Reduction of Chromosomes

Mitosis Cell duplication (or reproduction) where one cell creates two genetically identical daughter cells Cellular reproduction, growth and repair Occurs in all organisms Daughter cells are genetically identical There is one division Same number of chromosomes (2n) Steps: Interphase, Prophase, Metaphase, Anaphase and Telophase Cytokinesis in Telophase Centromeres split only once during Anaphase Division of a gonad (germ cell) with TWO nuclear fissions and creating four gametes with half the number of chromosomes Sexual reproduction Occurs in humans (animals), plants and fungi Daughter cells are genetically different There are TWO divisions Half the number of chromosomes Steps: Interphase, Prophase I, Metaphase I, Anaphase I, Telophase I, Prophase II, Metaphase II, Anaphase II and Telophase II Cytokinesis in Telophase I and II Centromeres split during Anaphase II, not Anaphase I Meiosis

Interphase: -the cell replicates its chromosomes -each chromosome has two sister chromatids held together by a centromere

-chromosomes coil up and a spindle forms -homologous chromosomes come together matched gene by gene forming a tetrad Crossing Over may occur when chromatids exchange genetic material -this occurs two or three times per pair of homologous chromosomes Crossing Over results in new combinations of alleles on a chromosomes

the centromere of each chromosome becomes attached to a spindle fiber the spindle fibers pull the tetrads to the equator of the spindle homologous chromosomes are lined up side by side as tetrads

homologous chromosomes separate and move to opposite ends of the cell centromeres do not split this ensures that each new cell will receive only one chromosome from each homologous pair

the spindle breaks down and the chromosomes uncoil the cytoplasm divides to yield two new cells each cell has half the genetic information of the original cell because it has only one homologous chromosome from each pair

a spindle forms in each of the two new cells and the fibers attach to the chromosomes

the chromosomes are pulled to the center of the cell and line up randomly at the equator

the centromere of each chromosome splits the sister chromatids separate and move to opposite poles

-nuclei re-form the spindles break down the cytoplasm divides identical to mitosis (Meiosis II)

four haploid sex cells (n) from one original diploid cell (2n) each haploid cell contains one chromosome from each homologous pair one from Mom and one from Dad haploid cells will become gametes transmitting genes to offspring