Cell division, variation and evolution — Edexcel International GCSE Science (Double Award)

Revision notes

7 short notes, in the order of the specification. Each one in short:

1. Mitosis

   Mitosis is the division of a diploid cell to produce two cells that contain identical sets of chromosomes. The DNA is copied before the cell divides and the copies are shared equally, so both daughter cells are diploid and genetically identical to the parent cell and to each other. In humans, each daughter cell has 46 chromosomes.

2. Growth, repair, cloning and asexual reproduction

   Mitosis occurs during growth, repair, cloning and asexual reproduction, because all of these need new cells that are genetically identical to the parent cell. A multicellular organism grows by making more cells, damaged cells such as cut skin are replaced, and clones and asexual offspring are made with no mixing of genetic information.

3. Meiosis, fertilisation and chromosome number

   Meiosis is the type of cell division that makes gametes, in the ovaries and testes. One diploid cell divides to produce four haploid gametes, each with half the number of chromosomes and genetically different from each other. In humans, 46 chromosomes become 23. Random fertilisation restores 46 in the zygote and produces genetic variation.

4. Genetic and environmental variation

   Variation is the differences between individuals of the same species. It can be genetic, caused by the alleles inherited, such as blood group and eye colour, or environmental, caused by surroundings and way of life, such as a scar. Many features depend on both: a person's height depends on their alleles and on their diet.

5. Mutations and the phenotype

   A mutation is a rare, random change in genetic material that can be inherited. Usually it is a change in the base sequence of DNA. Mutations are one source of genetic variation, and a mutation that happens in a gamete, or in a cell that makes gametes, can be passed on to the offspring.

6. Darwin's theory of evolution by natural selection

   Natural selection is the process by which evolution happens. Individuals in a population show genetic variation, more offspring are born than can survive, and they compete for resources. Those best suited to their environment are more likely to survive, reproduce and pass on their alleles, so the proportion with advantageous alleles increases over many generations.

7. Antibiotic resistance

   Antibiotic resistance evolves by natural selection. A mutation produces an allele that makes a bacterium resistant to an antibiotic. When the antibiotic is used, it kills the non-resistant bacteria, but the resistant bacterium survives, reproduces with less competition and passes on the allele. The population becomes mostly resistant, so infections are difficult to control.