The sperm and oocyte are gametes, which are also called sex cells. Unlike other cells in the human body, gametes contain just 23 different chromosomes-half the usual amount of genetic material. Somatic cells contain 23 pairs or 46 chromosomes. Gametes are haploid, which means that they have only one of each type of chromosome. Somatic cells are diploid, signifying their double chromosomal load.
Halving the number of chromosomes during gamete formation makes sense. If the sperm and oocyte each contained 46 chromosomes, then when they joined, the fertilized ovum would contain twice the normal number of chromosomes, or 92. Such a genetically overloaded cell usually does not support normal development. About one in a million newborns has three or four sets of chromosomes, but they have problems in all organ systems and live only a few days.
Gametes form from special cells, called germ-line cells, in a type of cell division called meiosis that halves the chromosome number. A further process, maturation, sculpts the distinctive characteristics of sperm and oocyte, which are completely different in appearance. The organelle-packed oocyte has 90,000 times the volume of the streamlined, top-heavy sperm.
Stages of meiosis
Meiosis entails two divisions of the genetic material. The first division is called reduction division because it reduces the number of chromosomes from 46 to 23. The second division, called the equational division is like mitosis, producing four cells from the two cells formed in the first division.
As in mitosis, meiosis occurs after an interphase period when DNA is replicated. The cell in which meiosis begins has homologous pairs of chromosomes, or homologs for short. Homologs look alike and carry the genes for the same traits in the same sequence. One homolog comes from the person’s mother, and one from the father. When meiosis begins, the DNA of each homolog replicates, forming two chromatids joined at two centromeres. The chromosomes are not yet condensed enough to be visible under a microscope.
After interphase, prophase I begins as replicated chromosomes condense and become visible. A spindle forms. Toward the middle of prophase I, the homologs line up next to one another, gene by gene, in a phenomenon called synapsis. A mixture of RNA and protein holds the chromosome pairs together.
Toward the end of prophase I, the synapsed chromosomes separate but remain attached at few points along their lengths. At this time, the homologs exchange parts in a process called crossing over. After crossing over, each homolog contains genes from each, each homolog contains genes from each parent. New gene combinations arise from crossing over when the parents carry different forms of the same gene, called alleles.
To understand how crossing over mixes trait combinations, consider a simplified example. Suppose that homologs carry genes for hair color, eye color, and finger length. One of the chromosomes carries alleles for blond hair, blue eyes, and short fingers. Its homolog carries alleles for black hair, brown eyes, and long fingers. After crossing over, one of the chromosomes might bear alleles for blond hair, brown eyes, and long fingers, and the other bears alleles for black hair, blue eyes, and short fingers. The daughter cells that result from meiosis will carry a mix of the parent cell traits.
Meiosis continues in metaphase I, when the homologs align down the center of the cell. Each member of a homolog pair attaches to a spindle fiber at opposite poles. The pattern in which the chromosomes align during metaphase I is important in generating genetic diversity. For each homolog pair, the pole the maternally-or paternally-derived member goes to is random. The situation is analogous to the number of different ways that 23 boys and 23 girls could line up in boy-girl pairs. The greater the number of chromosomes, the greater the genetic diversity generated.
For two pairs of homologs, four different metaphase configurations are possible. For three pairs of homologs, eight configurations can occur. Our 23 chromosome pairs can therefore line up in 8,388,608 different ways. This random arrangement of the members of homolog pairs in metaphase is called independent assortment. It accounts for a basic law of inheritance.
Homologs separate in anaphase I and finish moving to opposite poles in telophase I. during a second interphase, chromosomes unfold into very thin threads. Proteins are manufactured, but the genetic material is not replicated a second time. It is the single DNA replication, followed by the double division of meiosis, that halves the chromosome number.
Prophase II marks the start of the second meiotic division. The chromosomes are again condensed and visible. In metaphase II, the replicated chromosomes align down the center of the cell. In anaphase II, the centromeres part and the chromatids move to opposite poles. In telophase II, nuclear envelops form around the four nuclei, which then separate into individual cells. The net result of meiosis s four haploid cells, each carrying a new assortment of genes and chromosomes.