In the previous tutorial, you learned about the basics of mitosis. Remember, mitosis results in cells that are genetically identical to the original parent cell. This short video will review the basics of mitosis:
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Diploid organisms have two sets of chromosomes, one set inherited from the father and the second set from the mother. Thus, each diploid individual has a duplicate set of each chromosome. A pair of duplicate chromosomes is known as a homologous pair. Each generation, in the production of sperm and eggs, males and females reduce their chromosome numbers in half (to the haploid number) but must ensure that each gamete has a complete set of half of the chromosomes.
This video introduces the process of meiosis and shows how the chromosomes move into the 4 resulting daughter cells in an organized way that results in genetic variation:
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The primary function of meiosis in diploid organisms is the proper distribution of homologous chromosomes into gametes. Meiosis has three phases, (1) pairing of homologous chromosomes, (2) synapsis of the chromosomes together (resulting in crossing over), and (3) segregation of the chromosomes into daughter cells. As shown in the image below, a diploid cell that goes through meiosis produces 4 haploid daughter cells. Just as in mitosis, before the cell goes through meiosis, the chromosomes must replicate themselves (resulting in sister chromatids and nonsister chromatids ). Replication is followed by two divisions called meiosis I and meiosis II. At the conclusion of meiosis, four daughter cells are formed, each with one-half the normal number of chromosomes. One consequence of meiosis is that new genetic diversity arises due to two factors: the independent assortment of homologous chromosomes to gametes, and crossing over (recombination) in prophase I of meiosis.
The first source of genetic variation, independent assortment , occurs in metaphase I, when the homologous chromosomes line up on the metaphase plate. Orientation of the pairs is random; therefore, which chromosome (maternal or paternal) goes to each daughter cell is random. A particular daughter cell has a 50% chance of getting either the maternal or paternal chromosome for each homologous pair. The image illustrates two pairs of homologous chromosomes and how their random alignment at the metaphase plate can lead to different combinations of chromosomes in the daughter cells that are produced. The chromosome pairs align independently of one another. Therefore, the number of different combinations of chromosomes in the daughter cells is equal to 2n , where n = the haploid number of chromosomes (or the number of chromosome pairs). In this example there are two chromosomes, so there are 22 = 4 different chromosome combinations in the daughter cells. If we look at the whole human genetic complement (n = 23), the number of different combinations is 223 = 8,388,608. Due only to the independent assortment of chromosomes in meiosis I, over 8 million genetically distinct gametes can be produced.
To calculate the number of chromosome combination possibilities use the number of chromosome pairs, not the total number of chromosomes.
A second source of genetic variation during meiosis is the exchange of genetic material between the maternal and paternal chromosomes, a process called crossing over or recombination . While the homologous chromosomes are paired together in prophase I, pieces of one chromosome may be exchanged with the identical portion of the other chromosome. This means that the resulting chromosomes are not entirely maternal or paternal, but rather a mixture of both. In humans, crossing over occurs about 2-3 times per chromosome pair, between nonsister chromatids only (not between sister chromatids).