3.4.3

• Mutagenic agents increase the rate of gene mutation (above the rate of naturally occurring mutations). E.g UV light and alpha particles
• Changes in one base can lead to changes in the sequence of amino acids in the primary structure of the polypeptide.
• Base substitution is a change in one base that changes one triplet.
• Base substitution - Due to the degenerate nature of the genetic code, the new triplet may still code for the same amino acid so the sequence of amino acids in the primary structure of the polypeptide remains unchanged.
• Meiosis I separates homologous pairs, arranges chromosomes into homologous pairs, and creates genetic variation in gametes through crossing over (prophase I).
• A mutation can lead to the production of a non-functional protein or enzyme.
• Base substitution can lead to a change in base or triplet sequence of DNA or gene, changing the sequence of codons on mRNA and the sequence of amino acids in the primary structure of the polypeptide.
• Base deletion can lead to a change in base or triplet sequence of DNA or gene, changing the sequence of codons on mRNA and the sequence of amino acids in the primary structure of the polypeptide By changing position of hydrogen ionic or disulphide bridges in the tertiary structure- shape changes to non- functional or superior proteins
• Mutation can happen with A change in base or triplet sequence of DNA or gene can change the sequence of codons on mRNA and the sequence of amino acids in the primary structure of the polypeptide.
• A gene mutation is a change in the base sequence of DNA (on chromosomes) that can arise spontaneously during DNA replication (interphase) and involves base deletion or substitution.
• Base deletion involves one nucleotide or base removed from DNA sequence, changing the triplet or codon sequence from the point of mutation (frameshift) and changing the sequence of codons on mRNA after point of mutation.
• base deletion- A change in the tertiary structure by changing position of hydrogen/ionic/disulphide bridges of a protein can alter the active site if the protein is an enzyme, preventing the substrate from binding and forming an enzyme-substrate complex.
• In humans, non-disjunction involving chromosome 21 can lead to a child being born with Down’s syndrome.
• Failure of chromosomes to separate during meiosis II can result in one gamete having an extra copy of this chromosome and the other having none.
• What happens before meiosis starts?

  DNA replicates so there are two copies of each chromosome, called sister chromatids, joined by a centromere

  
• • During meiosis 1, the sister chromatids cross over (swap alleles of genes)
  • Every time the DNA divides, independent segregation occurs (chromosomes line up randomly)
  • This results in genetically different daughter cells

  
• What happens in meiosis 2?

  Separation of sister chromatids. This results in 4 haploid cells

  
• What is crossing over ?

  Genetic recombination between homologous chromosomes during meiosis. - ALLELES exchange between chromosome.

  • creates new combinatio of maternal and paternal alleles on chromosomes
• What is independent segregation?

  Independent segregation refers to the random assortment of homologous chromosomes during meiosis, resulting in the independent distribution of alleles into gametes.

  Random alignment of homologous pairs at equator → random which chromosome from each pair goes to each daughter cell

  • Creates different combinations of maternal and paternal chromosomes and alleles in daughter cells

  
• How meiosis creates genetic variation?

  Crossing over and independent assortment of chromosomes during meiosis create genetic variation.

  • random fertilisation when two gametes fuse to form a zygote.
• Importance of meiosis?

  Genetic diversity and production of gametes.

  • Two divisions - creates haploid gametes (half number of chromosomes)
  • Diploid number restored at fertilisation
  • Maintains chromosome number from one generation to the next
  • Independent segregation and crossing over creates genetic variation
• Mutation in the number of chromosomes ?

  • happens by chromosomes non-disjunction during meiosis.
  • Homologous chromosomes fail to separate during meiosis I OR sister chromatids fail to separate during meiosis II
  • One gamete has an extra copy of this chromosome and the other has none
  • Upon fertilisation, zygote has one fewer (dies) or one extra chromosome (survives)
  • Arises spontaneously
  • Causes genetic diseases e.g. down’s syndrome in humans – extra copy of chromosome 21
• What is polyploidy?

  Polyploidy is the condition of having more than two complete sets of chromosomes in an organism.
• What is aneuploidy?

  Aneuploidy is a condition in which an individual has an abnormal number of chromosomes, either too many or too few.
• The number of possible different combinations of chromosomes in daughter cells following meiosis is calculated as 2n, where n is the number of pairs of homologous chromosomes.
• The number of different combinations of chromosomes following the random fertilisation of two gametes is calculated as (2n)², where n is the number of pairs of homologous chromosomes.
• The number of possible chromosome combinations produced from the fertilisation of two gametes from separate individuals whose diploid number is 12 can be calculated as (2x6)² = 4096.