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Meiosis is a special type of cell division that is a reduction division. It results in the production of gametes or sex cells (sperms or eggs) with half the chromosome number that the organism contains in its body cells. The number of chromosomes in the gametes is called the haploid amount as compared to the diploid amount found in body cells. Meiosis occurs in gonads--sex organs which are testes in males and ovaries in females. It starts with a diploid body cell. The amount of chromosomes is replicated before meiotic division occurs. This results in paired chromosomes called chromatids. For example, if this were a human with a diploid number of chromosomes of 46, after replication, there would be 46 pairs of chromosomes. The cell goes through two rounds of cell division--prophase I, metaphase I, telophase I, anaphase I and then, a second round of division - prophase II, metaphase II, anaphase II and telophase II. The end result of meoisis is-- four haploid cells. It is important to note that during prophase I, homologous chromosomes form groups of four called tetrads. These will exchange sections of DNA with eachother in a process called crossing over during synapsis. This is much like shuffling a deck of cards and results in a rearrangement of genes. This produces sperms or eggs with much variation and explains why unless someone is an identical twin, your DNA is unique. Once meiosis is completed, in the case of females, only one cell becomes the ovum or egg, it is slightly larger than the other three cells, but contains half the chromosomes of the parent cell. The other three haploid but very small cells are called polar bodies. These are not used as gametes. In males, the end result is four haploid sperm cells. Both the human egg and sperm will have 23 chromosomes in each. Fertilization, restores the diploid amount of chromosomes (46) found in body cells. In sexually reproducing organisms, meiosis is the special cell division that forms haploid gametes. These haploid gametes preserve the species number of chromosomes in a particular organism, once they unite during fertilization.
Meiosis involves the formation of gametes or sex cells. Sex cells are sperm and egg cells. If you remember, sperm and eggs cells fuse together to form a zygote that will eventually diffentiate and divide to form a baby.
*Don't meiosis confused with mitosis because they are two totally separate processes! Meiosis only occurs in sex cells and mitosis occurs in stomatic cells (normal body cells). Mitosis purpose is to make more body cells so that we don't run out of cells! Mitosis creates 2 cells and meiosis creates 4 cells. Also, mitosis creates cells that are diploid (meaning each cell contains 46 chromosomes) and meiosis creates cells that are haploid (meaning each cell contains 23 chromosomes)
IMPORTANT: The primary purpose of mitosis is to create 4 sex cells with half the number of original number of chromosomes (haploid) that are used in the process of reproduction. You get one set of chromosomes from your mother and one set from your father (23 chromosomes from your father+23 chromosome from your mother= 46 chromosome total)
There are 8 steps and 2 phases in meiosis. The steps in the first phase are the same in the second phase. There are 4 steps to remember. The acryomn PMAT (similar to GMAT business school exam) can be used for memorization purposes.
The way I like to teach this concept is by using an analogy of a relationship between a boyfriend and a girlfriend.
The boyfriend and girlfriend are in a relationship they are preparing to live together and have a long happy life together (nucleus is expanding and nuclear membrane is braking down/disappearing). They become interconnected (crossing over/recombination of chromosomes)*and looking foward to their new life together
After sometime, their relationship starts to fall apart because they start to feel like they want to go their separate ways (formation of the microtubules in the centrosome and the microtubles attach to the kinetichore or centromere of homologous chromosomes + chromosomes line up in the middle of the cell called the metaphase plate)
The relationship does not work out so they decided to move out of their home. (pairs of homologous chromosomes are moved to opposite sides the cell with the help of the microtubules)
Their separation is finalized and they want nothing to do with each other anymore. They officially move into their own apartments. (cleavage furrow forms in the middle of the cell allow the cell to divide in half + chromsomes are on opposite poles of the cell). Cytokinesis is the process of dividing into two separate cells.
IMPORTANT: Key components of meiosis are crossing over, two phases, and creation of 4 haploid cells.
Note: At the end of meiosis I, there are 2 pairs of chromatids (2 chromatids= 1 chromosome) in each cell. At the end meiosis II, there is 1 chromatid in each cell.
*Recombination is the process where two homologous chromosomes exchange genetic information with each other. This process is responsible for genetic diversity.
I put a link to an animation of meiosis by McGraw Hill that explain the whole process if you are a visual and auditory learner. It's located in the reference section.
Meiosis is the replication of all genetic material without mixing and matching of genes and results in two diploid daughter cells.
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