Genetic diversity as a result of mutation or meiosis

• What is a gene mutation?

  • A change in the base sequence of DNA (on chromosome)
  • Can arise randomly during DNA replication (interphase)
• What is a mutagenic agent?

  A factor that increases rate of gene mutation e.g. ultraviolet light
• Explain how a mutation can lead to the production of a non-functional protein or enzyme

  1. Changes sequence of base triplets in DNA (in a gene) so changes sequence of codons on mRNA
  2. So changes sequence of amino acids in the polypeptide
  3. So changes the position of hydrogen / ionic/ disulphide bonds between amino acids
  4. So changes protein tertiary structure
  5. Enzymes active site changes shape so substrate can't bind so enzyme-substrate complexes can't form
• Explain the possible effects of a substitution mutation

  1)Base / nucleotides in DNA replaced by a different nucleotide / base

  2)This changes one triplet so changes one mRNA codon

  3)So one amino acid in polypeptide changes

  • Tertiary structure may change if position of hydrogen / ionic /disulphide bonds change

  or amino acid doesn't change

  • Due to degenerate nature of genetic code (Triplet could code for same amino acid) or if mutation is in an intron
• Explain the possible effects of a deletion mutation:

  1. One nucleotide / base removed from DNA sequence
  2. Changes sequence of DNA triplets from point of mutation (frameshift)
  3. Changes sequence of mRNA codons after point of mutation
  4. Changes sequence of amino acids in primary structure of polypeptide
  5. Changes position of hydrogen / ionic / disulphide bonds in tertiary structure of protein
  6. Changes tertiary structure of protein
• Describe the features of homologous chromosomes:

  • Same length
  • Same genes
  • May have different alleles
• What is a diploid cell?

  Has 2 complete sets of chromosomes, represented as 2n
• What is a diploid cell?

  Has a single set of unpaired chromosomes, represented as n
• Describe how a cell divides by meiosis:

  1)Meiosis 1 (first nuclear division) separates homologous chromosomes

  • Chromosomes arrange into homologous pairs
  • Crossing over between homologous chromosomes
  • Independent segregation of homologous chromosomes

  2)Meiosis 2 (second nuclear division) separates chromatids
• What is the outcome of Meiosis?

  • 4 genetically varied daughter cells
  • Daughter cells are normally haploid (if diploid parent)
• Explain why the number of chromosomes is halved during meiosis

  Homologous chromosomes are separated during meiosis 1
• Explain how crossing over creates genetic variation

  • Homologous pairs of chromosomes associate / form a bivalent
  • Chiasmata form (point of contact between non sister chromatids)
  • Alleles of equal length of non sister chromatids exchanged between chromosomes
  • Creating new combinations of maternal and paternal alleles on chromosomes
• Explain how independent segregation creates genetic variation?

  • Homologous pairs randomly align at the equator -> so random which chromosomes from each pair goes into each daughter cell
  • Creating different combinations of maternal and paternal chromosomes / alleles in daughter cells
• Other than mutation and meiosis, explain how genetic variation within a species is increased:

  • Random fertilisation / fusion of gametes
  • Creating new allele combinations / new maternal and paternal chromosome combinations
• Explain the different outcomes of mitosis and meiosis:

  1)Mitosis produces two daughter cells whereas meiosis produces four daughter cells

  • As one division in mitosis whereas two divisions in meiosis

  2)Mitosis maintains the chromosomes number (e.g. diploid -> diploid or haploid -> haploid) whereas meiosis halves the chromosome number (e.g. diploid -> haploid)

  • As homologous chromosomes separate in meiosis but not mitosis

  3)Mitosis produces genetically identical daughter cells whereas meiosis produces genetically varied cells

  • As crossing over and independent segregation happens in meiosis
• Explain why meiosis is important:

  • Two divisions creates haploid gametes (halves No. of chromosomes)
  • So diploid number is restores at fertilisation -> chromosome number maintained across generations
  • Independent segregation and crossing over ensures genetic variation
• How can you recognise where meiosis and mitosis occur in a life cycle?

  • Mitosis occurs between stages where chromosome number is maintained (e.g. diploid (2n) -> diploid (2n) OR haploid (n) -> haploid (n))
  • Meiosis occurs between stages where chromosome number is halved (e.g. diploid (2n) -> haploid (n))
• Describe how mutations in the number of chromosomes arise

  • Randomly by chromosome non-disjunction during meiosis
  • Homologous chromosomes (meiosis 1) or sister chromatids (meiosis 2) fail to separate during meiosis
  • So some gametes have an extra copy (n+1) of a particular chromosome and others have none (n-1)