CELL DIVISION mitosis & meiosis

Forms of DNA When the cell is NOT dividing = chromatin When the cell IS dividing = chromosome Monovalent chromosome Bivalent chromosome

Why do cells divide? Replacement - cells wear out & die Repair - tissues can be damaged by injury or infection Growth of the organism Reproduction

Types of cell division Mitosis New cells are identical to the old cells Chromosome number is the same in parent & daughter cells (diploid -2n) Growth, repair, & asexual reproduction

MITOSIS

Meiosis Parent cell divides twice to produce 4 cells Produces gametes (sex cells –sperm/eggs) Daughter cells have half the original number of chromosomes (haploid -1n)

4.When two gametes join to form a zygote, it will have the normal # of chromosomes

MEIOSIS

5. Daughter cells are not Genetically identical a.) Homologous chromosomes separate, sending one version of each to the new cells. b.) Crossing –Over: sometimes chromosomal segements are exchanged between Homologous Chromosomes.

Phases of Mitosis Interphase Growth & normal cell activities DNA = chromatin *as the cell prepares for division, DNA is duplicated

Prophase Chromatin thickens & reveals duplicated chromosomes (bivalent) Centrioles move to poles Nuclear membrane dissolves

Metaphase Bivalent chromosomes line up along equator of cell Chromosomes attach to spindle fibers

Anaphase Spindle fibers pull monovalent chromosomes to opposite ends of cell **an identical set of chromosomes moves to each pole

Telophase Chromosomes uncoil & become chromatin again – Nuc. Memb Reforms Cytokinesis: cell membrane pinches in to form 2 separate cells

Phases of Meiosis Cells go through two rounds of division during meiosis, resulting in 4 haploid cells

Meiosis I & II Section 11-4 Diff. genes Same genes