In sexual reproduction, two parents give rise to an offspring with an unique gene combination from either of them — each parent gives 1/2 of his/her genes to the offspring. A gene is a discrete unit of information on the DNA that codes for one protein, perhaps one of the many enzymes needed by our bodies.
Somatic cells have two sets of chromosomes; one set from each parent. For example, in humans one set = 23 chromosomes, so our somatic cells have 46 chromosomes arranged in 23 pairs. The two chromosomes in each pair are referred to as being homologous chromosomes, so we could say that humans have 23 pairs of homologous chromosomes. The two chromosomes of each pair carry genes for the same trait (for example, eye color) at the same location, but not necessarily the same form of that gene (for example, brown vs. blue eyes).
An important exception to this is the sex chromosomes, the X and Y chromosomes. Although these chromosomes pair with each other, they are not the same size. The X-chromosome is longer and has genes for many traits with no match on the Y-chromosome. A person with XX would be female and someone with XY would be male (although, that’s not true of all other organisms). All the other chromosomes are called autosomes.
Somatic cells have two sets of autosomes (however many pairs that is) and one pair of sex chromosomes so are called diploid or 2n cells. Thus, humans would have 44 + XX or 44 + XY chromosomes, and fruit flies would have 6 + XX or 6 + XY. Gametes or sex cells (eggs from female and sperm from male) have one chromosome from each autosome pair and one sex chromosome (one set of chromosomes), thus are called haploid or 1n. Human eggs would have 22 + X chromosomes, and human sperm would have 22 + X or 22 + Y chromosomes. Similarly, fruit fly eggs would have 4 + X chromsomes and their sperm would have 3 + X or 3 + Y chromosomes.
Meiosis is a special type of cell division that produces gametes with half as many chromosomes. The opposite process would be syngamy or fertilization, which is the union of the egg and sperm to restore the 2n number. This results in a zygote, the first cell formed by fertilization, a completely new and different organism with unique genetic information different from either parent. The zygote divides and grows to form an embryo which developes into a young organism, then an adult.
Life cycles of all sexually-reproducing organisms follow this pattern of alternation of generations. The 2n adult produces 1n gametes by the process of meiosis. These unite in the process of syngamy to produce a new 2n generation. Thus, the life cycles alternate between 1n and 2n stages, and between the processes of meiosis and syngamy. It is because of the way in which genes recombine in meiosis and syngamy that we have the whole study of genetics.
The steps in meiosis are similar to mitosis and even have the same names. However, there is a significant difference in how the chromosomes line up initially. In mitosis, chromosomes line up individually, while in meiosis, the two chromosomes in each homologous pair line up next to each other. This pairing process is called synapsis, and the resulting homologous pair is called a bivalent in reference to the two chromosomes or a tetrad in reference to the four sister chromatids involved.
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Interphase is the same in both mitosis and meiosis, but in meiosis, it is followed by two cell divisions. These two division processes are referred to as Meiosis I and Meiosis II, and result in a total of four daughter cells, each with a 1n chromosome number.
In prophase I, notice the difference in how the homologous chromosomes behave. They come together and match up (synapsis) in pairs (tetrads or bivalents). In human females, this stage happens prior to birth when the ovaries are forming, and then stops. A baby girl is born with all the precursor egg cells she will ever have in a sort-of "suspended animation" until puberty (hence abdominal x-rays are dangerous for any young to middle-aged human female, not just pregnant women, and hence there is a greater likelihood that a 40-yr-old mother will have a baby with Down Syndrome – due to incorrect meiosis — than a 20-yr-old mother).
In metaphase I, the bivalents line up, not individual chromosomes, so there’s a 50:50 chance of which chromosome of each pair faces which pole of the cell. Human “eggs” go about this far through meiosis before they are shed from the ovaries at ovulation.
In anaphase I, the homologous chromosomes separate, and one of each pair travels to each of the two poles of the cell, thereby reducing the chromosome number from 2n to 1n. Note that the sister chromatids stay together.
Two daughter cells are formed during telophase I. These usually go immediately into the second cell division (meiosis II) to separate the sister chromatids.
Meiosis II is pretty much like mitosis, in that the sister chromatids are separated. This results in four daughter cells, each with an 1n chromosome number. In human females, meiosis II in the precursor egg cells never happens until/if a sperm first enters the egg to fertilize it. Fertilization triggers Meiosis II, and then the sperm nucleus unites with the resulting egg nucleus. Thus, the unfertilized “eggs” that a woman sheds each month are not true eggs. Also in human females, division of the cytoplasm is not even. This provides a way of keeping as much cytoplasm as possible with the future egg/zygote. Rather than equal-sized gametes, one big egg and three smaller polar bodies with minimal cytoplasm are formed.
Interestingly, because the homologous pairs line up during Metaphase I, there is a 50:50 chance of which one of each pair will go to each of the poles of the cell (like flipping a coin, where you can get either heads or tails). Therefore, in humans with 23 pairs of chromosomes, a gamete (egg or sperm) could have 223 or 8,388,604 possible combinations of chromosomes from that parent. Any couple could have 223 × 223 or 70,368,744,177,644 (70 trillion) different possible children, based just on the number of chromosomes, not considering the actual genes on those chromosomes. Thus, the chance of two siblings being exactly identical would be 1 in 70 trillion. In addition, something called crossingover, in which the two homologous chromosomes of a pair exchange equal segments during synapsis in Meiosis I, can add further variation to an individual’s genetic make-up.
Note this comparison between mitosis and meiosis.
Copyright © 1996 by J. Stein Carter. All rights reserved.
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