sexual reproduction and meiosis#

• gametes


  are the sperm cells in men and the egg cells in women

  in sexual reproduction two gametes fuse together to form a zygote

  this divides and develops into a new organism

  gametes have a haploid number of chromosomes there's only one copy of each chromosome
• sexual reproduction


  normal body cells have the diploid number of of chromosomes

  meaning each cell contains two of each chromosome (a pair) one from each parent

  the chromosomes that make up each pair are the same size and have the same genes, although they could have different versions of those genes (called alleles) these pairs of matching chromosomes are called homologous chromosomes
• meiosis


  is a type of cell divisions that happens in the reproductive organs to produce gametes

  cells that divide by meiosis are diploid to start, but the cells that form are haploid - the chromosome number halved
• meiosis - interphase


  the whole of meiosis begins with interphase

  during interphase, the cell's DNA unravels and replicates to produce double-armed chromosomes called sister chromatids
• meiosis 1


  meiosis involves two divisions

  after interphase, the cells enter meiosis 1

  it is the reduction phase (it halves the chromosome number)
• meiosis - prophase 1

  the chromosomes condense, getting shorter and fatter

  homologous chromosomes pair up - number 1 with 1 and etc

  crossing-over occurs

  just like in mitosis, centrioles start moving to opposite ends of the cell, forming the spindle fibres

  the nuclear envelope breaks down
• meiosis - metaphase 1 


  the homologous pairs line up across the centre of the cell and attach to the spindle fibres by their centromeres
• meiosis - anaphase 1


  the spindles contract, pulling the pairs apart (one chromosome goes to each end of the cell)
• meiosis - telophase 1


  a nuclear envelope forms around each group of chromosomes

  cytokinesis occurs and two haploid daughter cells are produced
• meiosis 2


  the two daughter cells go through prophase 2, metaphase 2, anaphase 2, telophase 2 (and cytokinesis)

  pretty much the same as the stages in meiosis 1, except with half the number of chromosomes

  in anaphase 2 the sister chromatids are separated

  each new daughter cell inherits one chromatid from each chromosomes

  four haploid daughter cells are produced
• genetic variation - meiosis 


  is the differences that exist between individuals' genetic material

  the reason meiosis is important is that it creates genetic variation - it makes gametes that are all genetically different

  then during fertilisation any egg can fuse with any sperm which also creates variation

  means individuals are genetically unique