Sex cells are also called gametes. The gametes are the sperm and the egg; the more scientific terms for these cells are the spermatozoa and ova, respectively. These cells are made only in gonads, and they are made by a type of cell division that doesn't occur anywhere else in the body-- meiosis.
What is meiosis?
You have probably learned about meiosis in a previous class, but I will review it a little bit here... There are two types of cell division. The type of cell division other than meiosis is mitosis and it goes on all over the body. Mitosis is cell division that starts with one cell and ends with two identical cells. The starting cell is called the parent cell and the ending cells are called the daughter cells. In mitosis, the daughter cells are not only identical to each other, but they are also identical to the parent cell. We use mitosis all over our bodies for repair after injury, replacement of cells as some get worn and die, and growth. Mitosis is also used for embryonic development from the single, fertilized egg to the multicellular fetus and neonate.
Meiosis, on the other hand, is only used for making gametes! Why do we need a special type of cell division just for making gametes? There are two ways to answer this question. The first is that when we produce our next generation, we do not wish to make identical individuals to ourselves... instead, we make unique individuals, different from either parent. In order to do that, we have to be able to mix up our own genetic information into new combinations; it is these new combinations that come together during fertilization, producing an entirely unique, new individual. Mitosis does not allow for any mixing up of genetic information-- instead, it preserves genetic information perfectly to make identical daughter cells. Meiosis does the opposite-- meiosis jumbles up our genetic information into new combinations.
The second reason that we have to use meiosis is because we need our spermatozoa and ova to contain only half of the normal amount of genetic information. Keep in mind that during fertilization, the spermatozoan and ovum fuse together into one, combined cell (called the zygote). That means that their genetic information also comes together. When each gamete has half of the normal amount of genetic information, they can come together and make a whole. From that point forward the embryo (and then fetus and then neonate...) has a new set of genetic information, half from each parent, that is preserved for all the cells of its forming body.
How does meiosis occur?
Meiosis begins with one cell, called a germ cell (because it specifically makes gametes). As this germ cell prepares to divide, it replicates its genetic material, in the same way that this occurs before mitosis. The germ cell is going to have to go through two cell divisions in order to get from having double the genetic information within it to having only half the normal amount of genetic information within it.
Our cells have two copies of every type of chromosome. Chromosome number is typically represented by "N." We have two of every chromosome because we get one of each chromosome type from our mothers and one from our fathers. Therefore, we have "2N" chromosomes, which just means two of every type. N = 23 in humans, since we have 23 different types of chromosomes... and because we are 2N, we actually have 46 chromosomes in every cell. Organisms and cells with 2 copies of every chromosome type are called diploid, while those with only one of every chromosome type are called haploid.
Use the diagram to follow the description in the next paragraph.
Before our germ cells undergo meiosis, they proceed through DNA replication, during which every chromosome is copied. This means that we go from 2N to 2 x (2N), or 4N. The germ cells then proceed through two cell divisions: Meiosis I and Meiosis II. During meiosis I, the genetic information splits so that each cell has 2 x N instead of 2 x 2N. During meiosis II, the genetic information splits again, leaving each gamete with only N chromosomes. In humans, that would mean that each gamete has 23 chromosomes, and only one of each type (not 2N, but only N). When a spermatozoan and an ovum fuse, the 23 chromosomes from the spermatozoan join up with the 23 chromosomes from the ovum to make the zygote, which now has 46 chromosomes. Note that the 46 chromosomes in the zygote are actually 2 sets of 23 chromosomes or 2N (not 46 completely different chromosomes).
Another way to discuss this is to say that our germ cells are diploid (2N), and that they make haploid gametes (N) through meiosis.
There are many steps to each meiotic division (interphase, prophase, metaphase, anaphase, and telophase). I will assume that you remember enough about each of these phases to be able to use these terms in other web pages. If not, review them a little bit... you can review them in mitosis because they look the same in meiosis as they do in mitosis.
Gamete production differs between males and females
Both males and females undergo meiosis to produce their gametes. However, males have to produce very high numbers of gametes while females only need to produce up to one gamete per month. Therefore, the specific ways that gametes are made in the two sexes differ. The production of gametes in general is called gametogenesis. Gametogenesis in males is more specifically referred to as spermatogenesis, while gametogenesis in females is called oogenesis. These two types of gametogenesis are explained on the webpages linked from this page.
© 2011 STCC Foundation Press