Unit 3 Part II – The Cell

 A form of cell division happening in sexually reproducing organisms.  Consists of 2 consecutive cell divisions (meiosis I and meiosis II).  Produces 4 sex cells.  Distributes maternal and paternal chromosomes evenly and randomly.

 Meiosis makes sex cells.  For women it creates egg cells in the ovaries.  For men it produces sperm cells in the testes.

 Meiosis is necessary to halve the number of chromosomes going into the sex cells.  Why halve the chromosomes in gametes? At fertilization the male and female sex cells will provide ½ of the chromosomes each – so the offspring has genes from both parents.

 Homologous Chromosome corresponding (matching) chromosomes from a female and male parent.  Diploid Cell (2n) – a cell that contains both sets of homologous chromosomes.  Haploid Cell (n) – a cell that contains one set of homologous chromosomes.  Gene – segment of DNA that codes for a trait.  Replicated Chromosome – a chromosome that has doubled it’s DNA.  Non-Replicated Chromosome – a chromosome that has split and contains only one copy of DNA.

1. Meiosis takes a cell with two copies of every chromosome (diploid) and makes cells with a single copy of every chromosome (haploid). 2. Meiosis scrambles the specific forms of each gene that each sex cell (egg or sperm) receives through crossing over and independent assortment.

 Same steps as Mitosis, but it divides twice to produce 4 daughter cells instead of 2.  Daughter cells are not identical but similar.  Meiosis I PMAT  2 similar cells  Meiosis II PMAT  4 similar cells

 Interphase Cells duplicate their DNA.  Prophase I sister chromatids pair up forming a homologous chromosome.  Crossing Over When two chromosomes exchange portions of their chromatids. Occurs during Prophase I.

 Metaphase I Spindle fibers attach to the centromeres on each chromosome.

 Anaphase I Spindle fibers pull the homologous chromosomes toward opposite ends of the cell.

 Telophase I  Meiosis I results in 2 haploid (n) daughter cells. Each with half the number of chromosomes as the original cell.  Still replicated chromosomes.

 Prophase II  2 haploid (n) daughter cells. Each with half the number of chromosomes as the original cell.

 Metaphase II  The chromosomes line up in a similar way to the metaphase stage of mitosis.

 Anaphase II  The sister chromatids separate and move toward opposite ends of the cell.

 Telophase II  Meiosis II results in four haploid (n) daughter cells.  Daughter cells are non-replicated.

OOGENESIS EGG PRODUCTION SPERMATOGENESIS SPERM PRODUCTION

 The haploid sperm (carrying 1 set of chromosomes) combines with the haploid egg (carrying 1 set of chromosomes).