Cell Division: Meiosis

MEIOSIS gametes are formed through meiosis gametes - specialized reproductive cell involved in sexual reproduction (sperm or egg cells) meiosis occurs in sexual reproduction when a diploid cell produces 4 haploid daughter cells that can mature to become gametes (sperm or egg)

MEIOSIS homologous – description of chromosomes that occur in pairs; having a corresponding structure homologous - term used to refer to chromosomes that each have a corresponding chromosome from the opposite-sex parent homologous chromosomes are similar in shape, position & information, but not identical to each other.

MEIOSIS diploid (“two sets”) – description of a cell that contains a double set of chromosomes, one from each parent haploid - containing a single set of chromosomes

Chromosomes Human’s have 46 chromosomes 23 from your mother’s egg 23 from your father’s sperm “n” refers to the # of SETS of chromosomes A normal human cell is a diploid cell (2N) = 46 A sex cell or gamete (egg or sperm) is a haploid cell (N) = 23 Sperm (23) + Egg (23) = Zygote (46)

“Putting It All Together” - Fertilization

MEIOSIS meiosis – process of reduction division in which the # of chromosomes per cell is cut in half & the homologous chromosomes that exist in a diploid cell are separated; process that produces haploid gametes from diploid cells

Meiosis produces 4 genetically different haploid cells. Meiosis consists of 2 phases: Meiosis I Meiosis II To begin, you have one diploid cell In the end, you have four haploid cells

MEIOSIS Goal: reduce genetic material by half Why? n (mom) + n (dad) = 2n (offspring) from mom from dad child too much! meiosis reduces genetic content Just right!

MEIOSIS: CELL DIVISION IN 2 PARTS Sister chromatids separate Haploid Diploid Meiosis I (reduction division) Meiosis II Chromatids separate (equational division) 2n n n Result: 1 copy of each chromosome in a gamete

MEIOSIS I : REDUCTION DIVISION Prophase I (early) (diploid) (late) Metaphase I Anaphase I Telophase I Nucleus Spindle fibers Nuclear envelope

nuclear envelope fragments PROPHASE I early prophase chromatids pair crossing over occurs late prophase chromosomes condense spindle forms nuclear envelope fragments

RECOMBINATION During prophase I, each chromosome pairs with its homologous chromosome to form a tetrad tetrad - structure containing 4 chromatids that forms during meiosis

RECOMBINATION crossing-over - process by which homologous chromosomes exchange portions of their chromatids during meiosis crossing-over physical exchange of chromosomal (genetic) material between chromatids of homologous (sister) chromosomes Result: Generation of new combinations of genes (alleles).

RECOMBINATION occurs in prophase of meiosis I generates diversity Creates chromosomes with new combinations of alleles for genes A to F.

RECOMBINATION letters denote genes case denotes alleles A a a A B b b F F f f

RECOMBINATION A a A a alleles have crossed over to produce new gene combinations B b B b C C c c D D d d E E e e F F f f

METAPHASE I chromatid pairs align along equator of the cell

ANAPHASE I chromosomes separate & move to opposite poles sister chromatids remain attached at their centromeres

TELOPHASE I & CYTOKINESIS I nuclear envelopes reassemble spindle disappears cytokinesis divides cell into 2

MEIOSIS II Gene X sister chromatids carry identical genetic Meiosis II produces gametes with 1 copy of each chromosome & thus 1 copy of each gene neither cell replicates its chromosomes each cell splits (similar to mitosis) produces 4 daughter cells sister chromatids carry identical genetic information

MEIOSIS II : EQUATIONAL DIVISION Prophase II (haploid) Metaphase II Anaphase II Telophase II Four Non-identical haploid daughter cells

PROPHASE II nuclear envelope fragments spindle forms

METAPHASE II Chromosomes align along equator of cell

sister chromatids separate & move to opposite poles ANAPHASE II sister chromatids separate & move to opposite poles

TELOPHASE II & CYTOKINESIS II nuclear envelope assembles chromosomes unravel spindle disappears cytokinesis divides cell into 2

1 copy of each chromosome RESULTS OF MEIOSIS 4 haploid cells 1 copy of each chromosome

Comparing Mitosis and Meiosis Similar processes with very different & distinct outcomes Mitosis results in production of 2 genetically identical diploid cells (2N), whereas meiosis produces 4 genetically different haploid cells (N).

Mitosis vs. meiosis Meiosis KM

genetically identical? yes no MITOSIS MEIOSIS # of divisions 1 2 # of daughter cells 4 genetically identical? yes no chromosome # same as parent results in identical diploid (2n) daughter cells half of parent reduction division results in haploid (n) cells where somatic (body) cells only in cells in gonads (sex organs: ovary/testes) when throughout life at sexual maturity role growth and repair asexual reproduction produces gametes (sex cells: egg/sperm) sexual reproduction

MITOSIS MEIOSIS occurs in somatic (body) cells occurs only in gonads (sex organs: ovary/testes) produces cells for repair, maintenance, growth, asexual reproduction only produces gametes (sex cells: egg/sperm) results in identical diploid (2n) daughter cells reduction division results in haploid (n) cells

Meiosis KM