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The Wonderful World of Meiosis!


Welcome Mr. Krummel to Crystal’s Biology (Meiosis) Web Site!






Why do we need meiosis?

If our cells did not use meiosis both of our parents would give us a full set of chromosomes. We would have twice the number of Chromosomes needed and our children would have double that amount. Over time we would not even be considered human anymore.


PHASES OF MEIOSIS!


Interphase



Prior to meiosis, all the chromosomes within the cell are duplicated, forming homologous chromosomes. However the chromosomes are not the only things that are duplicated in an animal cell. Located outside the nucleus of an animal cell is a centrosome, containing a pair of centrioles Two centrosomes are produced by the duplication of a single centrosome during interphase.



Prophase I



During prophase I all of the duplicated chromosomes coil up and become shorter and thicker, they then become visible under the light microscope.

The nucleus breaks down and the nuclear membrane disappears. In the cytoplasm the meiotic spindle forms between the two pairs of centrioles and the spindles migrate to opposite poles of the cell.

Also during prophase I a process called synapsis occurs. During synapsis each pair of homologous chromosomes come together forming a four-part structure called a tetrad.



After synapsis crossing over may occur. During crossing over the chromatids in each tetrad pair, produced during synapsis, are held tightly together. The arms of the non-sister chromatids then can wind around each other, and genetic information may be exchanged.

Prophase I is the longest phase of meiosis, usually consuming 90% of the time for the two divisions.



Metaphase I



In Metaphase I the tetrads are as tightly coiled and condensed as they ever will be in meiosis.

During metaphase I the tetrads line up along the equator of the spindle. The spindle fibers from one pole of the cell attach to one chromosome of each pair as the spindle fibers from the opposite pole do the same to the rest of the chromosomes.



Anaphase I



Anaphase I begins when the two chromosomes of each tetrad separate and start moving toward opposite poles of the cell. The sister chromatids remain attached at their centromeres and move together toward the poles, this ensures us that each new cell will contain only one chromosome from each homologous pair.



Telophase I



The events of telophase I occur in reverse order of the events in prophase I. The homologous chromosome pairs have completed their migration to the two poles. Now a haploid set of chromosomes is at each pole, with each chromosome still having two chromatids. The nuclear envelope reforms around each chromosome set, the spindle disappears, and cytokinesis follows.

In animal cells, cytokinesis involves the formation of a cleavage furrow, resulting in the pinching of the cell into two cells. After cytokinesis, each of the two cells has a nucleus with a haploid set of replicated chromosomes.



Prophase II



While chromosome duplication took place prior to meiosis I, no new chromosome replication occurs before meiosis II.

During prophase II the centrioles duplicate and the two pairs of centrioles separate into two centrosomes. Then the nuclear envelope breaks down and a spindle forms in each new cell.



Metaphase II



The chromosomes become arranged on the metaphase plate (the chromosomes are still made up of sister chromatids) and become attached to the now fully formed spindle.



Anaphase II



During anaphase II the centromeres separate and the two chromatids of each chromosome move to opposite poles on the spindle.



Telophase II



In telophase II the nuclear envelope forms around each set of chromosomes, the spindle breaks down, and the cytoplasm divides. Cytokinesis also occurs, producing four daughter cells, each with a haploid set of chromosomes.



Genetic Variation

One way meiosis generates genetic variability is through the different ways in which chromosomes are combined in the daughter cells.



The number of possible chromosome combinations in the haploid nuclei, produced during meiosis, is potentially very large. The number of possible chromosome combinations is represented by 2n, where n is the number of chromosome pairs.

For example: fruit flies have 4 chromosome pairs. When the number of possible chromosome combinations is 2n you would substitute the n with the 4. The answer would then come out to be 16 possible arrangements.

Another example: In humans there are 23 chromosome pairs. With the equation 2n, we would put 23 in place of the n and there are over 8 million possible arrangements.



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