A Genetic Glossary by Bert Thompson, Ph.D.

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A Genetic Glossary

by	Bert Thompson, Ph.D.

As one science writer observed, “modern genetics is a formidable thicket of jargon” (Matt Ridley, Genome: Autobiography of a species in 23 Chapters, New York, Harper Collins, 1999, p. 5). Realizing that, I have tried to keep technical terms to a minimum in the two-part series on “Cracking the Code—The Human Genome Project in Perspective.” Some, however, are unavoidable. Therefore, in order to assist those who may not be familiar with scientific terminology, I am providing the following glossary.

[Words and phrases in bold type within these definitions also appear in the glossary.]

Alleles—In diploid organisms, different forms of the same gene (arranged as homologous pairs, one having been donated by each parent) on the DNA molecule.

Amino Acids—The basic building blocks of proteins; organic compounds containing an acidic carboxyl (COOH) group, a basic amino (NH₂) group, and a distinctive side group (“R” group) that varies in each amino acid and that determines the individual chemical properties of each. Twenty common amino acids are found in proteins.

Autosome—Any eukaryotic chromosome not involved in sex determination. Autosomes constitute the vast majority of an organism’s chromosomal complement.

Base—A nitrogen-containing (nitrogenous) molecule that, in combination with a pentose sugarand a phosphoric acid (phosphate) group, forms a nucleotide.

Chromosome—Threadlike structure into which DNA is organized, and on which genes (and other DNA) are carried. In eukaryotes, chromosomes reside in a membrane-bound cell nucleus; in prokaryotes, the chromosome consists of a single circle of naked DNA. From Greek chromos(“color”) because colored stains originally were used to visualize chromosomes. Number of chromosomes is characteristic of a species (humans have 23 matched pairs—22 autosome pairs; one sex chromosome pair).

Codon—The basic coding unit in DNA/RNA; composed of a triplet of nucleotides.

Cytogenic Map—The visual appearance of a chromosome when stained and examined microscopically. Visually distinct regions (“light” and “dark” bands) give each chromosome a unique appearance; important in determination of aberrations.

Cytoplasm—The inside of a cell, excluding the nucleus and organelles, that is a matrix containing dissolved/suspended ions and other molecules necessary for life.

Diploid—The number of chromosomes in somatic cells (as opposed to gametes) of humans and animals. In diploid cells, each chromosome is present in duplicate (or twice the haploid number). Diploid cells normally are produced by mitosis, which does not reduce chromosome number (as in meiosis) but maintains original number.

DNA—Deoxyribonucleic acid; a nucleic acid containing the genetic information found in most organisms and which is the main component of chromosomes of eukaryotic organisms. The DNA molecule is composed of two winding polynucleotide chains that form a double helix. Each chain is composed of individual units made of a base (adenine, cytosine, guanine, or thymine) linked via a pentose sugar (deoxyribose) to a phosphate molecule.

Double Helix—The structural arrangement of DNA, which looks something like a long ladder twisted into a coil (helix). The sides of the “ladder” are formed by a backbone of pentose sugarand phosphate molecules, and the “rungs” are composed of nucleotide bases joined weakly in the middle by hydrogen bonds.

Endoplasmic reticulum—A system of membranous sacs traversing the cytoplasm of eukaryoticcells. Provides transportation for delivery of synthesized proteins or for secretion of substances to the cell’s exterior in conjunction with Golgi bodies.

Eukaryote—A cell characterized by membrane-bound organelles (such as the nucleus, ribosomes, et al.). Animals, plants, fungi, and protoctists are eukaryotic.

Gamete—A haploid reproductive cell (spermatozoon or sperm cell in the male; oocyte or egg cell in the female) capable of fusing with another reproductive cell during fertilization to produce a diploid zygote. In sexual reproduction, each gamete transmits its parental genome to the progeny. In humans and most animals, the male gamete often is smaller than its counterpart in the female, is motile, and is produced in large numbers. The female gamete, by contrast, is much larger, immotile, and produced in relatively small numbers.

Gene—The physical hereditary unit passed from parent to offspring. Genes are sequences of nucleotides or pieces of DNA, most of which contain information for producing a specific protein. Genes code for the structures and functions of an organism.

Genetic Map—A map (also known as a chromosomal or linkage map) showing the linear arrangement of a particular species’ genes in relation to each other, rather than as specific points on each chromosome.

Genome—The total genetic makeup of an organism (from Greek, génos, “generation” or “kind”). Refers to DNA complement of a haploid cell, including DNA in the chromosomes as well as that within mitochondria. [“Nuclear genome” refers solely to DNA within the nucleus; “human genome” refers to all the DNA contained in an entire human (haploid) cell, rather than just in the nucleus.]

Genotype—The genetic identity of an individual that does not show as outward characteristics, but instead is a description of all genes that are present in the genome regardless of their state of expression or modification. Phenotype often is apparent to the naked eye; genotype can be determined only by specific genetic testing.

Germ cell—see Gamete.

Golgi Body—An organelle present in eukaryotic cells that functions as a collection and/ or packaging center for substances that the cell manufacturers for transport. Especially useful in protein distribution.

Haploid—The number of chromosomes in a spermatozoon or oocyte; half the diploid number. Haploid cells normally are produced by meiosis, which reduces the chromosome number by half during the formation of gametes.

Meiosis—The ordered process of cell division by which the chromosome number is reduced by half. Meiosis is the key element in the production of haploid gametes.

Mitochondria—The cellular organelles found in eukaryotic cells where energy production and respiration occur.

Mitosis—The ordered process by which a cell divides to produce two identical progeny, each with the same number of chromosomes as the original parent cell.

Nucleic Acid—see Polynucleotide.

Nucleotide—One of the structural components of DNA and RNA; composed of one sugar molecule, one phosphoric acid molecule, and one nitrogenous base molecule (adenine, cytosine, guanine, or thymine). [“Base” and “nucleotide” are used interchangeably in referring to residues that compose polynucleotide chains of DNA or RNA.]

Oocyte—The mature, female reproductive cell (also known as an egg cell).

Organelle—A subcellular structure characteristic of eukaryotic cells that performs a specific function. Largest organelle is the nucleus; others include Golgi bodies, ribosomes, and the endoplasmic reticulum.

Pentose Sugar—A sugar that has five carbon atoms in each molecule [e.g., ribose (in RNA) or deoxyribose (in DNA)].

Phenotype—The external, physical appearance of an organism that includes such traits as hair color, weight, height, etc. The phenotype is determined by the interaction of genes with each other and with the environment, whereas the genotype is strictly genetic in orientation. Phenotypic traits (e.g., weight) are not necessarily genetic.

Phosphate—Also known as phosphoric acid; element essential to living creatures. Required for energy storage and transfer (ion state also serves as a biological buffer).

Physical Mapping—Shows specific physical location of a particular species’ genes on each chromosome. Physical maps are important in searches for disease genes.

Polynucleotide—Also known as a nucleic acid. One of the four main classes of macromolecules (proteins, nucleic acids, carbohydrates, lipids) found in living systems. Polynucleotides—long chains composed of nucleotide—form backbone of DNA, in which two polynucleotide chains interact as their nitrogenous bases connect to form what is known as the DNA double helix.

Proteins—One of four main classes of macromolecules (in addition to nucleic acids, carbohydrates, and lipids) in living systems. Proteins are composed of amino acids and perform a wide variety of activities throughout the body.

RNA—Ribonucleic acid; a nucleic acid that functions in various forms to translate information contained in DNA into proteins. Similar in composition to DNA, in that each polynucleotidechain is composed of units made of a base (adenine, cytosine, guanine, or, in the case of RNA, uracil, rather than thymine as in DNA) linked via a pentose sugar (in this case, ribose rather than deoxyribose) to a phosphate molecule. Generally is single stranded (as opposed to DNA’s double helix), except on occasions where it (rather than DNA) serves as the primary genetic material contained in certain double-stranded RNA viruses. Numerous forms of RNA, including messenger RNA (mRNA), transfer RNA (tRNA), and ribosomal RNA (rRNA) are responsible for carrying out a variety of different functions.

Ribosomes—The intracellular, molecular machines that carry out protein synthesis. Associated with RNA and often attached to the endoplasmic reticulum.

Sex Cell—see Gamete.

Sex Chromosomes—The chromosomes that determine the sex of organisms which exhibit sexual differentiation (e.g., humans, most animals, some higher plants). In humans, the X chromosome determines female genetic traits; the Y chromosome determines male traits. Since a single chromosome is inherited from each parent during reproduction, XX is female, and XY is male.

Somatic Cells—All the cells (often referred to as body cells) of a multicellular organism other than the sex cells (gametes). Somatic cells reproduce only by the process of mitosis; changes in such cells are not heritable, since they are not involved in germ-line reproduction as sex cellsare.

Spermatozoon—The mature, male reproductive cell (also known as a sperm cell).

Zygote—The diploid cell resulting from the fusion of the male and female gametes that will grow into the embryo, fetus, and eventually the neonate (newborn).