Deoxyribonucleic acid (DNA) is the building block of life. The backbone of DNA is made up of four different bases: thymine (T), guanine (G), adenine (A), and cytosine (C). Each base is then attached to a phosphate group and a sugar, forming a single nucleotide. Genetic information is encoded by the sequence of nucleotides in the filament; therefore, the quantity and sequence of nucleotides in a DNA strand differs depending on the organism. The chemistry of each base makes it specifically complementary with another base (AT and CG). When two complementary strands of DNA join together, base pairs are formed between the nucleotides, resulting in the familiar double-stranded double helix structure. The sequence of nucleotides encodes genetic information through what is known as the “central dogma” of molecular biology (DNA  RNA  protein). This process in which the sequence of a DNA strand (a “gene”) is translated into a protein is known as “gene expression”. Ribonucleic acid (RNA), a single-stranded molecule, is formed within the nucleus with bases complementary to the nucleotide sequence within the DNA strand to be encoded. This process is known as "transcription" because the RNA molecule, through its complementary sequence, essentially transcribes the nucleotide sequence of the gene located on that specific section of the DNA strand. The RNA strand then exits the nucleus into the cytoplasm of the cell, carrying the DNA sequence information with it. Once outside the nucleus, the RNA strand codes for the formation of a protein. Each group of three nucleotide bases in the RNA sequence codes for an amino acid, the building block of proteins. The amino acids bind sequentially to the RNA molecule and in the process bond with each other. After forming along the RNA strand, the protein is released. The sequence of amino acids in the protein determines its function, such as an enzyme, antibody, hormone, or structural molecule. Mutations in DNA can occur through several mechanisms. Nucleotides may be deleted or added to the sequence, or they may be in the wrong order. These mutations may be hereditary or due to environmental factors that cause DNA damage.B. GENETIC TESTS The presence of certain genes can be detected through gene-specific tests.