Meiosis:
Meiosis is the process by which gametes are formed. Unlike normal body cells, a gamete is haploid and contains only a single set of DNA. The union of a haploid male gamete (
spermatozoan) and a haploid female gamete (
ovum) at
conception forms a diploid
zygote that has the potential to grow into an adult human. We will consider meiosis in more detail when we discuss the Reproductive System.
Meiosis I:
The first part of meiosis is virtually indistinguishable from mitosis at first glance: a single cell duplicates its DNA and then divides into two daughter cells, each of which is diploid. A closer look reveals some important differences, however. For one thing, the two daughter cells produced during Meiosis I are
not genetically identical.
During
Prophase I of meiosis, the homologous chromosomes pair up with each other to form structures known as
tetrads. (Since, at this point, each of the homologous chromosomes consists of two identical DNA molecules, a tetrad contains
four DNA molecules.) The truly unique event during Prophase I, though, is that the homologous chromosomes literally swap parts of themselves with each other, apparently at random. This is
crossing over and it ensures that the daughter cells produced after the first cytokinesis will not be genetically identical.
Another thing that distinguishes Prophase I of meiosis from prophase of mitosis is that spindle fibers attach to only
one side of each chromosome’s centromere. This means that the tetrads will be split apart during the first anaphase, and so each daughter cell will wind up with only one of the two homologous chromosomes, but the chromosomes themselves will remain intact.
It is, apparently, random chance that determines which of the two homologous chromosomes will wind up in a given daughter cell. These two factors (crossing-over and the fact that the homologous chromosomes are
randomly sorted into the daughter cells) guarantee that the two daughter cells that result from the first meiotic division will not be genetically identical.
Metaphase I in meiosis is very much like metaphase in mitosis, except that spindle fibers are attached to only one side of each chromosome’s centromere.
During
Anaphase I in meiosis, the homologous chromosomes are separated, but because spindle fibers are attached to only one side of each centromere, the chromosomes are not broken apart into their separate chromatids like they are in anaphase of mitosis.
Following
Telophase I of meiosis, the parent cell undergoes cytokinesis and divides to form two diploid daughter cells. But because of crossing over during Prophase I and random (and
independent) assortment of the homologous chromosomes, the daughter cells are not genetically identical. (What is meant by independent assortment of the homologous chromosomes is that how one chromosome pair is sorted has no influence on how any of the others are sorted. So each daughter cell will wind up with a random sampling of chromosomes, some inherited from the person’s father and some from the person’s mother.)
After the first cytokinesis, the daughter cells do not go into interphase, nor do they duplicate their DNA. Instead, they go right into Meiosis II.
Meiosis II:
During
Prophase II, spindle fibers attach to
both sides of each chromosome’s centromere. This ensures that when cytokinesis occurs, the centromeres will be broken and the chromatids will be separated.
During
Anaphase II, the centromeres split, just as they do in anaphase of mitosis.
After the second cytokinesis, four cells are formed. Thanks to the fact that there was no duplication of DNA between Meiosis I and Meiosis II, each of these four cells is haploid, rather than diploid. And thanks to crossing over and the random, independent assortment of chromosomes, none of the four cells is genetically identical.
Given the tens of thousands of genes in the human genome, and given the fact that the process of crossing over seems to involve (more or less) random swapping of genetic material between homologous chromosomes, it’s a safe bet that
none of the tens of thousands of ova an individual human female produces during her lifetime will be genetically identical. Perhaps one or two of the hundreds of billions of spermatozoa a human male produces in his lifetime will happen to be genetically identical. Still, the literally astronomical number of possible gene combinations a single man and a single woman can produce ensures that there is a tremendous amount of potential genetic variability in their children.
A somewhat stylized depiction of meiosis. Note how crossing-over occurs
during prophase I, and how the homologous chromosomes are sorted into
the daughter cells independently of each other. This ensures that the
daughter cells are already genetically distinct at the time of the first division.
Meiosis
Linear Mode
