Chromosomal abnormalities and mutations

• Chromosomal abnormalities


  Mistakes made in Meiosis
• Meiosis


  1. Diploid parent cell divides to make 4 haploid gametes

  2. Homologous chromosomes or sister chromatids fail to separate during Anaphase I or II

  3. One gamete will fail to receive a copy of that chromosome and ends up with too few chromosomes

  4. Another gamete will receive 2 copies of that chromosome and ends up with too many chromosomes
• Non-Disjunction


  The type of chromosomal abnormality in which homologous chromosomes or sister chromatids fail to separate during Meiosis
• Trisomy


  When there are 3 copies of a chromosome
• Monosomy


  When there is only 1 copy of a chromosome
• Trisomy Examples (47 chromosomes in total)

  • Down Syndrome: person has an extra chromosome 21 (Trisomy 21) and therefore 47 chromosomes
  • Klinefelter Syndrome: Male has an extra X Chromosome so he is XXY (and therefore 47 chromosomes)
• Monosomy Example (45 chromosomes in total)

  • Turner Syndrome: A female is missing her second X chromosome
• Mutation


  A change in the genetic material of an organism, usually in the DNA
• Mistakes can occur when DNA is being copied, resulting in a change in base sequence
• Mutations can happen by chance, or from damage to a strand of DNA (E.g. from radiation, some chemicals and viruses)
• Mutations is a key process in increasing genetic variation in a population
• Mutation


  • A permanent change in the genetic code of a cell
  • May create a new allele for the affected gene
  • If a mutation occurs in gametes, it can be passed onto offspring of affected individual
  • If it occurs in somatic (body) cells, it is not able to be inherited by offspring
• Causes of Mutations


  • Spontaneous mutations: when there are random mistakes in DNA replication
  • Induced mutations: caused by environment, mutagens
• Mutagens


  Agents that cause mutation
• Carcinogen


  Mutation leads to cancer
• Point mutation


  Change to a base in a DNA molecule
• Codon


  A sequence of 3 bases that codes for an amino acid
• Substitution


  An incorrect nucleotide is substituted for the correct one
• Deletion


  A nucleotide is removed from the DNA sequence
• Insertion


  A nucleotide is added to the DNA sequence
• Silent mutations


  Changes do not affect the protein
• Missense mutations


  One amino acid or amino acid sequence is changed and has an effect on the protein. E.g. Sickle-cell anaemia
• Nonsense mutations


  A stop codon is created and as a result sequence of amino acids is cut short. Protein is incomplete and cannot function at all. E.g. Cystic fibrosis
• Frameshift mutations


  Nucleotides are inserted or deleted, all amino acids after this point is changed. Will cause major changes in protein formed and result in severe genetic diseases. E.g. Tay-Sachs disease