Genetic diversity can arise as a result of...

• Gene mutations

  • Involve a change in the DNA base sequence of chromosomes.
  • They can arise spontaneously during DNA replication.
• Base deletion

  • One base is lost and there is a frame shift
  • Very harmful as the mutation changes the whole amino acid sequence
  • The resulting protein will be different and probably dysfunctional
• Why may base substitutions have no impact?

  The genetic code is degenerate so the new codon may code for the same amino acid.
• Gene mutations occur randomly.
• Most mistakes are corrected by a proofreading mechanism within the cell.
• Mutagenic agents

  Increase the rate of mutations in the DNA base sequence.

  e.g. ionising radiation, UV radiation, some chemicals, biological agents (viruses)
• What is the product of meiosis?

  4 haploid, genetically different daughter cells
• Haploid cells in humans

  Gametes- the sperm and the egg cell
• Human chromosomes

  46 chromosomes, 23 pairs
• Process of fertilisation

  • Haploid sperm cell and haploid egg cell fuse to form a diploid zygote
  • The zygote has half the chromosomes from the father and the other half from the mother
• How does fertilisation increase genetic variation?

  It is random, so the zygotes have different combinations of chromosomes from each parent.
• Homologous chromosomes

  Carry the same genes at the same loci, but could have different alleles.
• Purpose of meiosis

  Produces haploid daughter cells, needed for sexual reproduction. Occurs in reproductive organs.
• Before meiosis:

  DNA unravels and replicates so there are two copies of each chromosome called chromatids.
• During meiosis:

  • DNA condenses to form double-armed chromosomes, each made from two sister chromatids, joined by a centromere.
  • Meiosis I: cell divides to produce two diploid cells.
  • Meiosis II: two diploid cells divide to produce four haploid cells.
• How does meiosis cause variation?

  1. Independent segregation
  2. Crossing over
• Independent segregation

  • Each homologous pair of chromosomes is made up from one chromosome from the father and one from the mother.
  • When homologous pairs are separated during Meiosis I, it is random which chromosome from each pair ends up in which daughter cell.
• Crossing over

  • During Meiosis I
  • Homologous pairs of chromosomes align and pair up
  • Chromatids twist around each other and bits of chromatid cross over
  • Each daughter cell produced contains chromatids with a different combination of alleles.
  • Genetic variation among daughter cells
• Non-disjunction

  • Mutations in the number of chromosomes can arise spontaneously by chromosome non-disjunction during meiosis
  • Sister chromatids do not separate properly during meiosis (I or II), leading to uneven distribution of chromosomes.