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Before studying mitosis and meiosis, first become familiar with all the associated terminology. Before mitosis and cell division begins, the DNA must make a copy of itself (replication). This process occurs during Interphase, a stage of normal cell metabolism, when the cell is not involved in mitosis or meiosis. The two sides of the DNA double helix open up and each strand produces its complementary strand. This produces chromosomes with two identical chromatids. As mitosis and/or meiosis begin, the DNA strands of chromatin begin twisting and winding around histones (protein). The diagram below illustrates the levels of organization brought about by the coiling of the DNA.
Located at some point along each chromosome is a centromere, a point of attachment of each chromatid. Note that an even number of chromosomes exists in all diploid (2N) organisms, because we each receive one of each chromosome (23 pairs in humans) from our father and mother. These are homologous pairs, because each partner of a pair contains similar genetic material for specific physical, chemical, or behavioral characteristics.
Below is an overview of mitosis, the process that produces body cells. The drawings and photos below are of plant cells. Animal cells divide (cytokinesis) by pinching into two cells (cleavage), rather than forming a cell plate (which forms new cell walls) in the daughter cells. Animals also have centrioles, which form asters and the spindle apparatus.
Below are details of each phase, which you must know in order to identify stages of mitosis or meiosis. Prophase is the first visible evidence of cell division.
Metaphase, the time when chromosomes line up along the 'equator' of the cell.
A view of metaphase in a plant; note the absence of centrioles.
Anaphase, which separates the sister chromatids.
Telophase is the last phase of division, and the cell begins to divide by cytokinesis.
Below are stages of meiosis, which produces sex cells in higher organisms. The photos below are of the formation of pollen grains, the male gametes of flowering plants. Pollen fertilizes ova, which develop into seeds within a fruit of plants. You must observe the drawings and photos and study your text very thoroughly in order to understand and recognize the differences between mitosis and meiosis. In meiosis, two cell divisions are required before the gametes (usually sperm and ova) contain one set of single-stranded chromosomes. In the process of fertilization, the two haploid (1 N) cells fuse to form a diploid (2N) zygote (fertilized egg). During Interphase the DNA is replicated, as in mitosis. In the drawings below 2N = 4, and N = 2. There are approximately 20-22 chromosomes in the photos.
Meiosis I is often called the reduction division, since the diploid number of chromosomes is reduced to two haploid cells. This is possible, because the homologous chromosomes synapse (pair up) in Prophase I. During this time (which is divided into five "sub phases"), crossing over occurs. In Metaphase I, the homologous chromosomes line up by pairs.
Chiasmata (from the Greek letter Chi, which is written as X) are regions where homologous chromosomes cross one another and often break, exchanging parts with their homologue). This process insures even more genetic variability that can occur with the various arrangements of chromosome pairs.
The result of crossing over, as seen in meiotic Prophase I, is unidentical sister chromatids, which can be seen below; the alternating dark and light areas represent areas of each chromatid that have crossed over. Because of crossing over, it is very unlikely that any two sperms or eggs would ever carry the exact same arrangements of DNA.
Notice that these cells begin with two double stranded chromosomes, and end with two single chromatids. The second division of meiosis resembles mitosis, except that only haploid numbers are3 involved. A short Interkinesis stage exists between meiosis I and II, but there is no replication of DNA at this time. While males produce four sperm cells for each cell that undergoes meiosis, females produce one mature ova (which retains most of the original cytoplasm of the cell) and three polar bodies, containing the discarded chromosomes.
Summary of Meiosis:
Each pair of chromosomes lines up independently of all other pairs during Metaphase I. Note the example below; with 3 pairs of chromosomes, the number of different gametes possible is 2 X 2 X 2 =8. It is essential that you understand this process before attempting to work genetics problems, because the most difficult part of genetics is determining the number and kinds of gametes from each parent.
In humans there are 8,388,608 different possible arrangements of the chromosomes in sperms and eggs. Thus, two individuals have over 7 trillion possible combinations of chromosomes that could occur in their children. This does not include all the trillions of possibilities of crossing over, in which portions of chromosomes are exchanged with their homologue.
Use the letters A, B, and C to represent three genetic characteristics on the three purple chromosomes, and use a, b, and c to represent alternative forms of those genes on the blue-green chromosomes. Diagram the possible arrangements of the three pairs of genes in the gametes. See the illustrations below.
A dihybrid cross, involving two characteristics that are being observed.
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