Deoxyribonucleic acid (DNA) is the building block of life. The DNA backbone 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 strand; 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 to another base (AT and CG). When two complementary strands of DNA come together, base pairs form between the nucleotides, resulting in the familiar double-stranded double helix structure. The nucleotide sequence encodes genetic information through what is called 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 called "gene expression." Ribonucleic acid (RNA), a single-stranded molecule, is formed inside the nucleus with bases complementary to the nucleotide sequence in the encoded DNA strand. 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 leaves the nucleus into the cell's cytoplasm, 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 the protein. The amino acids bind in sequence to the RNA molecule and, in doing so, bind to each other. After formation along the RNA strand, the protein is then released. The sequence of amino acids in the protein determines its function, such as an enzyme, antibody, hormone, or structural molecule. DNA mutations can occur by several mechanisms. Nucleotides may be deleted or added to the sequence, or they may be in an incorrect order. These mutations can be either hereditary or due to environmental factors leading to DNA damage.B. GENETIC TESTINGThe presence of certain genes can be detected via gene-specific tests.