Academic journal article Alcohol Research

Concepts and Terms in Genetic Research-A Primer

Academic journal article Alcohol Research

Concepts and Terms in Genetic Research-A Primer

Article excerpt

It is further remarkable that drunkenness resembles certain hereditary . . . diseases.

-Benjamin Rush, Inquiry into the Effects of Ardent Spirits upon the Human Body and Mind, 1785

People have long suspected that alcoholism has a genetic component. Research over the last few decades has indicated that 40 to 60 percent of the susceptibility to alcoholism is inherited. Understanding the genetic mechanisms of addiction vulnerability is therefore one of the highest priorities of today's alcohol research. The knowledge base in this area is increasing exponentially, fueled by advances in genetic technology and data analysis. This sidebar reviews some basic genetic concepts mentioned in this issue of Alcohol Research & Health and tries to place them in a scientific and historical context. This discussion is offered with the caveats that traditional genetic concepts are changing so rapidly that most print and many online data sources rapidly are becoming obsolete, and this review-as well as many of the definitions in the accompanying glossary-is greatly simplified and subject to numerous exceptions.

DNA: The Molecule of Life

All the genetic information needed to sustain life is encoded in a long, threadlike molecule called deoxyribonucleic acid (DNA), which makes it possible to transmit this information from one generation to the next. A single strand of DNA is composed of a chain of building blocks called nucleotides. Each nucleotide consists of two subunits: (1) a modified sugar molecule (i.e., deoxyribose phosphate) and (2) one of four molecules known as organic bases called adenine (A), thymine (T), guanine (G), and cytosine (C). Sequential nucleotides are held together by strong chemical bonds between adjacent sugar phosphate subunits, leaving the bases exposed.

DNA typically occurs as a pair of nucleotide strands that are intertwined to form a double helix, a three-dimensional configuration resembling a spiral staircase. The sugar phosphate backbones form the outside of the helix. The bases within the helix are joined together by relatively weak chemical bonds, forming the steps of the "staircase." Strict rules govern the formation of base pairs between two DNA strands. Because of their chemical structure, adenine always binds to thymine, and guanine always binds to cytosine. Bases that can bind to each other are called complementary; similarly, the two strands of a DNA double helix also are complementary (see figure). The principle of complementarity is the basis of DNAs ability to store biological information and pass it on from generation to generation.

The DNA of most organisms is swaddled by protein molecules and tightly packaged into larger structures that during a certain phase of the cell cycle can be visualized as rod-shaped chromosomes. Chromosomes vary in size and shape and occur in matched pairs inside the nucleus of almost every cell of the body. The number of chromosomes per cell depends on the organism; humans, for example, have 23 pairs. When body cells proliferate, their chromosomes duplicate before the cell divides, ensuring that each daughter cell will receive a complete set of paired chromosomes. Reproductive cells, by contrast, are produced by a specialized type of cell division that distributes only one member of each chromosome pair to each egg or sperm cell. When an egg and a sperm cell fuse during fertilization, their chromosomes combine so that the developing offspring contains a full set of chromosomes with an equal share of genetic material inherited from each parent.

Proteins and the Genetic Code

Proteins are the basic structural and functional molecules of living things. As physical components of the cell's architecture, they not only contribute to an organism's basic form (i.e., are structural proteins), but simultaneously fulfill a myriad of functional roles. For example, nonstructural proteins play major roles in biochemical and metabolic events within cells; carry messages between cells; or circulate through the entire body via the bloodstream, functioning as hormones, immune system components, and transporters of oxygen and other vital substances. …

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