DNA
Figure 1. DNA is made up of nucleotides abbreviated as A, T, C and G. They pair as A–T and G-C 
DNA is the hereditary material of all living organisms, including humans. DNA stores information through a code made up of four chemical bases: A (adenine), T (thymine), C (cytosine), and G (guanine), and it is the order (or sequence) of these four bases that determines the information provided by DNA. The information is used as an instruction for building and maintaining all the cells of an organism. Figure 1. illustrates how these four bases are organized. The human genome consists of approximately 3.2 billion base pairs.
DNA structure
Figure 2. DNA is organized in a double-helix form. It is said to be antiparallel because one strand runs in the 5′→3′ direction and the other runs in the 3'→5' direction
DNA is composed of 2 strands running in opposite directions that are twisted together, also called the double helix. The strands are composed of nucleotides and deoxyribose sugars. A phosphodiester link connects the deoxyribose sugars with each other, and the hydrogen bonds between phosphates cause the DNA strand to twist. The two strands are bound with weak hydrogen bonds between the complementary nucleotides. The nitrogenous bases are “inside” like rungs on a ladder. Adenine binds with thymine and cytosine binds with guanine (Figure 2).
Collecting a DNA sample
DNA is stored in the nucleus of each cell. This means that DNA can be harvested from any sample containing eukaryotic cells. Usually, a blood sample is collected, and DNA is then extracted from the white blood cells (red blood cells do not contain a nucleus). If blood sampling is impossible or too risky (children, animals, forensics), smaller amounts of DNA can also be extracted from a mouth swab (containing epithelial cells) or a few hair follicles.
There are several types of DNA, including genomic, complementary, mitochondrial, and noncoding DNA. Genomic DNA is the full-length DNA sequence, including coding (exons) and noncoding (introns). All cells in our body will have the same genomic DNA sequence unless there has been a somatic mutation or aberrations that has occured during the course of life. Complementary DNA is DNA that is created by reverse transcribing RNA; it is usually used in the quantitative PCR method to quantify the level of gene expression. Because complementary DNA is created on the basis of RNA templates, it only contains the exons of the gene. DNA present in the mitochondria is also called mitochondrial DNA. In humans, mitochondrial DNA can only be passed on from the mother. Noncoding DNA denotes DNA sequences that do not code for any specific proteins. Humans have a high percentage of noncoding DNA in the genome; in fact, almost 98% of our genome consists of noncoding DNA!
DNA extraction
DNA can be extracted from any cell. First, we need to disrupt the cell (also known as cell lysis). This can be achieved using chemical and physical methods, grinding, or sonicating the cell. Once the cell is disrupted, we need to remove the membrane lipids by adding surfactants or detergents, digest the protein using proteases, and remove RNA using RNase. Following this, we can precipitate DNA (for example, using ethanol) and isolate DNA from all other materials present in a cell. There are several methods that one can choose to extract DNA; one of the most popular methods is the use of phenol and chloroform; however, newer methods that are easier and safer have been developed, for example, using column separation.