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OCR GCSE Biology A · J247
OCR J247 · B5.1h Check the specification (PDF) (opens in a new tab)
In sexual reproduction, two join at . A body cell is usually : it has two sets of chromosomes, with one chromosome of each pair inherited from each parent. A gamete must instead be , carrying just one set. Otherwise, joining two gametes would double the chromosome number in every generation. In humans, diploid cells have 46 chromosomes and gametes have 23; fertilisation brings 23 and 23 together to restore 46 in the new cell, called a zygote.
Meiosis is the cell division that produces haploid cells for sexual reproduction. Before it begins, the cell copies its DNA. It then divides twice. In the first division, the two chromosomes of each pair separate, so each resulting cell receives one chromosome from every pair. In the second division, the copied parts of those chromosomes separate. The simplified outcome is four haploid cells from one diploid starting cell. Copying the DNA first does not change the fact that each final cell has only one chromosome from each original pair.
Meiosis halves the chromosome number; different combinations of inherited DNA make the resulting haploid cells genetically different.
Meiosis does more than reduce chromosome number. The chromosome pairs can separate in different combinations, so gametes do not all inherit the same mixture of chromosomes. Paired chromosomes can also exchange sections of DNA before they separate, making new combinations of inherited information. As a result, the haploid cells produced by meiosis are genetically different. Fertilisation then combines genetic information from two gametes, adding further variation among offspring. Meiosis rearranges existing genetic information; it does not mean that every difference between gametes is a new mutation.
Diploid starting cell → meiosis → haploid gametes. In humans: . Fertilisation joins two haploid gametes, restoring the diploid number: .
Meiosis produces genetically different gametes because chromosome pairs separate in different combinations and can exchange sections of DNA. Fertilisation combines information from two gametes.
State that meiosis halves the chromosome number in gametes; fertilisation restores the diploid number.
Explain genetic variation by referring to different combinations of chromosomes and the exchange of DNA during meiosis.
Do not say that chromosome number doubles in each generation or that meiosis produces genetically identical gametes.
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Meiosis
Cell division that reduces the chromosome number by half to produce genetically different haploid cells.
Gamete
A reproductive cell, such as a sperm or egg cell, that carries one set of chromosomes.
Haploid
A cell with one set of chromosomes, containing one chromosome from each pair.
Diploid
A cell with two sets of chromosomes, arranged in pairs.
Fertilisation
The joining of two gametes to form a zygote.
Genetic variation
Differences in genetic information between individuals or cells.
Put your knowledge into practice — try past paper questions for Biology A
Meiosis
Cell division that reduces the chromosome number by half to produce genetically different haploid cells.
Gamete
A reproductive cell, such as a sperm or egg cell, that carries one set of chromosomes.
Haploid
A cell with one set of chromosomes, containing one chromosome from each pair.
Diploid
A cell with two sets of chromosomes, arranged in pairs.
Fertilisation
The joining of two gametes to form a zygote.
Genetic variation
Differences in genetic information between individuals or cells.