B: Diversity and Selection

• normal body cells are diploid - each cell contains 2 copies of each chromosome
• gametes are haploid cells - they contain one copy of each chromosome
• in sexual reproduction gametes join together at fertilisation to form a zygote
• Meiosis produces daughter cells that are genetically different from each other
• What happens to DNA during Prophase I of meiosis?

  DNA condenses and becomes visible as chromosomes
• What are homologous pairs called during Prophase I?

  They are called bivalents
• What occurs to the nuclear envelope during Prophase I?

  The nuclear envelope breaks down
• What happens to the centrioles during Prophase I?

  Centrioles move to opposite poles of the cell and spindle forms
• How do bivalents align during Metaphase I?

  Bivalents line up along the equator
• What occurs during Anaphase I of meiosis?

  Homologous pairs are separated
• What happens to the nuclear envelope during Telophase I?

  The nuclear envelope reforms around two groups of chromosomes
• What is cytokinesis in the context of Telophase I?

  Cytokinesis is the division of the cytoplasm
• What happens to the nuclear membrane during Prophase II?

  The nuclear membrane breaks down
• How do chromosomes align during Metaphase II?

  Chromosomes line up along the equator
• What occurs during Anaphase II of meiosis?

  The centromere divides and individual chromatids are pulled to opposite poles of the cell
• What happens to the nuclear membrane during Telophase II?

  The nuclear membrane reforms around the group of chromosomes
• What is the final step that occurs after Telophase II?

  Cytokinesis occurs
• in meiosis two nuclear divisions result in the formation of four haploid daughter cells from a single diploid parent cell
• crossing over


  in meiosis I homologous chromosomes pair up and the chromatids twist around each other causing bits of chromatid to swap over

  the chromosomes now contain the same genes but a different combination of alleles

  
• independent segregation


  in meiosis I when homologous chromosomes separate it is random which chromosome from each pair ends up in which daughter cell. this means the four daughter cells produced will have different combinations of maternal and paternal chromosomes leading to genetic variation
• causes of genetic variation in meiosis


  • independent segregation
  • crossing over
• Gene mutations involve a change in the base sequence of chromosomes
• types of genetic mutation


  • base substitution - one base is substituted with another
  • base deletion - one base is deleted
• Due to the degenerate nature of the genetic code, not all base substitutions cause a change in the sequence of encoded amino acids.
• base deletion will cause a shift in all the base triplets after it
• Mutagenic agents can increase the rate of gene mutation
• example of mutagenic agents


  • UV radiation
  • ionising radiation
  • some chemicals
  • some viruses
• Mutations in the number of chromosomes can arise spontaneously by chromosome non disjunction - a failure of the chromosomes to separate properly in meiosis
• Genetic diversity as the number of different alleles of genes in a population
• Genetic diversity is a factor enabling natural selection to occur - if a population has a low genetic diversity it may not be able to adapt to a change in the environment
• genetic diversity is increased by


  • genetic mutations forming new alleles
  • gene flow - different alleles are introduced to a population when individuals from a different population migrate into it and reproduce