DNA Typing Health Article

Definition

DNA typing is a laboratory procedure that detects normal variations in a sample of DNA (deoxyribonucleic acid). DNA typing is most often used to establish identity, parentage, family relationship and appropriate matches for transplantation of organs and tissues.

DNA

DNA is a molecule that stores genetic information required for the development of the body and the control of cellular processes. Each strand of DNA is made of individual nucleotides that are joined together. Each nucleotide is made up of a phosphate group, a five-carbon sugar (deoxyribose), and an organic base. Adenine, thymine, guanine, and cytosine are the four bases found in DNA. The sequence of bases in DNA determines the genetic code of an individual. Every person excluding identical twins has a different sequence of bases.

The DNA sequence is made up of coding and noncoding regions. A coding region is a section of DNA, called an exon, that contains the instructions for the production of a particular protein. The primary structure of proteins is determined by the sequence of bases in the exons of a gene. Exons are located within the genes but are separated by non-coding regions called introns. Genes make up about 5% of human DNA and the other 95% consists of non-coding regions. The function of the non-coding regions is for the most part unknown.

Normal variations can occur in the DNA sequence of the coding and non-coding regions. Sequence polymorphism and length polymorphism are the two main forms of DNA variation. Sequence polymorphism results from differences in the sequence of bases at a particular locus. A locus is a specific location on a DNA molecule. Length polymorphism results from differences in the length of DNA at a particular locus. Differences in the length of the DNA are due to variations in the number of times that a certain sequence of bases is repeated. The number of times that a certain sequence is repeated at a specific locus will often vary between individuals. A locus that has a repeated unit of nine to ninety-eight bases is called a variable number tandem repeat locus (VNTR) or minisatellite. Loci that have a repeating unit of only two to seven bases are called short tandem repeats (STRs) or microsatellites. For example CAGACAGACAGA is an STR of four bases that is repeated three times.

Nuclear and mitochondrial DNA

Two strands of single stranded DNA wind together to form double stranded DNA in the form of a double helix. The DNA strands are held together by hydrogen bonds that form between the bases. Adenine joins with thymine and guanine joins with cytosine. Two sequences are said to be complementary if they have a sequence that allows them to join together to form double stranded DNA.

Most DNA is packaged with proteins to form microscopic structures called chromosomes. Chromosomes are found in the nucleus of each cell of the body and can be visualized under the microscope. Each cell of the body, except for the egg and the sperm cells, normally contains 22 pairs of chromosomes and two sex chromosomes (46 chromosomes in total). The egg and sperm cells each contain 23 chromosomes.

DNA is also found in the mitochondria. The mitochondria are energy producing organelles found in most cells. There are many mitochondria found in each cell. Each mitochondria contains one copy of circular DNA. Since there are many mitochondria in each cell, a lot of mitochondrial DNA may be present in only a small sample of cells. Mitochondrial DNA is found in the egg cells but not the sperm cells. Mitochondrial DNA is, therefore, only passed down from a mother to her offspring.