Meiosis and mutations

• When does meiosis happen?

  In sex organs to produce gametes
• Describe crossing over.

  All chromosomes are copied in interphase, they are connected by centromeres

  The two chromosomes wrap around each other and form a chiasmata and bivalent.

  Once a chiasmata is formed parts of DNA can be swapped due to DNA strands being complimentary

  These are now called recombiant chromosomes and can now exchange alleles. Making genetic variation
• What happens in the two stages of meiosis?

  Meiosis 1 - Homologus chromosomes are seperated

  Meiosis 2 - Sister chromatids are seperated
• Describe meiosis 1

  Prophase 1 - Chromosomes condense and become visible. They can now link and form a chiasmata. Nuclear envelope breaks down and the spindle fibres can form a apparatus.

  Metaphase 1 - Chromosomes align along the apparatus

  Anaphase 1 - Spindle fibres shorten and chromosomes are pulled to opposite poles of the cell. The chiasmata now breaks.

  Telophase 1 - Chromosomes now reach opposite poles, nuclear membrane reforms and chromosomes decondense

  Cytokenisis can now happen
• When do the cells from meiosis become a haploid?


  After telophase in meiosis 2
• Describe meiosis 2.

  Prophase 2 - Chromosomes condense and become visible again, nuclear envelope breaks down and spindle fibres develop and apparatus

  Metaphase 2 - Chromosomes align along the spindle apparatus

  Anaphase 2 - Centromere of each chromosome divides, spindle fibres shorten and chromosomes are now pulled to opposite poles

  Telophase 2 - Chromosomes reach poles of the cell, nuclear envelope can reform and chromosomes decondense.
• Why is meiosis called reduction division?

  Chromosome number halves
• What is independent segregation.

  Happens during metaphase 1, we cannot predict which chromosome moves into which cell.
• What is random fusion?

  We don't know which gamete will fuse to which egg
• What is a gene mutation?

  A change to the quantity, or base sequence, of DNA in an organism.
• Why not a gene result in a negative change to the organism?

  3 bases can code for more than one gene / amino acid. Making the code degenerate.
• What are the 3 main types of mutations?

  Substitution: A change in 1 base sequence

  Addition: An extra nucleotide is inserted in the sequence

  Deletion: One or more base is deleted
• What is the 3 possible consequences of a substitution mutation?

  Mis-sense mutation: Results in a different amino acid being coded for

  Non-sense mutation: Results in a stop codon being formed

  Silent mutation: Altered base still codes for the same amino acid.
• What are the 2 rarer types of mutation?

  Inversion: A portion of the sequence is inverted

  Duplication: A repeat of a whole triplet
• Why can a mutation be positive?

  A better performing protein is made

  Results in better reproductive success

  Aids in natural selection
• Why can a mutation be nuetral?

  Occurs in a phenotypically insignificant strand of DNA

  Does not change the polypeptide chain

  Does not change the shape of the protein
• Why can mutations be negative?

  Changes the shape of a protein

  Changes the polypeptide chain

  Gives an increased risk of disorders
• What are mutagenic agents and give examples.


  Agents that increase the rate of mutations.

  Examples: Bromine, x-rays, gamma rays and UV rays
• What is a chromosomal mutation and where does it occur in Down's syndrome?

  Chromosomal mutations occur on the chromosome and change either the whole set of chromosomes or the number of chromosomes.

  Down's syndrome happens on chromosome 21, creating 23 copies due to fusion.
• What is non-disjunction.

  A change in the original number of chromosomes
• What is polyploid?

  Two complete sets of chromosomes passing on, common in plants or wheat.