Genes: The basic biological units of heredity . Segments of deoxyribonucleic acid (DNA) needed to contribute to a function.

An official definition: According to the official Guidelines for Human Gene Nomenclature, a gene is defined as "a DNA segment that contributes to phenotype/function. In the absence of demonstrated function a gene may be characterized by sequence, transcription or homology."

DNA: Genes are composed of DNA, a molecule in the memorable shape of a double helix , a spiral ladder. Each rung of the spiral ladder consists of two paired chemicals called bases. There are four types of bases. They are adenine (A), thymine (T), cytosine (C), and guanine (G). As indicated, each base is symbolized by the first letter of its name: A, T, C, and G. Certain bases always pair together (AT and GC). Different sequences of base pairs form coded messages.

The gene: A gene is a sequence (a string) of bases. It is made up of combinations of A, T, C, and G. These unique combinations determine the gene's function, much as letters join together to form words. Each person has thousands of genes -- billions of base pairs of DNA or bits of information repeated in the nuclei of human cells --which determine individual chacteristics ( genetic traits).

The chromosome: Genes are arranged in precise arrays all along the length of 23 pairs of much larger structures: the chromosomes . One chromosome in each pair comes from the mother and the other one from the father . The chromosomes in any particular pair look like each other, except in a boy. There is one pair of chromosomes, which usually settles the sex of the individual. This pair has two X chromosomes in females and one X and one Y chromosome in males.

The X and Y chromosomes: These chromosomes -- the X and Y are always capitalized -- are the sex chromosomes. All the other chromosomes in the human chromosome complement are numbered from 1 to 22 and are called the autosomes (literally, the other chromosomes).

History of the gene: 1869-1970:

• 1869 - The chemical material DNA is discovered in cells but its real functions are not known.
• 1909 - The term "gene" is first used and the chemical composition of DNA is discovered.
• 1920 - Chromosomes are proposed as the mechanism by which inherited characteristics are passed on.
• 1944 - DNA is first connected to the inheritance of traits.
• 1951 - The first sharp X-ray diffraction photographs of DNA are obtained.
• 1953 - Crick and Watson describe the structure of DNA.
• 1956 - DNA is made artificially.
• 1966 - DNA is found to be present not only in chromosomes but also in the mitochondria.
• 1969 - The first single gene is isolated.
• 1970 - The first artificial gene is made.

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