Genetics is the study of genes at all levels from molecules to populations. As a modern discipline, it began in the 1860s with the work of Gregor Mendel, who first formulated the idea that genes exist. We now know that a gene is a functional region of the long DNA molecule that constitutes the fundamental structure of a chromosome. DNA is composed of four nucleotides, each containing deoxyribose sugar, phosphate, and one of four bases: adenine (A), thymine (T), guanine (G), and cytosine (C). DNA is two nucleotide chains, oriented in opposite directions (antiparallel) and held together by bonding A with T and G with C. In replication, the two chains separate, and their exposed bases are used as templates for the synthesis of two identical daughter DNA molecules.
Most genes encode the structure of a protein (proteins are the main determinants of the properties of an organism). To make protein, DNA is first transcribed by the enzyme RNA polymerase into a single-stranded working copy called messenger RNA (mRNA). The nucleotide sequence in the mRNA is translated into an amino acid sequence that constitutes the primary structure of a protein. Amino acid chains are synthesized on ribosomes. Each amino acid is brought to the ribosome by a tRNA molecule that docks by the binding of its triplet (called the anticodon) to a triplet codon in mRNA.
The same gene may have alternative forms called al-leles. Individuals may be classified by genotype (their al lelic constitution) or by phenotype (observable characteristics of appearance or physiology). Both genotypes and phenotypes show variation within a population. Variation is of two types: discontinuous, showing two or more distinct phenotypes, and continuous, showing phenotypes with a wide range of quantitative values. Discontinuous variants are often determined by alleles of one gene. For example, people with normal skin pigmentation have the functional allele coding for the enzyme tyrosinase, which converts tyrosine into the dark pigment melanin, whereas albinos have a mutated form of the gene that codes for a protein that can no longer make the conversion.
The relation of genotype to phenotype across an environmental range is called the norm of reaction. In the laboratory, geneticists study discontinuous variants under conditions where there is a one-to-one correspondence between genotype and phenotype. However, in natural populations, where environment and genetic background vary, there is generally a more complex relation, and genotypes can produce overlapping ranges of phenotypes. As a result, discontinuous variants have been the starting point for most experiments in genetic analysis.
The main tools of genetics are breeding analysis of variants, biochemistry, microscopy, and direct analysis of DNA using cloned DNA. Cloned DNA can provide useful probes for detecting the presence of related DNA and RNA.
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