Mutations and Meiosis

• What is a gene mutation?


  Any change to one or more nucleotide bases.

  They can occur spontaneously during DNA replication.

  If they occur during the formation of gametes, they may be inherited.
• What are the 3 types of mutations?


  Substitution, Insertion and Deletion.
• What is substitution?


  A nucleotide in a DNA molecule is replaced by another nucleotide that has a different base e.g A is changed to G.
• How can substitution lead to a non-functional protein?


  If the substituted base leads to a different triplet this leads to a different codon on mRNA. this leads to a different amino acid being produced which can cause a non-functional protein.
• What is Deletion?


  When a nucleotide is lost from the normal DNA sequence, resulting in frameshift.
• What is frameshift? and why can it cause a polypeptide to not function correctly?

  The entire sequence after deletion is changed and shifted to the left by one base.

  All the triplets will be read differently so it is unlikely the polypeptide will function.
• When does polyploidy happen?

  When gametes don`t successfully reduce the number of chromosomes during meiosis.
• What is haploid?


  One set of chromosomes e.g sex cells
• What is diploid? 


  2 sets of chromosomes
• What is polyploid?


  More than 2 sets of chromosomes.
• What is non-disjunction?

  Individual chromosomes can fail to separate during meiosis so this causes one gamete to have more or less of specific chromosomes. (this can lead to down syndrome)
• What is the difference between meiosis and mitosis?

  Meiosis is sexual reproduction used to make gametes compared to mitosis which is Asexual reproduction which is used for replacing normal body cells.

  Meiosis has 23 chromosomes after cell division and mitosis has 46.

  Meiosis produces 4 daughter cells and has 2 cell divisions but mitosis

  has 1 cell division and produces 2 daughter cells.

  Meiosis causes variation.
• What 2 things cause genetic variation in meiosis?


  Independent segregation and crossing over.
• What is crossing over/recombination?

  This happens during prophase 1 of meiosis 1 and chromosomes twist around each other and swap portions over.

  The same genes swap but this can result in different alleles.
• What is independent segregation?

  In meiosis 1, 23 homologous chromosomes line up side by side randomly .

  This means there are various combinations you can have creating variation.
• What is Meiosis 1?


  The first division (the process of crossing over and independent segregation) At the end of the process homologous chromosomes have separated into different cells.
• What is Meiosis 2?


  The second division (the same process repeats with half the number of chromosomes).

  Chromatids move apart into different cells.
• What happens in Interphase?(meiosis 1)

  DNA unravels and replicates making double armed chromosomes.
• What happens in prophase?(meiosis 1)

  Homologous chromosomes pair up,

  Crossing over occurs,

  Centrioles start moving to opposite ends of the cell forming the spindle fibers,

  Nuclear envelope breaks down.
• What happens in metaphase?(meiosis 1)

  Homologous pairs line up across the centre of the cell and attach to spindle fibers by their centromere.
• What happens in anaphase?(meiosis 1)


  Spindle fibers contract and pairs of chromosomes are pulled apart to opposite ends of the cell.
• What happens in Telophase?(meiosis 1)

  A Nuclear envelope forms around each group of chromosomes,

  cytokinesis (division of cytoplasm),

  2 genetically different haploid daughter cells are made.