New York State Chemistry RegentsNew York State Earth Science Regents

 
Meiosis Notes PDF Print E-mail
Heredity and Genetics
Interphase- replication of DNA

Meiosis - A type of cell division that gives rise to four reproductive cells (gametes) each with half the chromosome number of the parent cell; parent cell = 2N, daughter cell = N. The end results of meiosis in males and females differ in the fact that males have four sperm and females have one ovum (egg) and three polar bodies.
Meiosis I, Prophase I – The chromatin begins to coil into short rods, and homologous chromosomes are formed (synapsis). Paired chromosomes are called bivalents or tetrads- two chromosomes and four chromatids. Chiasmata are caused by genetic recombination. The spindle appears and the nucleoli break down. By the end of prophase I, the nuclear membrane (envelope) has completely dissolved and the tetrads are visible and have lined up along the equator of the cell. One kinetochore forms per chromosome rather than one per chromatid, and the chromosomes attached to spindle fibers begin to move.
Metaphase I – Kinetchore of each tetrad becomes attached to the microtubules of the spindle fibers. Bivalents/tetrads align at the metaphase plate. Orientation is random.
Anaphase I – The homologous chromosomes that form each tetrad are pulled apart (chiasmata separate). One pair goes to one end of the cell, and the other pair moves to the other. Each of the daughter cells is now haploid (23 chromosomes), but each chromosome has two chromatids.
Telophase I –The chromosomes reach the ends of the cell. Nuclear envelope reforms if cell does not directly go into Meiosis II. Cytokinesis (cytoplasmic division) takes place and the cell divides into two daughter cells.

At the end of meiosis I, each daughter cell contains half the number of chromosomes found in the parent cell. One chromosome of each homologous pair is present in each daughter cell. Meiosis I is reductive division. It reduces the number of chromosomes from the diploid (2n) to the haploid(n)
Meiosis II – Each daughter cell produced in meiosis I undergoes another nuclear and cytoplasmic division in meiosis II. Meiosis II is similar to mitosis but is not preceded by the replication of DNA. Chromatids of each chromosome are no longer identical because of recombination. Meiosis II also has four stages which produces two daughter cells each with 23 chromosomes (n) and each chromosome has only one chromatid.

Prophase II – Telophase I leads directly into prophase II. Dissolves nuclear envelope (if it reformed) A new spindle forms around the chromosomes.
Metaphase II –spindles move chromosomes along equator area and attaching to the opposite sides of the centromeres in the kinetochore region.
Anaphase II – The centromeres divide and the chromatids separate to opposite sides of the cell. Each chromatid now becomes a new chromosome with its own centromere.
Telophase II – The nuclear membrane forms around each set of chromosomes that gather into nuclei. The spindle breaks down and the cell undergoes cytokinesis.

Each of the daughter cells formed in meiosis I has divided in two, resulting in a total of four daughter cells produced in meiosis II. Each of the daughter cells produced in meiosis II is haploid (N)
 

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-8 #1 michael cuanico 2010-02-23 17:52
this is a test comment
 

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