meiosis

cell division of sex cells

a complete set of chromosomes arranged according to shape size and number in a organism

in the gonads (reproductive cells) , testes in the male and ovaries in the female

gametes [ gametes are formed through the process gametogenesis] in the male sperm cells are produced, through the process spermatogenesis and in the female egg cells are produced , through the process of oogenesis

spores

in the anthers to produce pollen grains in the male and in the ovary to produce ovules in the female

DNA replication takes place where the chromosomes duplicate and the genetic material is doubled the two copies of chromosomes are called homologous pairs

a reduction division where the diploid cell is separated to create 2 haploid cells.

• the spindle is formed • the chromatin network contracts and the single threaded chromosomes become visible • the nuclear membrane and and nucleolus disappear • the chromosomes arrange themselves and pair homologously • the homologous chromosomes lie next to each other and at the chiasma ( a point along the chromatid) the chromatids cross over and genetic material is exchanged

• the spindle extends across the whole cell • the homologous pair arrange themselves at the equator • the spindle fibers attach to the centrosome of the homologous pairs

the spindle contracts and the homologous pairs separate and they move tp the opposite ends of the cells poles

the spindle disappears the nucleus membrane and nucleolus form the cytoplasm divides thus forming 2 haploid cells with different genetic materials due to crossing over

• the 2 daughter cells from meiosis 1 divide again - the centromeres of these daughter cells separate resulting in 4 haploid cells

• nucleus membrane and nucleolus disappear • spindle threads form • the daughter cells are now visible as chromatids

• the spindle threads extends across the cell • the sister chromatids arrange themselves at the equator • the spindles attach to the centromeres of the daughter cells

the spindle fibres contract ,then separating the sister chromatids and pulling them to the opposite poles of the cell

-the spindles disappear -the nucleus membrane and nucleolus reform -the cytoplasm divides thus resulting in 4 haploid daughter cells the new daughter cells are genetically different from each other

set of genes in a organisms haploid set of chromosomes

• it ensures genetic variation • it prevents the doubling effect of fertilisation • it results in the formation of mature gametes in the process of gametogenesis

process whereby gametes are formed

• at the equator each homologous chromosome pair is randomly arranged , which results in the random separation of chromosomes • the paternal and maternal chromosomes randomly move to opposite poles of the cell, this ensures that the combination of genetic material received by each gamete is random

crossing over results in the exchange of genetic material between the paternal and maternal homologous chromosome pairs , a new genetic combination occurs between the exchanged chromosomes , this ensures that all the gametes have a unique gene combination so when the gametes of the same parents come together for fertilisation the resultant offspring will have a unique karyotype thus ensuring variation in the offspring

when one pair of chromosomes does not separate and the one cell receives two copies of chromosomes and the other cell doesnt

it happens during anaphase 1 when the homologous chromosomes do not separate or in anaphase when the sister chromatids do not separate