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