Meiosis

MeiosisUnit 3 Part II The CellWhat is MeiosisA 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.Where does Meiosis occurMeiosis makes sex cells.

For women it creates egg cells in the ovaries.

For men it produces sperm cells in the testes.Why do we need Meiosis?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 of the chromosomes each so the offspring has genes from both parents.

Terms to KnowHomologous 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 its DNA.

Non-Replicated Chromosome a chromosome that has split and contains only one copy of DNA.What does Meiosis accomplish?Meiosis takes a cell with two copies of every chromosome (diploid) and makes cells with a single copy of every chromosome (haploid).

Meiosis scrambles the specific forms of each gene that each sex cell (egg or sperm) receives through crossing over and independent assortment.

Steps in MeiosisSame steps as Mitosis, but it divides twice to produce 4 daughter cells instead of 2.

Daughter cells are not identical but similar.

Meiosis IPMAT2 similar cells

Meiosis IIPMAT4 similar cells

Interphase/Prophase IInterphaseCells duplicate their DNA.

Prophase I sister chromatids pair up forming a homologous chromosome.

Crossing OverWhen two chromosomes exchange portions of their chromatids.Occurs during Prophase I.

Metaphase IMetaphase ISpindle fibers attach to the centromeres on each chromosome.

Anaphase IAnaphase ISpindle fibers pull the homologous chromosomes toward opposite ends of the cell.

Telophase ITelophase 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 IIProphase II 2 haploid (n) daughter cells. Each with half the number of chromosomes as the original cell.

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

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

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

Female vs. MaleOogenesis egg productionSpermatogenesisSperm Production

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