meiosis

• one complete set of chromosomes in a cell (n=23)

• two complete sets of chromosomes in a cell (2n=46)

specialized cells called gametes (sperm cells and egg cells). they have a haploid number of chromosomes and are produced through meiosis

any cells in an organism excluding male and female gametes .they are diploid and are produced through mitosis

sex chromosomes(XX or XY)

the first 22 pairs of chromosomes which control the appearance ,structure and functionings of the body

• phases are the same

meiosis - occurs in sex organs - 4 daughter cells formed - crossing over occurs - karyokinesis and cytokinesis occurs mitosis - occurs in body cells - karyokinesis and cytokinesis occurs once - 2 daughter cells formed - crossing over does not occur

type of cell division used to produce gametes or sex cells

animals - occurs in the sex organs (ovaries and testes ) to produce gametes plants - mosses and spores produce spores used for reproduction - in flowering plants it takes place in the anther and in the ovule

• DNA replication takes place • chromosomes which are single threads become double • each chromosomes will now consist of two chromatids joined by a centromere • DNA replication helps to double the genetic material so that it can be shared by the new cells arising from cell division

it is caused by a non disjunction of chromosomes which results in the presence of an extra chromosome number 21

• production of haploid gametes • the halving effect of meiosis overcomes the doubling effect of fertilization thus maintaining a constant chromosome number from one generation to the next • mechanism to introduce genetic variation through crossing over and the random arrangement of chromosomes at the equator

in anaphase 1 and anaphase 2

• if there is non disjunction of chromosome pair 21 in humans , it leads to the formation of an abnormal gamete with an extra copy of chromosome 21 • if there is fusion between a normal gamete (with 23 chromosomes) and an abnormal gamete (with an extra copy of chromosome 21 ) ,it leads to down syndrome

• nuclear membrane and nucleolus disappears • centrosome splits and the two centrioles move apart forming spindle fibers • pairs of homologous chromosomes lie next to each other forming a bivalent • crossing over occurs .chromatids from each homologous chromosome overlap and touch each other at the chiasma • chromatid segments break off and are exchanged resulting in the exchange of genetic material

• homologous chromosomes move to the equator • they lie on opposite sides of the equator parallel to each other • this called random arrangement and brings about further variation • each chromosome in the homologous pair becomes attached to a spindle fiber

• one chromosome from each pair is pulled to opposite poles by contraction of the spindle fibers • this separates the homologous chromosomes ,one to each pole

• a nuclear membrane and nucleolus forms • cytokinesis (division of cytoplasm ) splits the mother cell into two daughter cells

• nuclear membrane and nucleolus disappears • centrosome splits into 2 centrioles and a spindle fiber forms

single chromosomes arrange themselves randomly along the equator - each chromosome becomes attached to a spindle fiber

• centromere splits separating each chromosome into 2 daughter chromosome each pulled to opposite poles

• a new nuclear membrane forms • cytokinesis splits the cells into 2 new cells • there will now be 4 daughter cells • these cells will be haploid and genetically different