Key Questions

• What is a gene mutation? 


  A change in the base sequence of DNA.
• When do gene mutations occur?

  During DNA replication.
• What types of gene mutation are there?

  A base could be deleted, an extra base could be inserted, or one base could be substituted for a different base.
• What is a substitution mutation also known as?

  A point mutation because the substitution happens at a single point (one base) on the DNA.
• The genetic code is degenerate. What does this mean?


  More than one triplet codes for the same amino acid.
• Do all substitution mutations alter the protein being coded?

  No, because the genetic code is degenerate so the same amino acid could be coded by the triplet containing the mutation.
• What level of protein structure will be affected if a mutation does have an effect?

  Primary - the sequence of amino acids in the polypeptide could be altered.
• What is a mutagenic agent?

  Anything that increases the rate of gene mutation above the normal level.
• Give some examples of mutagenic agents.

  Radiation (UV, gamma, X-rays, alpha and beta particles etc), heavy metals (mercury, lead) and some chemicals.
• What is a chromosome mutation?

  A change in the number of chromosomes that should be present in a cell nucleus.
• When do chromosome mutations occur?

  During meiosis.
• What does chromosome non-disjunction mean?

  Either two chromosomes or two sister chromatids do not split from each other when they should do during meiosis.
• State examples of inherited disorders that arise from chromosome non-disjunction.

  Down syndrome (trisomy 21), Edwards syndrome (trisomy 18) and Patau syndrome (trisomy 13).
• What does the term diploid mean?


  A cell that contains two complete sets of chromosomes, one from each parent. We can also call it 2n algebraically.
• What does the term haploid mean?


  A cell that contains only one complete set of chromosomes, from one parent. Also called n, algebraically.
• What does n refer to when discussing haploid and diploid cells?

  n is the number of pairs of homologous chromosomes.
• What types of cells are haploid?

  Gametes are always haploid cells. This means when they fertilise the resulting cell is diploid.
• Describe the daughter cells that arise from meiosis.

  Genetically different from each other and are haploid cells.
• How many daughter cells arise from a cell undergoing meiosis?

  Meiosis generates four haploid daughter cells from each diploid parent cell.
• Why is it important that meiosis is able to generate haploid cells

  Haploid gametes which can then be fertilised by another gamete maintains the chromosome number across generations.
• How many times does the nucleus usually divide during meiosis?

  During meiosis the nucleus usually divides twice to form four haploid daughter cells.
• What processes cause the haploid gametes to be genetically different?

  During meiosis, crossing over and independent segregation cause daughter cells to be genetically different.
• How does independent segregation cause genetic variation?

  Independent segregation recombines maternal and paternal chromosomes when they align at the metaphase plate.
• How does crossing over cause genetic variation?

  Crossing over recombines maternal and paternal alleles when the chromosomes align at the metaphase plate.
• Besides processes in meiosis, how else is genetic variation increased?

  Fertilisation is random, as any combination of egg and sperm can potentially fertilise and become a zygote.