A 90-minute discussion session with the lecturers, Washington, D.C.-area high school students, and three HIV-positive individuals—Adam Barrett, Zinhle Thabethe, and Phill Wilson—who share their personal experiences about living with HIV.

After describing the fundamental properties of circadian rhythms, Dr. Takahashi takes us on an exciting journey into a very special region of the brain—the suprachiasmatic nucleus (SCN). The SCN functions as a "master" biological clock that governs our physiology and certain behaviors. The clock regulates rhythms of sleep and wakefulness that make us morning larks, evening owls, or something in between. When the master clock is out of synchrony with other biological clocks in the body, symptoms of jet lag ensue.

Dr. Rosbash reveals that the fruit fly (Drosophila melanogaster) has a biological clock in its nervous system. Although tiny in size, the fruit fly has had a major impact on our understanding of circadian rhythms. The fruit fly served as the instrument with which scientists proved that certain behaviors such as rest and activity are under direct genetic control. Although much remains to be learned, the outlines of how the biological clock functions have emerged from research on this singular insect.

Dr. Rosbash discloses how scientists have persuaded Mother Nature to reveal the inner workings of the fruit fly's biological clock. From the almost 14,000 genes in this organism, scientists have painstakingly identified a handful that regulate the "ticktock" of the biological clock. In doing so, scientists have also brilliantly shown how the environment resets our biological clocks so that they are in synchrony with the cycles of nature.

Dr. Takahashi describes the powerful strategies that he and others have harnessed for understanding biological clocks in mammals. To tease out the secrets of how the clocks in higher organisms function, scientists had to overcome uncommonly high hurdles posed by the complexity of mice, hamsters, and humans. Many of these studies used the increasingly important research tools of genomics and computer-based informatics. One payoff already is a better understanding of human sleep disorders that are linked to specific genes.

Is it a boy or a girl? Dr. Page looks at how we define male and female and summarizes the development of human sex characteristics. He then explains the role of the sex chromosomes, X and Y, and, in particular, the SRY gene. Dr. Page demonstrates the differences between species that reproduce sexually and those that reproduce clonally without sex. A likely major advantage of sexual reproduction is that meiotic recombination and subsequent natural selection can weed out deleterious mutations.

Dr. Meyer explains the value of studying model organisms and introduces the nematode C. elegans Affectionally known as "the worm," it has two sexes: male, which possesses a single X chromosome, and hermaphrodite, which possesses two X chromosomes. Dr. Meyer explains that sex determination is controlled by the xol-1 gene. Xol-1 gene expression is regulated by sex-determining factors produced by the X chromosome.

Having too many chromosomes can lead to too much gene expression. If a male and a female have a different number of X chromosomes, a dosage-compensation mechanism is necessary to equalize the level of gene expression. In human females who have two X chromosomes, one X chromosome in each cell is inactive, while in C. elegans hermaphrodites, the activity of both X chromosomes is reduced by half. Dr. Meyer explains how the gene that controls dosage compensation in C. elegans works. Some genes involved in dosage compensation also have a role in cell division.

Dr. Page interprets the results of an audience-participation experiment comparing testosterone levels in males and females of varying ages. He then explains how the Y chromosome is inherited from father to son in a near clonal fashion. He demonstrates that successive inversions and deletions during mammalian evolution have reduced the Y chromosome to its present form--small and sparsely populated with genes. In some men, a deletion in the Y chromosome can lead to infertility. Dr. Page describes how intracytoplasmic sperm injection can help these men father children.

Life processes are fundamentally chemical reactions. Left to themselves, however, the reactions would occur too slowly and nonspecifically to sustain life. Cellular enzymes are catalysts that tame reactions by accelerating them, lending specificity, and regulating their time and place. Some principles of biological catalysis are demonstrated.