Genetics with the Basics of Selection - M.P. Myhun - 2008

CHAPTER V. Genetic Foundations of Ontogeny

5.4. Conclusion of Individual Development Processes

The final stage of ontogeny is the Aging of the Organism and its death. There is reason to believe that in higher organisms, the aging process and life span are largely genetically determined. The die-off of specific differentiated Cells is inherent in the normal development of higher organisms. Examples include: the destruction of larval Tissues and Organs during insect metamorphosis, tail and gill cells during the Transformation of a tadpole into a frog, cells in the interdigital webs of vertebrate embryos, and so on. In all these and other examples, genetically programmed Cell death occurs at a precisely defined stage of ontogeny and is controlled by genes. In some plants and animals, genetically programmed death occurs at a strictly defined moment of The life cycle. However, There are also other views regarding whether aging and death are genetically programmed components.

The MECHANISM OF ACTION of the genetic factors causing aging is not yet fully understood, although There are many interesting hypotheses on this matter. Occasionally, reports appear in the media about the discovery of genes responsible for aging and the like.

The hypothesis of programmed cell death. Based on his experiments, L. Hayflick concluded that the death of all cells belonging to an organism reflects the aging process at the level of the individual cell. It is hypothesized that a genetic program leading to death is irreversibly executed within The Cell through differential Gene Expression. It has also been proven that the number of successive divisions of somatic cells is genetically limited, after which they lose mitotic activity and die off. The number of cell divisions isolated from a human embryo was approximately 50, from an adult 20-30, and from an elderly person only a few. It has been proven that the number of divisions depends on the normal life span of the species' individuals. For instance, for mink cells—whose lifespan is about 10 years—the number of divisions is fewer than for human cells, and for mouse cells—which live approximately 3 years—the divisions are fewer than for mink cells. Upon closer study of cell divisions, it was noticed that long before they stop dividing, their Structure AND Functions change specifically. A complex of morphological, physico-chemical, and biochemical changes takes place. Age-related Changes in the cell play a major role in the manifestation of body aging and lead to the individual's death much earlier than its cells cease dividing.

If a sufficient number of age-related changes accumulates in vital organs (such as The Heart or Brain) within the complex set of interdependent cells that constitutes an organism, the body dies, even though it may still contain quite a few viable cells. Therefore, if one could slow down the Structural and functional changes occurring in cells, it would be possible to live up to the "genetic limit of aging." The accumulation of changes in cells leads to a loss of control over various processes in the organism. Many scientists believe that these losses occur at THE CELLULAR LEVEL, particularly in DNA.

There are several modern Theories of Aging, but most researchers believe that aging is caused simultaneously by multiple factors.

The error theory. Chemical metabolic reactions in the cell are not always precise; errors can occur during The formation of new DNA, RNA, and protein molecules. Repair mechanisms do not always correct the damage. An error in DNA leads to the synthesis of altered RNA, which in turn leads to the synthesis of altered protein-Enzymes that function poorly or not at all. As a result, metabolic reactions cease, and the cell either fails to perform its functions or dies. Scientists have found that 25% of enzymes in elderly people are defective and have certain flaws.

The free radical theory. DNA, RNA, and protein molecules inside the cell are constantly attacked by other molecules—metabolic products or environmental pollutants. A special type of molecule known as free radicals is formed. They have high reactivity and can damage cell membranes, DNA, and RNA.

The cross-linking theory. In the mid-20th century, Yu. Berksten noticed that the gelatin present in carbon paper "ages." He drew attention to the similarity of this process with the aging observed in the body—in Cartilage and ligaments. Both processes are related to reactions in Proteins that lead to a loss of elasticity. Stiffness in the Muscles and joints of elderly people reminded Berksten of the leather-tanning process, where proteins harden under METABOLISM/18.html">The Influence of certain chemicals. During tanning, specific chemical "bridges" called cross-links form between protein molecules. The aging of the organism can be explained by The Emergence of bridges between protein molecules. Such bridges cannot be broken by cellular enzymes. Subsequently, the presence of another type of cross-link was discovered in DNA molecules. Cross-links between two DNA strands cannot be destroyed by the cell. Such bridges interfere with the synthesis of RNA on DNA, which disrupts the processes of forming vital proteins. In addition, cross-links hinder DNA participation in Cell Division. The formation of cross-links in proteins and DNA can be triggered by many chemical substances.

The brain regulation hypothesis. The normal functioning of an integral organism requires the coordinated work of all its parts (Homeostasis). The primary condition for homeostasis is the coordinated activity of two regulatory systems: the endocrine and the nervous systems. Endocrine glands constantly monitor the internal environment of the organism and, in the event of deviations, release Hormones into the Blood to normalize the condition. Some scientists believe that the aging of cells and organisms is caused by the gradual loss of The ability to maintain homeostasis. At the same time, control over hormone production decreases, and life processes become unbalanced. This hypothesis also has experimental evidence.

The autoimmune theory. The body's immune system protects it from various diseases. The Main Components of The Immune System are white Blood Cells of two types—B AND T—which are specialized to fight Bacteria, Viruses, Cancer cells, and foreign cells... It is hypothesized that with age, cells of both types function less effectively and exhibit non-specific functions, "attacking" normal healthy cells. The destruction of the body by its own immune system is called autoimmunity. The increasing level of autoimmunity in the organism is presumably a cause of aging.

Aging and death conclude the individual life of the cell and the organism. At the same time, the individual life of non-dividing cells can be significantly extended. Research into the conditions under which Cell Aging and death can be prevented is of vital fundamental, theoretical, and practical importance, since an organism dies as a result of the death of a specific group of vital cells, while many cells remain alive and functionally sound after its death.