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Biology · Reproduction, Meiosis and Variation

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Biology · Edexcel Paper 1

Reproduction, Meiosis and Variation

Meiosis and Gametes

Your body cells have 46 chromosomes, but a sperm or an egg has only 23.

  • Meiosis A type of cell division that produces four genetically different haploid gametes.
  • Gamete A sex cell, such as a sperm or an egg, with half the number of chromosomes of a body cell.
  • Haploid Having one set of chromosomes, as in a gamete.
A diploid cell with two pairs of chromosomes divides by meiosis into four haploid gametes with different mixes of chromosomes.
Meiosis makes four cells, each with half the chromosomes and a different mix of genes.

Mitosis and meiosis

  • Mitosis Makes two daughter cells; Daughter cells are genetically identical; Used for growth, repair and asexual reproduction
  • Meiosis Makes four daughter cells; Daughter cells have half the number of chromosomes (haploid); Daughter cells are genetically different

What happens in meiosis

  • Start A diploid cell in the ovary or testis has pairs of chromosomes.
  • Copying The DNA is copied, so each chromosome has two identical copies.
  • First division The chromosome pairs separate, and the cell divides into two.
  • Second division The copies of each chromosome separate and the cells divide again.

Asexual and Sexual Reproduction (Separate only)

Some organisms reproduce with one parent and some need two.

  • Asexual reproduction Reproduction with one parent, in which the offspring are genetically identical to the parent.
  • Sexual reproduction Reproduction involving the fusion of male and female gametes, which produces genetically different offspring.
  • Clone An organism that is genetically identical to another.
Asexual reproduction gives identical offspring from one parent, while sexual reproduction gives different offspring from two parents.
Asexual reproduction gives clones. Sexual reproduction gives variation.

Asexual and sexual reproduction

  • Asexual reproduction One parent only; No gametes, and no fertilisation; Cells divide by mitosis
  • Sexual reproduction Two parents; Gametes are made by meiosis and fuse at fertilisation; Offspring are genetically different from both parents
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Biology · Edexcel Paper 1

Reproduction, Meiosis and Variation continued

Causes of Variation

No two people are exactly alike. This lesson explains how genes and the environment each cause differences between individuals.

  • Variation The differences between individuals of the same species.
  • Genetic variation Differences in characteristics caused by differences in genes.
  • Environmental variation Differences in characteristics caused by the environment, also called acquired characteristics.
A Venn diagram of genetic, environmental and combined causes of human variation with examples in each part.
Many characteristics, such as height, are caused by both genes and the environment.

Continuous and discontinuous variation

  • Continuous variation Any value in a range, with no clear groups; Examples: height, mass, leaf length; Usually controlled by many genes, and often by the environment as well
  • Discontinuous variation Falls into distinct categories, with no in-between values; Examples: blood group, sex, ability to roll your tongue; Usually controlled by one gene (or a few)

Mutations and Genetic Variation

Every new allele began as a mutation. This lesson explains what mutations are, what causes them, why most of them do nothing.

  • Mutation A random change in the base sequence of DNA.
  • Allele A different version of the same gene.
  • Mutagen Something that increases the rate of mutation, such as ionising radiation or certain chemicals.
A DNA sequence with one base changed, and a bar showing that most mutations have no effect, some have a small effect and a few have a large effect.
Most mutations make no difference to the organism.

Mutations in different cells

  • Mutation in a body cell Happens in a cell such as the skin; Can be copied into the new cells made by mitosis; May cause cancer if it affects the control of cell division
  • Mutation in a gamete Happens in a cell that makes gametes, or a gamete; Is present in every cell of the offspring; May cause an inherited disorder or a new variation