GED Science Practice Test: Meiosis

Remember that the eukaryotic cells in the bodies of animals are diploid, meaning that they have two copies of each chromosome.  Again, in humans, we have 46 chromosomes, or 23 pairs.  However, it is important for a human’s sex cells (egg and sperm) to only have one copy of each pair of chromosomes.  Sex cells are thus said to be haploid, because they only have half one copy, or half the normal number of chromosomes found in the cells of our body.  Egg and sperm cells have only 23 chromosomes each.  However, when an egg and sperm combine together, they together achieve the diploid number of chromosomes.

The production of sex cells, or gametes, is called meiosis. In contrast to mitosis, meiosis is an 8-step process which includes two rounds of cell division (meiosis I and meiosis II). The steps of meiosis I are prophase I, metaphase I, anaphase I, and telophase I; and the steps of meiosis II are prophase II, metaphase II, anaphase II, and telophase II. There are some differences in the stages as well. In prophase I, a process called crossing over occurs, through which genetic material is swapped the duplicate chromosomes.  Meiosis I is very similar to mitosis, resulting in cells that are diploid.  Meiosis II is also similar to mitosis, except it divides the chromosomes in diploid cells to make them haploid.  In other words, thinking again of our human example, humans have 46 chromosomes (diploid number).  During DNA synthesis, there are 92 chromosomes.  During meiosis I, the cell with 96 chromosomes divides and forms 2 diploid cells, each with 46 chromosomes.  During meiosis II, the two cells with 46 chromosomes divide to form 4 haploid sex cells, or gametes.  The following diagram shows the process of meiosis:

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While binary fission and mitosis result in daughter cells that are identical to the parent cell, this is not the case with meiosis.  There are two reasons that the daughter cells in meiosis are different from the parent.  One is the crossing over phase that happens during prophase I.  Chromosomes swap information, resulting in chromosomes that are not identical to the parent cell.  Additionally, the haploid gametes that result from meiosis only contain half of the parent’s genetic information.

Furthermore, when these gametes combine when a sperm fertilizes an egg, the resulting fertilized egg is not identical to either parent.  It has half of its genetic information from the sperm, and half of its genetic information from the egg.  While it might be tempting to describe the fertilized egg as “half mom/half dad,” this is not exactly how inheriting genetic information works, as you will learn in the next lesson.   Meiosis is a form of sexual reproduction, because it requires two parents.  Sexual reproduction is how humans reproduce; the generation of human offspring requires a mother and a father.  However, many plants also reproduce sexually as well.  When bees pollinate apple trees, for example, they are carrying pollen (which contains the male gamete) to other apple trees and flowers.  The resulting diploid seeds that grow in apples are the combination of the male and female sex cells combining.

 

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