4.3 genetic diversity (mutation or during meiosis)

• what is a gene mutation

  a change in the base sequence of DNA (on chromosomes)

  can arise spontaneously during DNA replication (interphase)
• mutagenic agent

  a factor that increases rate of gene mutation

  eg ultraviolet (UV) light or alpha particles
• explain how a mutation can lead to the production of a non-functional protein or enzyme

  changes sequence of base triplets in DNA (in a gene) so changes sequence of codons on mRNA

  so changes sequence of amino acids in the polypeptide

  so changes position of hydrogen / ionic / disulphide bonds (between amino acids)

  so changes protein tertiary structure (shape) of protein

  enzymes active site changes shape so substrate can’t bind so enzyme substrate complex can’t form
• effects of a substitution mutation

  base / nucleotide in DNA replaced by a different base / nucleotide

  this changes one triplet so changes one mRNA codon

  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
• effects of a deletion mutation

  one nucleotide / base removed from DNA sequence

  changes sequence of DNA triplets from point of mutation (frameshift)

  changes sequence of mRNA codons after point of mutation

  changes sequence of amino acids in primary structure of polypeptide

  changes position of hydrogen / ionic / disulphide bonds in tertiary structure of protein

  changes tertiary structure / shape of protein
• features of homologous chromosomes

  same length, same genes at same loci but may have different alleles
• difference between diploid and haploid cells

  diploid - has 2 complete sets of chromosomes, represented as 2n

  haploid - has a single set of unpaired chromosomes, represented as n
• describe how a cell divides by meiosis

  in interphase, DNA replicates → 2 copies of each chromosome (sister chromatids), joined by a centromere

  meiosis I (first nuclear division) separates homologous chromosomes

  chromosomes arrange into homologous pairs

  crossing over between homologous chromosomes

  independent segregation of homologous chromosomes

  meiosis II (second nuclear division) separates chromatids

  outcome = 4 genetically varied daughter cells

  daughter cells are normally haploid (if diploid parent cell)
• explain why the number of chromosomes is halved during meiosis

  homologous chromosomes are separated during meiosis I (first division)
• 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 / (equal) lengths of (non-sister) chromatids exchanged between chromosomes

  creating new combinations of (maternal & paternal) alleles on chromosomes
• explain how independent segregation creates genetic variation

  homologous pairs randomly align at equator → so random which chromosome from each pair goes into each daughter cell

  creating different combinations of maternal & 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

  mitosis produces 2 daughter cells whereas meiosis produces 4 daughter cells

  as 1 division in mitosis whereas 2 divisions in meiosis

  mitosis maintains the chromosome number (diploid → diploid or haploid → haploid) whereas meiosis halves the chromosome number (diploid → haploid)

  as homologous chromosomes separate in meiosis but not mitosis

  mitosis produces genetically identical daughter cells whereas meiosis produces genetically varied daughter cells

  as crossing over and independent segregation happen in meiosis but not mitosis
• explain the importance of meiosis

  two divisions creates haploid gametes (halves number of chromosomes)

  so diploid number is restored at fertilisation → chromosome number maintained between generations

  independent segregation and crossing over creates genetic variation
• how can you recognise where meiosis and mitosis occur in a life cycle?

  mitosis occurs between stages where chromosome number is maintained (diploid (2n) → diploid (2n) OR haploid (n) → haploid (n))

  meiosis occurs between stages where chromosome number halves (diploid (2n) → haploid (n))
• describe how mutations in the number of chromosomes arise

  spontaneously by chromosome non-disjunction during meiosis

  homologous chromosomes (meiosis I) or sister chromatids (meiosis II) fail to separate during meiosis

  so some gametes have an extra copy (n+1) of a particular chromosome and others have none (n-1)