meiosis

type of cell division that produces four genetically different haploid cells - involves two cellular divisions - introduces genetic variation - produces gametes (sperm and egg) which are used in sexual reproduction

• one chromosome inherited from mother (maternal chromosome) • one chromosome inherited from father (paternal chromosome) • carry most of the same genes but might have different versions of those genes in some places

• dna replication takes place • this produces replicated chromosomes • each replicated chromosome is made up of two identical strands called sister chromatids • these are held together at a central point known as the centromere • at this point there would be 23 pairs of homologous chromosomes in a human diploid cell that would have all been replicated (46 in total)

prophase i metaphase i anaphase i telophase i

prophase ii metaphase ii anaphase ii telophase ii

prophase i - homologous chromosomes pair up and condense making them visible under a microscope - spindle fibres start to form as centroles move to opposite ends of the cell - nucleolus disappears while the nuclear envelope breaks down

• homologous chromosomes line up at the equator • each chromosome attaches to spindle fibres by centromere

• spindle fibres contract and shorten • which pulls homologous chromosomes to opposite poles of the cell • chromatids in each chromosome stayed joined together at this point

• chromosomes start to uncoil when they reach the poles • new nuclear envelopes form around each set of chromosomes • new nucleoli start to appear too • after telophase i, the first round of cytokinesis begins

• divides cytoplasm and forms two new cells • each new cell has half the number of chromosomes of the original • each of these chromosome still contains two chromatids

• in both new cells chromosomes condense again • nucleoli disappear • nuclear envelopes break down • new spindle fibres form as the centrioles move to opposite poles of each cell

• replicated chromosomes line up at the equator instead of the homologous pairs of replicated chromosomes we saw in metaphase i • attach to spindle fibres by centromeres

• centromeres divide • each replicated chromosomes split into two chromatids • as the spindle fibres contract and pull chromatids to opposite poles of the cell

• chromatids reach poles and uncoil • four new nuclear envelopes and nucleoli form so we now have four distinct nuclei

• cytoplasm of both cells divides • this forms four genetically different haploid cells each of which has half the number of chromosomes compared to the original cell

one difference is that mitosis is mainly used for growth and repair whereas meiosis produces gametes for sexual reproduction. another difference is that mitosis is associated with asexual reproduction whereas meiosis is associated with sexual reproduction. one difference is that mitosis involves just one cellular division which produces two diploid cells with two sets of chromosomes whereas meiosis involves two cellular divisions which produces four haploid cells with one set of chromosomes.

mitosis - does not introduce any genetic variation so the daughter cells are genetically identical to the parent cell. meiosis - does introduce genetic variation so the daughter cells are genetically different to the parent cell. mitosis - has one stage of anaphase where it only separates the sister chromatids in replicated chromosomes which does not reduce chromosome number. however during anaphase i, meiosis = separates homologous chromosomes and this reduces chromosome number by half. then during anaphase ii, meiosis also separates the sister chromatids in replicated chromosomes.

double as many as haploid 46 chromosomes (two complete sets of chromosomes) all cells other than gametes example: skin cell white blood cell fertilised egg cell - there can be varying numbers of chromosomes - sheep have 54 chromosomes in diploid cells

half as many as diploid 23 chromosomes (single set of chromosomes) - example is gametes (egg cells + sperm cells) - there can be varying numbers of chromosomes - sheep have 27 chromosomes in haploid cells

23

one chromosome originally came from the offspring’s mother, and the other chromosome originally came from the father.

testes ovaries

Reproductive cells

diploid