3.4.3 Mutations and meiosis

• Example of genetic mutation in enzymes- if a mutation occurs in the coding section of DNA, there could be a change in the mRNA sequence, therefore there could be a change in the amino acid. This might change the R-group of the amino acid which would also change the bonds between R-groups. This changes the tertiary structure, which means the active site is changed and enzyme-substrate complexes won't be able to be formed.
• Common mutagens (things which can potentially cause mutations):

  • UV light
  • X-rays
  • Gamma, alpha, beta radiation
  • Smoking (carcinogens)
  • Chemical warfare
• Types of mutations:

  • Chromosomal mutations- deletion, inversion, duplication
  • Gene mutation- substitution, deletion, insertion, inversion
• Deletion- the type of gene mutation which is likely to cause the most disruption of the gene, because every triple code after the deletion also changes (frame shift)
• Substitution- only one triple code is affected, but it might have no effect if the changed triple code is a degenerate code
• Frame shift- where the following triple codes after a mutation are shifted to the left or right
• Examples of gene mutation: Sickle cell anaemia

  Caused by a substitution mutation, which causes a major protein structural change
• Meiosis- a type of cell division which produces four, haploid daughter cells, which are all genetically different from each other and the parent cell
• Chromosome- a thread-like (linear) structure made of DNA and histone proteins, where hereditary information is physically passed from one generation to the next
• Chromatid- one of the two strands of a chromosome that are joined together by a single centromere prior to cell division
• Homologous chromosomes- (only related to meiosis) a pair of chromosomes (one maternal, one paternal) that have the same gene loci so determine the same features. Not necessarily identical, but capable of pairing during meiosis
• Stage 1 of meiosis:

  • Prophase 1- chromosomes condense, homologous chromosomes pair up. Crossing over occurs, sister chromatids of one chromosome are no longer identical.
  • Metaphase 1- homologous chromosomes line up along the middle of the cell, in random orientations (independent segregation).
  • Anaphase 1- homologous chromosomes move across from each other to opposite ends of the cell. Chromosome is still intact- two sister chromatids.
  • Telophase 1- two cells are created with half the number of chromosomes as the original cell.
• Stage 2 of meiosis:

  • Metaphase 2- chromosomes line up along the middle of the cell.
  • Anaphase 2- sister chromatids are separated and pulled towards opposite ends of the cell.
  • Telophase 2- four genetically different haploid cells are formed.
• Number of chromosomes in cells after the stages of meiosis:

  A) pre-meiosis

  B) 6

  C) 1

  D) 3

  E) 3

  F) 2

  G) 3

  H) 3

  I) 3

  J) 3

  
• Genetic variation can be caused in two ways through meiosis, which occur in prophase 1. As a result of crossing over, or independent segregation of chromosomes. It can also happen outside of meiosis through random fertilisation when gametes combine.
• Differences between meiosis and mitosis:

  A) meiosis

  B) mitosis

  C) two

  D) one

  E) four

  F) two

  G) crossing

  H) no

  I) crossing

  J) haploid

  K) diploid

  L) independent

  M) no

  N) independent

  O) different

  P) identical

  
• Chromosomal mutation example: Trisomy 21 (Down syndrome)

  • Where a person has three of the same homologous chromosomes, or one extra chromosome.
  • People with this mutation are sterile (cannot produce gametes)- due to not being able to do meiosis since chromosomes can't pair up
• Non-disjunction event- when chromosomes or chromatids fail to separate during the phases of meiosis
• If non-disjunction happens in meiosis 1, one pair of homologous chromosomes do not separate. If non-disjunction happens in meiosis 2, the homologous chromosomes will separate, but if it occurs in anaphase 2, the sister chromatids fail to separate- so there is a 50% chance that there will be a normal amount of chromosomes.
• Differences between chromosomal and gene mutations:

  A) chromosomal

  B) gene

  C) chromosome

  D) structure

  E) number

  F) DNA

  G) base

  H) sequence

  I) no

  J) frame-shift

  K) no

  L) non

  M) protein

  N) functional

  
• Haploid cells can have either an odd or even number of chromosomes, while diploid cells can only have even numbers of chromosomes, so they can form homologous pairs.