More About Meiosis
At a genetic level, sexual reproduction is all about mixing up genes and putting together new combinations. The first step in this process is meiosis, a special form of cell division that produces gametesóthe egg and sperm cells. Most of the action during meiosis centers on the chromosomes.
Cells in the human body have 46 chromosomes, including 22 pairs of autosomes and one pair of sex chromosomes (XX in females, XY in males). Because there are two sets of chromosomes, one from each parent, the cells are considered diploid. Meiosis starts with a diploid cell and turns it into four haploid cells, cells with only one set of chromosomes. This means that when the chromosomes of egg and sperm cells combine at fertilization, the embryo regains the normal diploid number.
Meiosis mixes up the parental genes in two ways. First, the members of each chromosome pair come together and swap segments in a process known as crossing over, or recombination. Second, because each gamete gets only half the parental chromosomes, the exact combination in each egg or sperm can and does vary. This is because during meiosis the chromosomes assort independently, with a random member of each pair going to each daughter cell.
Because males have one X and one Y chromosome, half the cells get an X and half get a Y during the meiosis that leads to sperm production. (In females, all the eggs will get one or the other X.) In a general sense, the sex of the offspring is determined by the particular sex chromosome carried by the sperm. However, in the early weeks of development, all fetuses have preliminary structures for both sexes, and the immature gonads can become either testes or ovaries. In the seventh week of fetal development, a gene on the Y chromosome, if present, activates, and the bipotential gonads commit to becoming testes. In the absence of a Y chromosome, and the signal to form testes, the fetus develops as a girl.
At least that's the way it usually happens. In rare cases, an XX individual becomes a male or an XY individual becomes female. Researchers realized that studying the genes of these sex-reversed people could lead them to the master switch for sex determination. They subsequently identified a gene called SRY (sex-determining region on the Y chromosome).
Meiosis, the form of cell division unique to egg and sperm production, sets the stage for sex determination by creating sperm that carry either an X or a Y sex chromosome. But what is it about the X or Y that determines sex? Before a meiotic cell divides, its two sets of chromosomes come together and cross over, or swap, segments. The first animation shows normal crossing over, where the X and Y chromosomes exchange pieces only at their tips. The second animation shows a rare mistake in which the Y chromosome transfers a gene called SRY to the X chromosome, resulting in sex-reversed babies. Studies of sex-reversed individuals led researchers to identify the master switch for sex determination, the SRY gene, which tells a fetus to become a boy.
Part 1: Normal male meiosis
In the cell nucleus, chromosomes contributed by this male's mother (in red) and father (in blue) pair up. For clarity, only the X and Y sex chromosomes and 5 of the 22 pairs of autosomes (nonsex chromosomes) are shown.
Each chromosome has replicated and consists of two identical chromatids. Crossing over can occur anywhere along the autosomes, and here, they swap segments at each end. The X and Y chromosomes normally cross over only at their tips (indicated in blue on the Y). Note that SRY lies below this region.
The nuclear membrane breaks down, and the chromosomes line up along the cell's equatorial plane and then move to the poles. A random member of each chromosome pair goes to each haploid daughter cell. A second division separates the chromatids and produces four cells, which develop into sperm.
In the top panel, a sperm with an X chromosome fertilizes the egg; in the bottom panel, a sperm with a Y chromosome fertilizes the egg. The XX and XY fetuses develop along the same pathway through week six. Then SRY switches on; the XY fetus develops into a boy (bottom panel), and the XX fetus becomes a girl (top panel).
Part 2: Atypical male meiosis resulting in sex-reversed individuals
Meiosis begins just as in the previous example. However, this time the Y chromosome breaks below SRY, transferring SRY to the X chromosome. This produces two sperm with abnormal sex chromosomes. When they fertilize eggs, the XX (SRY+) embryo develops into a boy (top panel), and the XY (SRYñ) embryo develops into a girl. They are sex reversed.
Autosomes are chromosomes other than sex chromosomes and are the same in both sexes.
Mammalian chromosomes are DNA molecules bound up with proteins, particularly proteins known as histones, which form a core for the strand of DNA to wrap around. Chromosomes become visible in the cell nucleus only during cell division when they are condensed into tightly coiled rods. They typically have two arms on either side of a centromere, a condensed region critical for the movement and sorting of chromosomes during cell division. The two halves of a replicated chromosome are called chromatids.
During the first meiotic division, homologous chromosomes synapse, or pair up. At this point, each chromosome consists of two identical chromatids. Crossing over is a precise mechanism for cutting through the DNA of two chromatids and exchanging equivalent pieces without loss of information. In this way, the chromosomes transmitted to gametes can acquire mixtures of maternal and paternal genes.
Genes are lengths of DNA that code for proteins and are the basic units of heredity. Different types of the same gene are called alleles and are responsible for variation in inherited traits. Each gene can be mapped to a specific location on a chromosome, and the proximity of different genes determines their linkage, or the likelihood that they will be inherited together. Recombination is more likely to separate alleles that are further apart on a chromosome than those with little space for crossovers between them.
Meiosis I: During the first meiotic division, recombination occurs and the chromosome number is halved.
Prophase I: Chromosomes condense and become visible. Homologous chromosomes pair up and recombination (crossing over) occurs. Crossovers may be visible as chiasmata, x-shaped connections between chromatids.
Metaphase I: Paired chromosomes line up along the cell's equatorial plane.
Anaphase I: Homologous pairs separate and move to opposite poles.
Telophase I: Chromosomes are at poles; nuclear membranes may re-form.
Meiosis II: The second meiotic division closely resembles mitosis (the type of cell division that occurs in body cells), except that the starting and ending cells are haploid.
Prophase II, metaphase II, anaphase II: The chromosomes again move to the equatorial plane, and this time the chromatids separate to opposite poles.
Telophase II: Nuclear membranes re-form around the chromosomes.
Sex chromosomes differ between the sexes and are involved in sex determination, although they may have other functions as well.
Before the SRY gene was identified, scientists knew that there was a testes-determining factor on the Y chromosome. The challenge was to pinpoint its location. This was done by comparing first the observable physical structure of the chromosomes and second, when the technology allowed, the DNA sequences of sex-reversed individuals with those of the normal population. (Sex reversal occurs in about 1 out of 20,000 births.) Screening with Y-specific DNA (DNA that is found only on the Y chromosome) showed that XY females tended to be missing a certain segment of DNA on the short arm of the Y chromosome, whereas XX males carried DNA from that same region. Mapping that region yielded SRY. The protein encoded by SRY is apparently a transcription factor, and thus it regulates the function of another gene or genes.
Side-by-side animations showing mitosis and meiosis
Overview flowchart of haploid/diploid, meiosis/mitosis in life cycle
Diagram and overview of stages of meiosis
Simple diagram showing structure of DNA, from chromosomes in nucleus to double helix
Details on crossing over mechanism
Online glossary of terms related to genetics
Articles available on the Web with subscription (free trial):
O'Neill, G. 1990. The gene that makes a man of you. New Scientist 127(1726).
Concar, D. 1991. Sex-change engineering makes man of mouse. New Scientist 130(1768).
Technical Review Article
Goodfellow, P.N., and Lovell-Badge, R. 1993. SRY and sex determination in mammals. Annual Review of Genetics 27:71–92.
Griffiths, A.J.F., Miller, J.H., Suzuki, D.T., Lewontin, R.C., and Gelbart, W.M. An Introduction to Genetic Analysis. 7th ed. New York: W.H. Freeman and Co., 2000.
Hartl, D.L. Essential Genetics. Sudbury, Mass.: Jones and Bartlett, 1996.
Director: Dennis Liu, Ph.D.
Scientific Direction: David Page, M.D.
Scientific Content: Donna Messersmith, Ph.D., Jessica McKibben, Ph.D.
Animators: Chris Vargas, Eric Keller