Genetic Diversity - Mutation And Meiosis

• What causes genetic variation?

  Crossing of chromosomes

  Independent segregation of chromosomes ( Random which chromosome ends up in each daughter cell)

  Mutations

  Random fertilisation
• Differences between meiosis and mitosis

  Mitosis:

  Same number of chromosomes as parent cell

  Genetically identical daughter cells

  Produces two daughter cells

  
  

  Meiosis:

  Half the number of chromosomes as parent cell

  Genetically different daughter cells

  Produces four daughter cells
• Non-disjunction


  Failure of chromosomes to separate properly
• Define Gene Mutation


  = A change to one or more nucleotide bases/change to sequence of the bases in DNA (in a gene).
• How does gene mutation affect genetic diversity?

  Gene mutation involves a change in the base sequence of chromosomes. They can arise spontaneously during DNA replication and include base deletion and base substitution.
• Why don't all mutations affect the order of amino acids?

  Due to the degenerate nature of DNA
• Substitution


  When one base is substituted for another
• Deletion


  When one base is deleted
• Examples of mutagenic agents

  UV radiation

  Ionising radiation

  Some chemicals

  Some viruses
• Outline substitution mutations.

  Consequences?

  1. Nucleotide in a DNA molecule replaced by another nucleotide that has a different base.

  
  

  2. Can result in change of one amino acid in resulting polypeptide produced as one triplet codon is changed
• Why do some substitution mutations not lead to different polypeptides being produced?

  - Genetic code is degenerate - not all base substitutions cause a change in the sequence of encoded amino acids

  
  

  - If new base triplet codes for same amino acid as the initial triplet => silent mutation.

  
  

  - Mutation will have no effect on final polypeptide produced.
• Outline deletion mutations.

  Consequences?

  1. Nucleotide (and base) lost from normal DNA sequence.

  
  

  => frameshift mutation ---> when n bases deleted where n is not wholly divisible by 3.

  
  

  => amino acid sequence of polypeptide is completely different ---> produced protein is unlikely to function properly.

  
  

  => all triplets in sequence read differently as each has been shifted to the left by one base.
• Define Mutagenic Agent


  = A chemical agent which increases the rate of gene mutation
• List examples of mutagenic agents

  1. UV light.

  2. Ionising radiation.

  3. Chemicals.

  4. Some viruses.
• Define chromosome mutations

  = A change in the number/structure of whole chromosomes.
• Outline polyploidy chromosome mutations

  = Change in whole sets of chromosomes.

  
  

  - Occurs when organisms have three or more sets of homologous pairs of chromosomes rather than the usual 2.
• Outline non-disjunction chromosome mutations


  = Change in the number of individual chromosomes.

  
  

  = Gamete has +/- 1 of a particular chromosome.

  
  

  - Occurs when individual homologous pairs of chromosomes fail to separate during meiosis.
• How does meiosis differ from mitosis?

  Meiosis = 4 genetically different haploid daughter cells.

  Mitosis = 2 genetically identical diploid daughter cells.
• What happens pre-meiosis?

  INTERPHASE: ach chromosome replicates to form a new genetically identical chromatid. The sister chromatids are held together by the centromere.
• Outline the process of Meiosis I

  1. Homologous chromosomes pair up and their chromatids wrap around each other.

  
  

  2. Equivalent portions of chromatids can be exchanged in crossing-over.

  
  

  3. Homologous chromosomes separate, with one chromosome from each pair going to each of two daughter cells.
• Outline the process of Meiosis II

  1. Chromatids are separated by a further division into 4 cells, each with a single chromatid.
• Outline a diagram showing the chromosome content of the cells after the first and second meiotic division

  
• Why do we half the chromosome number in meiosis?

  1. To produce haploid gametes ---> so zygote and offspring formed have the correct diploid number of chromosomes as haploid gametes from mother and father fuse together, restoring diploid number of chromosomes.

  
  

  2. Ensures one allele from each of mother and father for each gene.

  
  

  => maintains a constant number of chromosomes in adult species.
• Define Reduction Division


  = The number of chromosomes in each gamete is reduced to half of the original number

  
  

  Diploid --> Haploid
• What factors of meiosis will lead to genetic variation?

  1. Independent segregation of homologous chromosomes.

  
  

  2. Genetic combination by crossing over.

  
  

  3. Haploid gametes fuse randomly at fertilisation

  ---> produces more genetic variation in offspring.

  
  

  4. Gametes usually come from different parents

  ---> 2 different genetic makeups combined so even more variety as a result.
• How does crossing over lead to more genetic variation?

  1. Each chromosome lines up alongside homologous partner in meiosis I.

  
  

  2. Chromatids of each pair twist around each other.

  
  

  3. Tensions created (due to twisting) + portions of chromatids break off.

  
  

  4. Broken portions might then rejoin with the chromatids of its homologous partner => recombination.

  
  

  5. Usually equivalent portions (same genes) of homologous chromosomes that are exchanged.

  
  

  6. New genetic combinations of maternal and paternal alleles produced.
• Why is crossing over so important?

  Without crossing over and recombination, only 2 genetically different types of daughter cell would be produced => crossing over increases genetic variation even further.