- Review concepts of DNA and genetics.
- Discuss genetic polymorphism and its implications.
- Outline nomenclature used in pharmacogenomics.
The double helix structure of deoxyribonucleic acid (DNA) was discovered in 1953 by Watson and Crick. The now famous structure is composed of two DNA molecules that run in opposite directions and are wound around each other in a clockwise direction. Each polynucleotide chain is composed of a phosphate backbone, sugars, and the four nucleotide bases adenine (A), cytosine (C), guanine (G), and thymine (T). One of the forces holding the two strands together are hydrogen bonds formed between the nucleotide bases of the opposite strands called base pairs (A and T form two and C and G form three hydrogen bonds). Within cells of higher organisms, genomic DNA (gDNA) is associated with many proteins, among them histones that help organize these macromolecules into chromosomes. Unless a eukaryotic cell is dividing, gDNA is highly condensed and almost exclusively located in the cell nucleus. In humans, there are approximately 3 billion base pairs (bp) that are organized in 46 chromosomes (two sets of 23, one set inherited from each parent). gDNA is also called the “blueprint” of life, because it encodes information for proteins and RNA. Specifically, the sequence of the four base “letters” A, C, G, and T determines the sequence of amino acids in proteins and the sequence composition of RNAs in the cell. However, only a small portion of the gDNA serves this purpose. Other areas of the gDNA harbor binding sites for regulatory proteins such as transcription factors or encode noncoding RNAs such as microRNA that play a role in the regulation of gene and/or protein expression.1,2 More insight is also being gained about the role of epigenetics. The term epigenetics refers to changes in gene expression that are not explained by the DNA sequence itself, but by DNA modifications such as methylation and acetylation. Biological consequences may also arise from structural variations across the genome such as copy number variations (CNVs).
Sources of DNA Used for Genetic Analyses
gDNA can essentially be extracted from any part of the human body. The most widely utilized sources of gDNA are blood and saliva or cheek brushings (also known as mouth swabs or buccal brushes), because they are relatively easy to collect and bear only minimal risk. Whole blood is easy to handle and store and high-quality and sufficient amounts of gDNA are typically obtained from specimen as little as a few drops. Blood may also be collected in concert with clinical samples to minimize blood draws. Mouth swabs using cotton or bristle brushes collect saliva and/or mucosal epithelium cells by placing them into the mouth or brushing along the side of the cheek. The amount of DNA retrieved from such samples may vary considerably and may allow for only a limited number of genetic tests to be performed. Also, the integrity of gDNA may ...