Meiosis

• How many divisions are there in meiosis

  2
• What is the number of chromosomes determined by

  The number of centromeres
• What happens in meiosis 1

  Homologous pairs of chromosomes are separated
• What happens in meiosis 2

  Pairs of chromatids are separated
• What 3 things cause variation in meiosis

  Independent segregation

  Crossing over

  Random fertilisation
• What happens in prophase 1

  Chromosomes supercoil

  Nuclear envelope disappears

  Centrioles cast spindle fibre

  Crossing over occurs
• What is crossing over

  An exchange of DNA between chromatids on homologous chromosomes
• how does crossing over occur

  Homologous chromosome line up and twist around eachother

  Parts of chromatids break off at the same point resulting in swapping

  Same genes are formed in the end but in different locations
• What does crossing over allow

  Allows both ‘grand‘ parents to pass some genes each to the offspring
• What does bivalent or letrad mean

  The two homologous chromosomes when they wrap together
• What is chiasma 


  Point of contact between two homologous chromosomes
• What happens during metaphase 1

  Chromosomes line up at the equator

  Spindle fibre attaches to centromere of each chromosome

  Each chromosome lies next to its homologous partner at the equator

  Within the homologous pair, the chromosomes are randomly organised on either side of the equator
• What is independent segregation

  When the chromosomes are randomly organised on either side of the equator
• What does independent segregation mean will be the result of the cells

  4 daughter cells have completely different combinations of chromosomes
• What happens during anaphase 1

  Involves the separation of homologous chromosomes to opposite poles

  1 chromosome from the homologous pair is pulled to opposite poles

  This is a random process
• What happens during telophase 1

  Involves reforming of the nuclear envelope

  Animal cells reform a nucleus for a short period after anaphase 1 in telophase 1
• What happens in cytokinesis 1 for animal cells

  Undergo cytoplasm splitting in cytokinesis
• What happens to plant cells during cytokinesis 1

  Plant cells go directly into the next stage of meiosis without splitting
• What happens during prophase 2

  The DNA supercoils, the nucleus disintegrates and new spindle fibres form

  This is the same as mitosis, except they are no longer identical
• What happens in metaphase 2

  the chromosomes line up at the centre through the attachment of spindle fibres to the centromeres

  Chromosomes are randomly assorted on either side of the equator
• What happens in anaphase 2

  Genetically different chromatids are independently segregated to opposite poles
• What happens in telophase 2

  The nuclei start to reform and the cell splits in cytokinesis
• What happens to animal cells in cytokinesis 2


  The two cells split in two, forming 4 cells
• What happens to plant cells during cytokinesis 2

  Nuclei aren’t separated by plasma membrane yet, so the cell splits in 4 ways to make 4 cells
• What does meiosis produce

  4 genetically similar haploid gametes
• What can go wrong in mitosis and meiosis

  Chromosomes might fail to separate
• What is chromosomes failing to separate known as 


  non - disjunction
• What does non-disjunction result in

  Cells with an abnormal number of chromosomes, potentially leading to genetic disorders