DNA

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For a non-technical introduction to the topic, see Introduction to genetics.

The structure of part of a DNAdouble helix

For other uses, see DNA (disambiguation).

Deoxyribonucleic acid (DNA) is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms and some viruses. The main role of DNA molecules is the long-term storage of information. DNA is often compared to a set of blueprints or a recipe, or a code, since it contains the instructions needed to construct other components of cells, such as proteins and RNA molecules. The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in regulating the use of this genetic information.

Chemically, DNA consists of two long polymers of simple units called nucleotides, with backbones made of sugars and phosphate groups joined by ester bonds. These two strands run in opposite directions to each other and are therefore anti-parallel. Attached to each sugar is one of four types of molecules called bases. It is the sequence of these four bases along the backbone that encodes information. This information is read using the genetic code, which specifies the sequence of theamino acids within proteins. The code is read by copying stretches of DNA into the related nucleic acid RNA, in a process called transcription.

Within cells, DNA is organized into long structures called chromosomes. These chromosomes are duplicated before cells divide, in a process called DNA replication.Eukaryotic organisms (animals, plants, fungi, and protists) store most of their DNA inside the cell nucleus and some of their DNA in organelles, such as mitochondriaor chloroplasts.^[1] In contrast, prokaryotes (bacteria and archaea) store their DNA only in the cytoplasm. Within the chromosomes, chromatin proteins such ashistones compact and organize DNA. These compact structures guide the interactions between DNA and other proteins, helping control which parts of the DNA are transcribed.

This article is part of the series on: Gene expression a Molecular biology topic (portal) (Glossary)

Introduction to Genetics

General flow: DNA > RNA > Protein

special transfers (RNA > RNA, RNA > DNA, Protein > Protein)

Genetic code

Transcription

Transcription (Transcription factors, RNA Polymerase,promoter) Prokaryotic / Archaeal / Eukaryotic

post-transcriptional modification (hnRNA,Splicing)

Translation

Translation (Ribosome,tRNA) Prokaryotic / Archaeal / Eukaryotic

post-translational modification (functional groups, peptides, structural changes)

gene regulation

epigenetic regulation (Genomic imprinting)

transcriptional regulation

post-transcriptional regulation (sequestration, alternative splicing,miRNA)

translational regulation

post-translational regulation (reversible,irreversible)

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Contents  [hide] 1 Properties 1.1 Grooves 1.2 Base pairing 1.3 Sense and antisense 1.4 Supercoiling 1.5 Alternate DNA structures 1.6 Quadruplex structures 1.7 Branched DNA 2 Chemical modifications 2.1 Base modifications 2.2 Damage 3 Biological functions 3.1 Genes and genomes 3.2 Transcription and translation 3.3 Replication 4 Interactions with proteins 4.1 DNA-binding proteins 4.2 DNA-modifying enzymes 4.2.1 Nucleases and ligases 4.2.2 Topoisomerases and helicases 4.2.3 Polymerases 5 Genetic recombination 6 Evolution 7 Uses in technology 7.1 Genetic engineering 7.2 Forensics 7.3 Bioinformatics 7.4 DNA nanotechnology 7.5 History and anthropology 8 History of DNA research 9 See also 10 References 11 Further reading 12 External links

Properties

Chemical structure of DNA. Hydrogen bonds shown as dotted lines.

DNA is a long polymer made from repeating units called nucleotides.^[2][3][4] The DNA chain is 22 to 26 Ångströms wide (2.2 to 2.6 nanometres), and one nucleotide unit is 3.3 Å (0.33 nm) long.^[5] Although each individual repeating unit is very small, DNA polymers can be very large molecules containing millions of nucleotides. For instance, the largest human chromosome, chromosome number 1, is approximately 220 million base pairs long.^[6]

In living organisms, DNA does not usually exist as a single molecule, but instead as a pair of molecules that are held tightly together.^[7][8] These two long strands entwine like vines, in the shape of a double helix. The nucleotide repeats contain both the segment of the backbone of the molecule, which holds the chain together, and a base, which interacts with the other DNA strand in the helix. A base linked to a sugar is called a nucleoside and a base linked to a sugar and one or more phosphate groups is called a nucleotide. If multiple nucleotides are linked together, as in DNA, this polymer is called a polynucleotide.^[9]

The backbone of the DNA strand is made from alternating phosphate and sugar residues.^[10] The sugar in DNA is 2-deoxyribose, which is a pentose (five-carbon) sugar. The sugars are joined together by phosphate groups that form phosphodiester bonds between the third and fifth carbon atoms of adjacent sugar rings. These asymmetric bonds mean a strand of DNA has a direction. In a double helix the direction of the nucleotides in one strand is opposite to their direction in the other strand: the strands are antiparallel. The asymmetric ends of DNA strands are called the 5′ (five prime) and 3′ (three prime) ends, with the 5' end having a terminal phosphate group and the 3' end a terminal hydroxyl group. One major difference between DNA and RNA is the sugar, with the 2-deoxyribose in DNA being replaced by the alternative pentose sugar ribose in RNA.^[8]

A section of DNA. The bases lie horizontally between the two spiraling strands.^[11]Animated version at File:DNA orbit animated.gif.

The DNA double helix is stabilized by hydrogen bonds between the bases attached to the two strands. The four bases found in DNA areadenine (abbreviated A), cytosine (C), guanine (G) and thymine (T). These four bases are attached to the sugar/phosphate to form the complete nucleotide, as shown for adenosine monophosphate.

These bases are classified into two types; adenine and guanine are fused five- and six-membered heterocyclic compounds calledpurines, while cytosine and thymine are six-membered rings called pyrimidines.^[8] A fifth pyrimidine base, called uracil (U), usually takes the place of thymine in RNA and differs from thymine by lacking a methyl group on its ring. Uracil is not usually found in DNA, occurring only as a breakdown product of cytosine. In addition to RNA and DNA, a large number of artificial nucleic acid analogues have also been created to study the proprieties of nucleic acids, or for use in biotechnology.^[12]