chapter 9 - genetic diversity

• spec point - mutations in the number of chromosomes can arise spontaenously by chromosome non-disjunction during meiosis
• what do gene mutations involve?

  a change in the base sequence of chromosomes
• when do gene mutations usually occur during?

  DNA replication
• why do not all mutations cause a change in the sequence of amino acids?

  because the genetic code is degenerate
• what are mutagenic agents?

  can increase the rate of gene mutation
• what is a mutation?

  a change in the base sequence of DNA within a gene or a quantity of DNA
• what are the types of mutagens & their effects?

  • physical e.g. ionising rays - breaks DNA strands
  • chemical e.g. deaminating agents - changes base into another base
  • biological e.g. viruses - inserts viral DNA into host cell
• what are the types of mutations?

  1. insertion - a base sequence is added to the sequence. causes a frame shift
  2. deletion - deletion of a base in the sequence, causes frame shift to the left
  3. substitution - where a base sequence is exchanged for another
  4. inverstion - a base sequence is added, causes frame shift
  5. duplication - where 1 or more bases is inserted in, causes frame shift
  6. translocation - a section of a gene moves from one location to another
• what is a silent mutation?

  a change in the nucleotide base sequence that has no effect
• EXAM QUESTION - explain how a mutation leads to a non-functional protein/enzyme

  • a mutation is a change in the base sequence of DNA
  • this would lead to a change in the primary structure
  • this would also lead to change in the tertiary structure which would lead to formation of bonds in different places
  • resulting in a different tertiary structure
  • this would also result in a change to the active site (enzyme extra mark)
• what is a non-disjunction?

  the chromosomes/chromatids do not split equally during anaphase
• what is a chromosome mutation?

  changes to structure/number of whole chromosomes
• what are the 2 forms of chromosome mutations?

  1. polyploidy - changes to whole sets of chromosomes, occurs when organisms have 3 or more sets of chromosomes rather than usual two, occurs mainly in plants
  2. aneuploidy - changes to number of individual chromosomes, on fertilisation with a gamete that has the normal number of chromosomes, the resultant zygote will have more / fewer chromosomes than normal in all their body cells
• spec point - meiosis produces daughter cells that are genetically different from each other
• what do 2 nuclear divisions usually result in the formation of in meiosis?

  4 haploid daughter cells from a single diploid parent cell
• what are the reasons for increased genetic variation?

  1. independent segregation
  2. crossing over
  3. mutations
  4. random fertilisation
• independent segregation:

  • meiosis 1, homologous pairs of chromosomes line up opposite each other at the equator of the cell
  • every pair of chromosomes is random
  • pairs are separated so 1 of each homologous pair ends up in the daughter cell
  • creates a large number of possible combinations of chromosomes
• when does independent segregation & crossing over occur in meiosis?

  meiosis 1
• crossing over:

  • when homologous pairs line up opposite each other at the equator in meiosis 1, parts of the chromatids can become twisted around each other, putting tension on the chromatids, causing the pairs to break
  • the broken parts of the chromatid recombine with another chromatid, resulting in new combinations of alleles
• meiosis:

  • 2 nuclear divisions
  • haploid cell
  • introduces genetic variations

  mitosis:

  • one nuclear division
  • diploid cell
  • creates genetically identical cells
• what is genetic diversity?

  number of different alleles of genes in a population
• what is natural selection?

  process that leads to evolution in populations
• what is evolution?

  the change in allele frequency over many generations in a population
• when can natural selection occur?

  if there is a genetic diversity within a population
• what does natural selection result in?

  species becoming better adapted to their environment & adaptations may be anatomical, physiological or behavioural
• EXAM QUESTION - concerning natural selection

  • new alleles for a gene are created by random mutations
  • if the new alleles increases the chances of the individual to survive in that environment, then they are more likely to survive & reproduce
  • this reproduction passes on the advantageous allele to the next generation
  • as a result, over many generations, the new allele increases in frequency in the population
• what are the types of selection?

  directional selection

  • 1 of the extremes has the selective advantage
  • occurs when there's a change in the environment
  • modal trait changes

  stabilising selection

  • the modal traits have the selective advantage
  • occurs when there's no change in the environment
  • modal trait remains the same
  • standard deviation decreases, as individuals with the extreme trait decrease
  • favors mean