An Introduction to DNA and Chromosomes
Part 7

A closer look at what makes up the human genome...

How is DNA passed on to the next generation?

When humans reproduce, they pass on their genetic information to their offspring. However, if each parent passed on his or her entire genetic code, their child would have twice as many chromosomes as each parent. If this pattern were to continue, the number of chromosomes would double each and every generation, which would quickly become unworkable for cells. In order for a baby to have a non-increasing number of chromosomes, he or she must receive half the normal number of chromosomes from each parent. Therefore, the reproductive cells known as eggs in adult females and sperm in adult males--collectively termed germ cells--must have only half the normal number of chromosomes. Hence, gametes have only 23 chromosomes instead of 23 pairs (46 chromosomes total) like the rest of the cells in your body. These cells are called haploid, as opposed to cells with two pairs of each chromosome that are called diploid.

A special kind of cell division called meiosis generates haploid gametes from diploid parental cells. Meiosis occurs only for the formation of eggs and sperm, but it is clearly a very important process. To get daughter cells with half the number of chromosomes, cells replicate their DNA and then divide twice, instead of once as in mitosis. (For more information on mitosis, click here). The result is four daughter cells that are normally genetically different from the parent cell and from each other.

Before we start describing the phases of meiosis, let’s take a moment to clarify the concepts regarding homologous chromosomes and sister chromatids. Homologous chromosomes each have the same type of information, but one was inherited from your mother and the other was inherited from your father. In other words, at the same location or "gene locus" on each homologous chromosome is the gene for a certain trait, such as eye color. Because each homologous chromosome comes from a different parent, however, the alleles, or versions of the gene, can be different. You could get a blue-eyed allele from your father and a brown-eyed allele from your mother, for example. Sister chromatids, on the other hand, only form once a cell has replicated its DNA. They are two identical copies of one chromosome, joined at the middle to form the familiar X-shape. Sister chromatids are pulled apart during mitosis (and, as we will see, during the second phase of meiosis). To summarize: each chromosome has a matching homologue, which carries similar but not identical information. A pair of identical sister chromatids is the result of a chromosome replicating itself.

Now we can begin with a closer look at meiosis. Cells that undergo meiosis first have an interphase, during which they replicate their DNA, followed by two special rounds of cell division. The stages of division have the same names as in mitosis, but are distinguished from each other by roman numerals: the first round, meiosis I, consists of prophase I, metaphase I, etc. and the second round, meiosis II, consists of prophase II, metaphase II, etc. The second division proceeds a lot like mitosis, with the separation of sister chromatids. The first division, however, is different from mitosis in important ways, as we will see.

Prophase I is more complex than mitosis prophase (or prophase II of meiosis). In prophase I, the X-shaped chromosomes (pairs of sister chromatids) also become visible, but this time homologous chromosomes pair up instead of remaining independent. Each pair is held tightly together, forming what is called a bivalent and allowing a process called "crossing over" to take place. Crossing over is a very important phenomenon in genetics. When chromosomes overlap, genetic material from one chromosome (inherited from the mother, say) can trade places with genetic material from the other chromosome (inherited from the father). For example, your mother’s brown-eyed allele could switch places with your father’s blue-eyed one. This process shuffles the genetic information, creating chromosomes that are unique combinations of maternal and paternal alleles, and not just copies from one parent or the other. For this reason, crossing over is said to promote genetic recombination. Crossing over is an important source of genetic variation, which helps make every single person genetically unique (unless you have an identical twin). Interestingly, cells can remain in this state of paired homologous chromosomes for a very long time, even for years. For example, a female baby’s reproductive cells begin meiosis before she is born, but they only progress as far as prophase I. Meiosis later resumes when she reaches puberty.

At the start of prometaphase I, the nuclear membrane breaks down and microtubules attach to the chromosomes, just like in mitotic prometaphase, and meiosis I proceeds. In metaphase I, all the bivalents line up on the equator of the cell. Then, during anaphase I, the homologues are pulled apart as the attached microtubules shorten and the centrosomes move outward. The cell then continues to divide until there are two daughter cells, marking the end of meiosis I.

Before the start of meiosis II, DNA replication does not occur. Instead, meiosis II begins like mitosis, with the chromosomes (still in the form of paired sister chromatids) lining up at the equator of the cell. They are then pulled apart by the microtubules, and the cell divides in two. The result of meiosis II is that now we have only 23 chromosomes in each daughter cell, whereas in mitosis there were a full 46 chromosomes in each daughter cell. Remember that at the beginning of meiosis II there are two cells each undergoing a division, so the final product will be four daughter cells. Observe that there are only 23 chromosomes (the haploid number) in each resulting germ cell and how each one has a unique combination of chromosomes.

Last Modified: 04/12/2007

An educational product of HOPES, not to be used in place of medical care. For more information about HOPES, click on the Logo. To contact HOPES with comments or questions, click here.