deoxyribonucleic acid - definition of deoxyribonucleic acid in the Medical dictionary - by the Free Online Medical Dictionary, Thesaurus and Encyclopedia.

deoxyribonucleic acid /de·oxy·ri·bo·nu·cle·ic ac·id/ (DNA) (-ri″bo-noo-kle´ik) the nucleic acid in which the sugar is deoxyribose; composed also of phosphoric acid and the bases adenine, guanine, cytosine, and thymine. It constitutes the primary genetic material of all cellular organisms and the DNA viruses and occurs predominantly in the nucleus, usually as a double helix (q.v.), where it serves as a template for synthesis of ribonucleic acid (transcription).

The DNA double helix. A, Diagrammatic model of the helical structure, showing its dimensions, the major and minor grooves, the periodicity of the bases, and the antiparallel orientation of the backbone chains (represented by ribbons). The base pairs (represented by rods) are perpendicular to the axis and lie stacked one on another. B, The chemical structure of the backbone and bases of DNA, showing the sugar phosphate linkages of the backbone and the hydrogen bonding between the base pairs. There are two hydrogen bonds between adenine and thymine, and three between cytosine and guanine.

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DNA.

Deoxyribonucleic acid (DNA)

The genetic material in cells that holds the inherited instructions for growth, development, and cellular functioning.

Mentioned in: Gene Therapy, Genetic Testing, Von Willebrand Disease, Wilson Disease

deoxyribonucleic acid (DNA)

[dē·ok′sirī′bōno̅o̅klē′ik]

a large, double-stranded, helical molecule that is the carrier of genetic information. In eukaryotic cells, it is found principally in the chromosomes of the nucleus. DNA is composed of four kinds of serially repeating nucleotide bases: adenine, cytosine, guanine, and thymine. Genetic information is coded in the sequence of the nucleotides. Also called desoxyribonucleic acid. See also nucleic acid, ribonucleic acid.

deoxyribonucleic acid (DNA) probes,

n a nucleic acid fragment labeled with a radioisotope that is complementary to a sequence in another nucleic acid fragment that will bind to it and thus identify it. It is used as a diagnostic tool to identity the species of microbe involved in an infectious process such as refractory periodontal disease.

deoxyribonucleic acid

a nucleic acid occurring in cells as the basic structure of the genes. DNA is present in all body cells of every species, including unicellular organisms and DNA viruses. The structure of DNA was first described in 1953 by J.D. Watson and F.H.C. Crick.

DNA molecules are long linear polymers of small molecules called nucleotides, each of which consists of one molecule of the five-carbon sugar deoxyribose bonded to a phosphate group and to one of four heterocyclic nitrogenous compounds referred to as bases. A single strand of DNA is made by linking the nucleotides together in a chain with bonds between the sugar and phosphate groups of adjacent nucleotides. It thus consists of a backbone of alternating sugar and phosphate groups with a base attached to each sugar as a side chain. The four bases are two purines, adenine (A) and guanine (G), and two pyrimidines, cytosine (C) and thymine (T). Single-stranded DNA can be synthesized with any specified sequence of bases, but in living cells the base sequence has a meaning; it specifies the amino acid sequence of all of the polypeptides and proteins made by the cell. And since all of the enzymes that catalyze biochemical reactions are proteins, the DNA contains the specifications for all of the biochemistry and structure of the cell.

The chemical basis of the genetic code lies in the ability of the bases to form hydrogen bonds with each other. Unlike the covalent bonds holding together the atoms of a single strand of DNA, hydrogen bonds are weak and easily broken and reformed. Hydrogen bonding is governed by the base pairing rule: A always bonds with T, and C always bonds with G. A and T (or C and G) are called complementary bases. The genetic information is read and preserved by the matching up of complementary bases.

In cells, the DNA is double-stranded. The configuration of the DNA molecule resembles a ladder in which the sides are the sugar-phosphate backbones, which are antiparallel (they run in opposite directions), and the rungs are hydrogen-bonded complementary bases; thus, the entire sequence along the two strands is complementary. This whole structure is twisted so that the two strands form a double helix. Once before each cell division, a group of proteins splits the two strands apart, and as complementary nucleotides bond to the bases of each strand they are jointed to form a new strand. This process is called replication. It results in the exact duplication of the DNA molecule, because each strand serves as a template (pattern) for the synthesis of its complementary strand. When the cell divides, one copy goes to each daughter cell. Thus, the genetic information is passed on from generation to generation without change except for rare mutations, which result from copying errors or incorrectly repaired breaks in the DNA molecule that change the base sequence.

The reading of the genetic code involves two processes: transcription and translation. In transcription, a length of DNA is used as a template to make a complementary strand of messenger RNA (mRNA). RNA (ribonucleic acid) is a nuceic acid like DNA. The only differences are that the sugar, ribose, has an extra oxygen atom, and the pyrimidine base, uracil (U), which also pairs with adenine, replaces thymine. In translation, the mRNA molecule is read by a structure called a ribosome, which produces the polypeptide specified by the mRNA message.

The genetic code is a triplet code. Every triplet of bases along the strand specifies a single amino acid. There are 64 possible triplets (codons) that can be formed from the four bases. Each one specifies that one of 20 different amino acids be inserted in a growing polypeptide chain or marks either the start or the end of a chain.

Two other types of RNA are involved in translation. Ribosomal RNA (rRNA) forms a large part of the ribosome. Transfer RNA (tRNA) is the means by which codons are matched with amino acids. tRNAs are small molecules with several self-complementary sections so that they fold up into a compact structure owing to bonding between complementary bases. One end of the molecule is a three-base anticodon, which bonds to its complementary codon on mRNA molecules. The other end is recognized by a specific enzyme which attaches the correct amino acid to it. During translation, the ribosome proceeds along the mRNA molecule and, as each codon is matched by a specific tRNA, the amino acid it carries is transferred to the growing polypeptide chain, and the process is repeated until the 'stop' codon is reached. Like the mRNA molecules, rRNA and tRNA molecules are formed on DNA templates; the genetic material contains not only the information for polypeptide sequences but also for rRNA and tRNA sequences.

The chromosomes of mammalian cells contain 3 × 10⁹ base pairs which is enough to code for the 100,000 or so enzymes and structural proteins. Less than 10% of the DNA codes for proteins and RNA, the rest is noncoding, also referred to as 'junk' DNA, and is of uncertain purpose. DNA is the molecule that directs all of the activities of living cells, including its own reproduction and perpetuation in generation after generation.

deoxyribonucleic acid

See DNA.