilisation, and the advantages and disadvantages of each type of reproduction.
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What this dot point is asking
OCR wants you to compare sexual and asexual reproduction, describe how gametes are made by meiosis, explain how meiosis halves the chromosome number and produces genetically varied cells, and weigh up the advantages and disadvantages of each type of reproduction.
Two types of reproduction
In sexual reproduction the male gamete (sperm in animals, or the nucleus in a pollen grain in flowering plants) fuses with the female gamete (an egg cell, or ovum) at fertilisation. Because each parent contributes a mixture of alleles, the offspring inherit a new combination and show variation. Examples include humans, most animals and flowering plants.
In asexual reproduction there is no fusion of gametes and no mixing of genetic information. The offspring are produced by mitosis from a single parent, so they are clones that are genetically identical to the parent and to each other. Examples include bacteria dividing, strawberry plants growing runners, and spores in fungi.
Gametes and the chromosome number
Body cells are diploid: they carry two copies of each chromosome, one from each parent. In humans a body cell has 46 chromosomes (23 pairs). Gametes are haploid: they carry only one copy of each chromosome, so a human gamete has 23 chromosomes.
How meiosis produces variation
Meiosis is a special type of cell division that takes place only in the reproductive organs (the ovaries and testes in humans). It differs from mitosis in two important ways: it halves the chromosome number, and it produces cells that are all genetically different.
The key stages to know are:
- The chromosomes are copied, so each chromosome is made of two identical strands.
- The cell divides twice in succession (mitosis divides only once).
- This produces four haploid gametes from one diploid cell.
- The chromosomes and alleles are shuffled as they are shared out, so each of the four gametes has a different combination of alleles.
The shuffling of alleles during meiosis, together with the random way two gametes meet at fertilisation, is the source of the genetic variation seen in sexually reproducing species. By contrast, mitosis (used in growth, repair and asexual reproduction) produces two identical diploid cells and no variation.
Advantages and disadvantages
OCR often asks you to evaluate the two strategies, so learn a balanced set of points.
- Sexual reproduction produces variation, so if the environment changes some offspring may be better adapted and survive (natural selection can act). The cost is that it is slower, needs two parents and uses more energy (for example finding a mate).
- Asexual reproduction is fast, needs only one parent and lets an organism rapidly populate a favourable, stable habitat. The cost is no variation, so if conditions change or a new disease appears, the whole identical population may be wiped out.
Many organisms use both: a strawberry plant reproduces asexually by runners in good conditions but sexually by flowers and seeds when conditions vary, getting the benefits of each.
Exam-style practice questions
Practice questions written in the style of OCR exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
OCR 20196 marksCompare sexual and asexual reproduction, including how the genetic material of the offspring is produced and one advantage of each type of reproduction to an organism.Show worked answer →
A 6-mark extended comparison, marked on linked points across both types of reproduction.
Sexual reproduction: involves two parents and the fusion of a male gamete with a female gamete at fertilisation. Gametes are made by meiosis, which halves the chromosome number and shuffles the alleles, so the offspring inherit a mixture of alleles from both parents and are genetically different from each other and from the parents. This variation is an advantage when the environment changes, because some offspring may be better suited to new conditions (the raw material for natural selection).
Asexual reproduction: involves only one parent and no gametes. The offspring are produced by mitosis and are genetically identical clones of the parent. An advantage is that it is fast and needs no mate, so a single organism can rapidly colonise a favourable, stable habitat.
Markers reward the number of parents, the role of meiosis versus mitosis, the link to variation versus identical clones, and a sensible advantage for each.
OCR 20213 marksA human body cell contains 46 chromosomes. State how many chromosomes are in a human gamete, and explain why meiosis must halve the chromosome number before fertilisation.Show worked answer →
A short B5 recall and reasoning question.
A human gamete contains 23 chromosomes, half the number in a body cell.
Meiosis halves the chromosome number so that when two gametes fuse at fertilisation, the full diploid number of 46 is restored (23 from each parent). If gametes had the full 46 chromosomes, fertilisation would double the number to 92, and it would keep doubling each generation. Reward the value 23 and the explanation that halving keeps the chromosome number constant from one generation to the next.
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