AQA GCSE Biology contents

Unit 6 · Inheritance, Variation & Evolution

Sexual & Asexual Reproduction & Meiosis

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Sexual and asexual reproduction

  • Sexual reproduction involves the fusion (joining) of male and female gametes: sperm and egg in animals, pollen and egg in flowering plants
    • There is mixing of genetic information, so the offspring show variation
    • Gametes are made by meiosis
  • Asexual reproduction involves only one parent and no fusion of gametes
    • No mixing of genetic information, so the offspring are genetically identical to the parent: clones
    • Only mitosis is involved
  • Genes are passed from parent to offspring in both types of reproduction; the difference is whether they come from one parent or two

Meiosis

  • Cells in the reproductive organs divide by meiosis to form gametes
  • Copies of the genetic information are made (the chromosomes replicate), then the cell divides twice to form four gametes, each with a single set of chromosomes (half the normal number, 23 in humans)
  • All the gametes are genetically different from each other, because which chromosome of each pair goes into each gamete is random
  • At fertilisation the gametes join, restoring the normal number of chromosomes (46). The new cell then divides by mitosis, the number of cells increases, and as the embryo develops the cells differentiate

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— making gametesThe parent cell has two pairs of chromosomes here: one of each pair came from each parent.1 the parent celltwo pairs of chromosomes2 every chromosome is copiedeach one is now a pair ofidentical strands3 the FIRST divisionone chromosome from each pair goes to each cell —WHICH one is random, so no two cells are alike4 the SECOND division — four gametesgamete 1gamete 2gamete 3gamete 4Meiosis happens only in the reproductive organs, and only for making gametes.One parent cell → FOUR gametes.Each gamete has HALF the number of chromosomes, and each one is genetically different.At fertilisation two gametes fuse, restoring the full number and giving a unique combination —which is where the variation in sexual reproduction comes from. The fertilised egg then dividesby MITOSIS to form an embryo.
Meiosis in a cell with two pairs of chromosomes: chromosomes replicate, the cell divides once and then again, giving four gametes with one chromosome from each pair, each combination different

Mitosis Meiosis
Where body cells reproductive organs
Divisions one two
Cells made two four
Chromosomes full set, same as parent half the number: one of each pair
Genetically identical all different
Used for growth, repair, asexual reproduction making gametes

Advantages of each type [Biology only]

Sexual reproduction Asexual reproduction
produces variation in the offspring only one parent needed
if the environment changes, variation gives a survival advantage by natural selection more time and energy efficient: no need to find a mate
humans can speed up natural selection by selective breeding to increase food production faster
many identical offspring when conditions are favourable
  • Some organisms use both: the malarial parasite reproduces asexually in the human host and sexually in the mosquito; many fungi reproduce asexually by spores and sexually for variation; many plants make seeds sexually and also reproduce asexually by runners (strawberries) or bulb division (daffodils)