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Mulitcellular Organisms


Sex and Death

Three Ages

The drawing in Figure 10-15 shows the three stages in the life cycle of organisms with sexual reproduction, human included - birth, growth, and ageing. Death is the final event. The evolution of sex and death is traced in the following.

The unicellular bacteria reproduced asexually in a simple process called fission. In this mode of reproduction, a given cell replicates its own DNA and then divides into two perfectly coequal clones of itself, each clonal offspring receiving one copy of the DNA. These cells mature and then repeat the cycle on and on ad infinitum. They are in effect immortal. In reality, they do die as the result of accident or starvation but not the kind of mortality common to all sexually reproduced organisms. A review on the subject of ageing can be found in the appendix.

For early single-cell eukaryotes reproducing sexually, sex may involve nothing more than two cells sticking together and swapping portions of their DNA in a process called conjugation. It is then followed by the fission of the participating cells. The new products are genetically different from the parent cells prior to conjugation. As a consequence, the older generation has disappeared - the new generation carrries another set of genes.

Figure 10-15 The Three Ages
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The more advanced single-cell eukaryotes like the ciliate has a more elaborate arrangement. It segregates the chromosomes into a micronucleus used exclusively for reproduction purpose and there is a macronucleus containing DNA used to direct the day-to-day operation of the cell. Haploid cells are made in the micronucleus through meiosis. After fertilization, the micronuclei of the two cells fuse, creating a diploid micronucleus which is the foundation of a new generation. In the subsequent replication, a new macronucleus is produced from the new micronucleus. And then the old macronucleus, sitting alone at one end of the cell, begins to degenerate and dies.

Once eukaryotes became multicellular, not only would reproductive DNA be kept in separate nuclei; it would be sequestered in just a few special cells in the body, which in humans and other animals are called germ cells5. Like micronuclei, germ cells have only one function: the transmission of DNA from one generation to the next via sexual reproduction. The rest of the cells in the body - the somatic cells - receive identical sets of chromosomal DNA, but they use this DNA only to carry out the body's daily, nonreproductive functions.

The somatic cells in our bodies divide only by simple fission. They do not exchange or recombine DNA with one another - they do not undergo meiosis or have sex. The only purpose of somatic cells, from nature's point of view, is to optimize the survival and function of the true guardians of the DNA, the germ cells. While the germ cells can renew themselves through fertilization, the somatic cells would keep on accumulating errors and mutations until they cannot function anymore and dies6. That is, the somatic entity is disposable.

From a human point of view, it is our somatic selves - embedded in which are things like mind, personality, love, will - that we cherish most and that define us, to ourselves and to others. We think of reproduction as only one of many activities we can choose to engage in. We want so desperately to be more than just a vehicle for DNA. Yet somatic cells will die at the end of each generation. In the larger scheme of things, it matters not a whit that some of these somatic cells contain all that we hold most dear about ourselves. In terms of the basic process of life itself, which is the transmission of DNA from one generation to the next, all our life is just so much sound and fury, signifying certainly very little, and quite possibly nothing.

5Male germ cells (sperms) are produced continuously in the testes throughout life. About 50,000 female germ cells (eggs) are prepared in the ovaries of the female embryo 12 days after fertilization. It is reported in 2004 that stem cells for oocytes (immature female reproductive cells) are found within the ovaries of mice in contrary to traditional believe.

6Even though if cellular damage by external causes can be avoided, an inherent problem is bound to arise by internal metabolism. The metabolic pathway in mitochondria relies on the transportaton of free electrons, which eventually attach to oxygen and are normally neutralized by hydrogen ion to form water. This is usually referred to as respiration. Sometimes the process breaks down, the electrons do not attach to the oxygen and instead, attach to other oxygen species. Specific molecules, each carrying a wayward electron, were created. They are call free radicals (such as hydroxyl, superoxide and peroxide - collectively called reactive oxygen species or ROS). These free radicals are highly reactive and can do tremendous damage to the cell. The ROS can indiscriminately damage anything that gets in their way - proteins, fats, RNA, and DNA. The effect is cumulative and a serious problem will occur if ROS numbers get too high. Not surprisingly, the cell has responded to this threat by creating various enzymes that bind to free-radicals and inactivate them. Collectively, molecules that destroy free radicals are called anti-oxidants. Aging is probably the result of the breakdown in the cellular safety nets - not enough anti-oxidants are produced naturally. In addition, modern life has enormously increased the number of toxic free-radicals introduced into our bodies every day from the surrounding environment. The most significant sources of excess free-radicals are dietary or environmental. Dietary sources are usually fats, either rancid or hydrogenated fats, and fats that have been heated to high temperatures during cooking. Environmental pollutants include automobile exhaust, cigarette smoke and numerous other chemicals, physical exercise, stress, and radiation. To increase the defence against these ROS, it is suggested that consumption of special kinds of foods, herbs, and dietary supplements (including vitamins C and E, beta carotene and selenium) will scavenge excess free-radicals and provide raw materials necessary for the body to produce antioxidant enzymes. Recently, it is observed that the most efficient way to delay aging is through caloric restriction. It is found that by reducing 30 - 40 percent the calories in an animal's diet, results in healthy and long-lived mice and rats. It seems that the animals have switched their metabolism from a pro-growth mode to a pro-repair mode. The latest research tries to manuplate the level of the IGF-1 (insulin-like growth factor-1) protein so that people can maintain the pro-repair mode without diet restriction.

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