1 September 2003
From the decoding of the human genome to the fiftieth anniversary of Watson and Crick’s discovery of its structure to the recent claim by Chinese scientists that they have created rabbit-human hybrid cells using cloning technology, DNA has been much in the news lately. Given that Watson and Crick’s discovery is ushering in a brave new world of biological revolution, it is only fitting that our word of the month should be:
Deoxyribonucleic acid, n., also DNA, a self-replicating chemical that carries the genetic information in the cell. DNA consists of two long chains of nucleotides twisted into a double helix and linked by hydrogen bonds between the complementary base pairs, adenine and thymine or cytosine and guanine. Thus the sequence of one strand can be replicated from its partner. The sequence of base pairs determines individual hereditary characteristics. The existence of nucleic acids has been known since the 19th century and DNA was differentiated from RNA in 1931. But DNA’s function in genetics was not fully understood until Watson, Crick, and Wilkins deciphered its structure and its process of self-replication in 1953.
What follows is a look at some of the terms associated with DNA. Most are technical and, unsurprisingly, 20th century in origin, although a few are older or based on ancient Latin or Greek roots.
Adenine, n., one of the four bases that make up the “letters” ATGC in DNA. Adenine always pairs with thymine. From the German adenin, after the Greek word for gland, 1885.
Allele, n., an alternative form of a given gene. Different alleles produce variation in inherited characteristics, such as hair and eye color. In use since 1931, it is a clipping of allelomorph, 1902, which is from Greek roots meaning one form.
Amino acid, n., one of 20 molecules that are combined to form proteins in living things. The sequence of amino acids in a protein and the protein function are determined by the genetic code. Amino is a combining form that denotes the group of atoms NH2, 1887.
Amplify, v., to make multiple copies of a DNA sequence, to use the DNA in a sample to synthesize copies of itself and increase the total amount of identical DNA in that sample, 1968.
Autosome, n., a chromosome other than a sex chromosome. Humans have 22 pairs of autosomes and one pair of sex chromosomes. From auto- (self, independent) + soma (Greek for body), 1906.
Base pair, n., two bases (either adenine and thymine or cytosine and guanine), which form one rung in the DNA ladder, 1956. The two strands of DNA are held together in the shape of a double helix by the bonds between base pairs.
Base, n., one of the molecules that form DNA and RNA. The bases are adenine, thymine, guanine, and cytosine.
Bioinformatics, n., the discipline of managing and analyzing biological data using advanced computing techniques. Especially important in analyzing genomic research data. From the Dutch bioinformatica, 1978.
Biotechnology, n., the use biological techniques and microorganisms to perform industrial or manufacturing processes and tasks. Applications include pharmaceuticals, agriculture, and environmental cleanup, 1972.
Cell, n., the basic structural unit of any living organism, consisting of one or more nuclei, cytoplasm, and various organelles, contained within a semi-permeable membrane, from the older sense of an enclosed cavity, 1672 (cells were identified this early, but their functions were not understood until the 20th century).
Chromosome, n., a DNA molecule containing genes found in the nucleus of a cell. Humans have 23 pairs of chromosomes. Every child gets half of their chromosomes from the father and half from the mother. From the German, ultimately of Greek roots chromo- (color) + soma (body), 1889.
Clone, n. & v., a genetically exact copy of biological material such as a DNA segment (e.g., a gene), a whole cell, or a complete organism. From the Greek word for twig, the word was first applied to the grafting of plants in 1903. It was applied cells and animal organisms in 1970. The verb senses are from 1959 for plants and 1974 for other organisms.
Codon, n., a sequence of DNA that determines which amino-acid shall be inserted at any given position in a protein, from code + -on, 1963.
Congenital, adj., a trait or condition present at birth, resulting from either genetic or non-genetic factors, a modern form based on the Latin congenitus or born with, 1796.
Cytosine, n., one of the four bases that make up the letters ATGC in DNA. Cytosine always pairs with guanine. From the German, based on the Greek root for hollow, 1894.
Diploid, adj., pertaining to a full set of genetic material consisting of paired chromosomes, one from each parent, formed from the Greek for double, 1908. Cells, other than gametes (sperm and egg) have a diploid set of chromosomes. Cf. haploid.
DNA fingerprinting, n., the identification of multiple, specific alleles in an individual’s DNA to produce a unique identifier for that person, 1980, a.k.a. genetic fingerprinting, 1984.
Dominant, adj., denotes a gene that expresses itself even if only one copy is present in an individual, i.e., it is inherited from only one parent, 1900. Cf. recessive.
Eugenics, n., improving a species, esp. humans, by artificial selection, from the Greek for well-born, 1883.
Fraternal twins, n., siblings born at the same time as the result of fertilization of two eggs by two sperm. They share the same genetic relationship to each other as any other siblings, 1904. Cf. identical twin.
Gamete, n., a reproductive cell (sperm or egg) with a haploid set of chromosomes, a modern word from the Greek root for marriage, 1886.
Gene therapy, n., the replacement or repair of nonfunctional or defective genes with healthy ones, 1971.
Gene, n., a physical and functional unit of heredity, when the term was coined the structure and mechanisms of genes were unknown, genes are now understood to be a sequence of nucleotides which determines the primary structure of some protein, also the location in the nucleotide where the sequence resides, from the German gen, 1911.
Genetic code, n., the system by which DNA stores genetic information, now known to be triplets of nucleotides read in sequence, 1961.
Genetic engineering, n., originally the production of desired characteristics in an organism through selective breeding, now altering or replacing the genetic material in an organism to obtain a desired characteristic, 1949.
Genetic, adj., pertaining to origin, 1831, pertaining to the mechanisms of biological heredity and variation, 1908, from Genesis.
Genetics, n, the branch of biology concerned with heredity and variation, 1905.
Genome, n., the haploid set of chromosomes in an organism, all the DNA contained in an organism, from the German genom, 1930.
Genomics, n., the branch of molecular biology that deals with the organization and evolution of genomes, using nucleotide sequencing and gene mapping, 1987.
Genotype, n., the genetic constitution of an organism, as distinguished from its physical appearance, from the German genotypus, 1910, cf. phenotype.
Guanine, n., one of the four bases that make up the letters ATGC in DNA. Guanine always pairs with cytosine.
Haploid, adj., pertaining to a single set of unpaired chromosomes, as in a gamete, from the German based on the Greek for single, 1908. Cf. diploid.
Helix, n., anything structured in spiral form, from the Latin and ultimately the Greek, 1643. In 1954, Watson and Crick first postulated that DNA consisted of a double helix, a twisted ladder-like structure. The sides of the ladder are made of sugar and phosphate molecules; the rungs consist of nucleotide bases joined by hydrogen bonds.
Heredity, n., the quality of being heir, heritage, c.1540, the property of living organisms that allows them to pass characteristics to their offspring, 1863, from the French hérédité.
Identical twins, n., monozygotic siblings, siblings born at the same time as a result of a single zygote dividing, identical twins have the same genotype, 1889. Cf. fraternal twin.
In vitro, adj., outside a living organism, in a test-tube, petri dish, etc., from the Latin vitrum, glass, 1894.
In vivo, adj., within a living organism, from the Latin vivus, alive, 1901.
Intron, n., a sequence of DNA that does not code, junk DNA, in[tra] + -on, 1978.
Junk DNA, n., sequences of DNA that do not code for genes, the majority of the genome is non-coding, but may have regulatory and other functions.
Mendelian, adj., relating to genetics, from Gregor Mendel (1822-84), Austrian monk and botanist, who discovered the principles of heredity, 1902.
Messenger RNA, n., an RNA molecule produced by transcription of a gene, which is used as a template for protein manufacture, abbrev. mRNA, 1961.
Mitochondrial DNA, n., genetic material found in mitochondria, the organelles that generate energy for the cell, mitochondrial DNA is inherited solely from the mother, 1964.
Mitochondrial Eve, n., the female carrier of the mitochondrial DNA sequence from which all modern human mitochondrial DNA sequences are derived, postulated to have lived in Africa 150-200,000 years ago, the last common female ancestor of all modern humans, 1987.
Mutation, n., a heritable change in DNA sequence, from the Latin via French, in biological use since 1894.
Nucleic acid, n., a molecule composed of nucleotides, DNA or RNA, so named because they are found in the nuclei of cells, 1892.
Nucleotide, n., a subunit of DNA and RNA consisting of one base (adenine, guanine, cytosine, or thymine), one molecule of sugar (deoxyribose in DNA and ribose in RNA), and one molecule of phosphoric acid, from the German, 1908.
Nucleus, n., the organelle found in most cells that contains the chromosomes and controls metabolic activity and synthesis with the cell, from the Latin for kernel or inner part, in biological use from 1831.
Phenotype, n., externally observable traits of an organism, e.g., hair and eye color, phenotypes may or may not be inherited, from the German after the Greek root pheno- meaning shining, to cause to appear, 1911.
Polymerase chain reaction, n., also PCR, a method DNA amplification, 1985.
Protein, n., a large molecule composed of chains of amino acids in a specific order that is determined by the base sequence of nucleotides in the gene that codes for the protein. Proteins determine the structure, function, and regulation of the organism’s cells, tissues, and organs. From the German after the Greek for primary, prime because proteins are the fundamental constituents of plants and animals, 1844.
Recessive, adj., applies to a gene or trait which is only expressed if there are two identical copies or, for a male, if one copy is present on the X chromosome, a gene or trait that must be inherited from both parents to be expressed, 1900. Cf. dominant.
Recombinant DNA, n., DNA segments that have been artificially joined together (recombined) and which can be introduced into a cell to replicate, 1942.
Ribonucleic acid, n., also RNA, a chemical found in the nucleus and cytoplasm of cells that functions in protein synthesis, RNA is structurally similar to a single strand of DNA (single helix) except that uracil ("U") substitutes for thymine, so named because the sugar it contains is ribose, 1931.
Sequence, v., to determine the order of base pairs in a segment of DNA, 1970.
Sex chromosome, n., a chromosome that determines an individual’s sex, humans have two sex chromosomes (X and Y, so called because of their shape), females have two X chromosomes and males have one X and one Y chromosome, 1906.
Sex-linked, adj., relating to genes located on one of the sex chromosomes, sex-linked traits are generally only seen in males, 1912.
Stem cell, n., an undifferentiated cell, a cell, usually found in embryos or in certain adult tissues, such as bone marrow or the stomach lining, that are capable of growing into any other type of cell, 1959.
Thymine, n., one of the four bases that make up the letters ATGC in DNA. Thymine always pairs with adenine.
Transcription, n., the process by which genetic information in DNA is copied into molecules of RNA, which are synthesized with the DNA serving as a template, 1961, also reverse transcription, or the synthesis of DNA from an RNA template, 1977.
X chromosome, n., see sex chromosome.
Y chromosome, n., see sex chromosome.
