Meiosis

Meiosis Overview
The process of meiosis involves two cell divisions and produces cells that are different from the parent cell - that is, meiosis produces cells that have one-half the number of chromosomes as the parent cells. Meiosis takes place during sexual reproduction when sex cells (gametes), called the egg and sperm, are produced in sex organs (gonads). A complex series of changes in the cell nuclei form sex cell nuclei that have one-half the number of chromosomes found in a species normal body cell. One-half the number of chromosomes is called the haploid (or n) number. In humans, the haploid number (n) is 23. When the egg and sperm unite during fertilization, the species normal chromosome number, called the **diploid** or **2n** number, is restored (brought back). The diploid number in humans is 46. Thus, // fertilization restores the species normal chromosome number that was halved in meiosis //(**n + n = 2n**). If meiosis did not take place, the fertilized egg would have double the chromosome number (2n + 2n = 4n). This would cause phusical abnormalities in the offspring. If this process continued, each generation of offspring would have double the chromosome number of its parents.

__**Some Terms to Know for Meiosis**__
//** Chromatin **// - what DNA is called when it is uncoiled and in thin strands //** Chromatid **// - each strand of a double stranded chromosome; chromatids are made when single stranded chromosomes replicate during interphase. //** Homologous Chromosomes **//- chromosomes that are similar in size, shape, and genetic content; have genes for the same traits, arranged in the same order; similar, but not identical. //** Gamete **// - a sex cell. Either a sperm or an ovum (egg). //** Fertilization **//- the fusion of sperm and ovum. //** Somatic Cells **//- body cells. These are diploid, or 2n. They have the full set of chromosomes for the species.
 * // Chromosome //** - long, thread-like structures located in the nucleus of the cell
 * // Crossing Over //**- when sister chromatids exchange genetic information; occurs during gamete formation (meiosis). Increases genetic variation.
 * // Nondisjunction //**- the failure of chromosomes to separate properly during Meiosis I. Results in gametes containing either too many chromosomes or too few chromosomes.
 * // Diploid //**- having 2 of each kind of chromosome (2n). The human diploid number is 46.
 * // Haploid //** - having 1 of each kind of chromosome (n). The human haploid number is 23.

__﻿A Look at Meiosis...[[image:Interphase.png align="right"]]__

 * //Interphase //**
 * Chromosomes coil and copy.
 * Cell prepares for division.


 * // Prophase I //**
 * Each pair of chromatids line up with its homologous pair. They become fastened at their cetromeres. Th[[image:Prophase_I.png align="right"]]is pairing process is called ** synapsis **.
 * Each group of four chromatids is called a **tetrad **.
 * **Crossing over **occurs. Nuclear membrane breaks down. Spindles form.
 * Tetrads move towards equator




 * //Metaphase I //**- centromeres of tetrads line up on equator of cell.

//** Anaphase I **//
 * Tetrads separate (** disjunction **). One set of chromatids moves to one side of cell, and the other set of c[[image:Anaphase_I.png align="right"]]hromatids moves to the opposite side of the cell.
 * The cluster of DNA around each pole will be haploid. (However, each chromosome is double stranded. The sister chromatids do not separate during meiosis I).

//** Telophase I/Cytokinesis **//
 * Cytoplasm divides, forming two new cells.
 * Each new cell is haploid.

//** Prophase II **//


 * Each daughter cell forms spindles and double-stranded chromosomes move towards the equator.


 * // Metaphase II //**
 * Centromeres line up on equator.
 * Spindles attach to centromeres.

//** Anaphase II **//
 * Centromeres divide.
 * Two chromatids separate and move towards opposite poles.


 * // Telophase II / Cytokinesis[[image:Telophase_II_and_Cytokinesis.png align="right"]] //**


 * Both daughter cells divide, forming four haploid cells.
 * Nuclear membrane reforms.