2. meiosis + genetic variation

• "describe and explain the appearance of one of the chromosomes in cell x"

  • chromosome consists of 2 sister chromatids
  • held together by a centromere
  • because DNA replication has occurred

  
• "describe what has happened during division 1 of the figure below"

  • one of each chromosome in the homologous pair
  • gone into the daughter cells to form haploid cells

  
• "identify one event that occurred during division 2 but NOT during division 1"

  • separation of sister chromatids

  
• "2 ways in which meiosis produces genetic variation"

  • crossing over
  • independent segregation
• "A mutation can lead to the production of a non-functional enzyme. Explain how."

  • mutation causes a change in the DNA base sequence
  • therefore this changes the primary amino acid sequence
  • so a change in the hydrogen, ionic and disulphide bonds
  • the tertiary structure of the active site of the enzyme is changed
  • overall no enzyme-substrate complexes will be formed
• Explain how the chromosome number is halved during meiosis

  • homologous chromosomes pair
  • one of each pair goes to each daughter cell
• Explain the process of crossing over in meiosis I

  • homologous pairs of chromosomes pair up
  • chromatids twist around each other and tension causes part to break off
  • They still contain the same genes, but different alleles
• 2 reasons why a mutation changing the nucleotide sequence of a gene doesn't change the structure of a polypeptide:

  • the genetic code is degenerate so more than one codon can code for the same amino acid
  • mutation may occur in introns/ non-coding sequence and are spliced out anyway