Chapter 9: Genetic diversity

• mutation


  a change in the sequence of bases in DNA that may result in an altered polypeptide
• Substitution mutation


  base swapped for a different base
• non sense mutation (substitution) 


  formation of stop codon- polypeptide chain terminates early
• missense mutation (substitution) 


  formation of a codon for a different amino acid - may or may not affect tertiary structure of polypeptide chain
• silent mutation (substitution)


  formation of different codon that codes for the same amino acid - no effect on polypeptide
• deletion mutation 


  loss of a nucleotide base creating a frame shift to the left

  • changes every triplet further on the DNA sequence and therefore amino acid so may have dramatic effect on the tertiary structure of the polypeptide
• addition mutation


  an extra base is inserted leading to a frame shift to the right

  • changed every triplet further in the DNA sequence therefore every amino acid so may have dramatic effect on the tertiary structure of the polypeptide
• mutagenic agents 


  environmental factors that increase the mutation rate of cells
• mutagenic agents EXAMPLES

  • high energy radiation eg UV light
  • ionising radiation such as x-rays
  • toxic chemicals eg nitrous acid changes C->U by removing -NH2 group
• mutations can spontaneously occur during DNA replication
• mutagen 


  anything that can lead to a mutation
• somatic mutation 


  occur after conception, so aren’t inherited
• gamete mutation 


  occurs during conception, from the gamete’s of the parents, so is inherited and passed onto offspring
• non disjunction mutation 


  pairs of homologous chromosomes fail to separate in meiosis
• haploid cells 


  contain one set of chromosomes
• diploid cells


  contain two sets of chromosomes
• genetic variation by meiosis comes as a result of

  • independent segregation of chromosomes
• crossing over 


  the process by which non sister chromatids exchange alleles
• independent assortment 


  the production of different combinations of alleles in daughter cells due to random alignment of homologous pairs along the equator of the spindle during metaphase
• stages of meiosis:

  1. DNA replicates forming chromosomes with 2 sister chromatids
  2. homologous chromosomes pair up and crossing over occurs
  3. homologous chromosomes separate
  4. individual chromosomes line up at the equator
  5. sister chromatids are pulled apart forming new haploid cells
• allele frequency 


  how common an allele is in a population
• gene pool


  all alleles of all the genes of all individuals in a population of a given time
• adaptions

  • behavioural
  • physiological
  • anatomical
• behavioural 


  acts which all organisms do to increase their chance of survival and reproduction
• physiological 


  process inside of an organisms body that increases its chances of survival and reproduction
• anatomical 


  structural features which increases an organisms chance to survive and reproduce
• natural selection 


  mechanism by which evolution takes place
• environmental selection pressures

  • disease
  • competition
  • natural disasters
  • predators
• selection pressures determine the frequency of alleles within the gene pool and results in differential survival and reproduction
• evolution by natural selection depends on two factors

  1. genetic variation within the population (variety of phenotype)
  2. overproduction of offspring - more offspring are produced that can survive therefore those with most advantageous traits will survive and reproduce