- Abbreviation of:
- deoxyribonucleic acid
DNA
What does DNA do?
What is DNA made of?
Who discovered the structure of DNA?
Can you edit DNA?
What’s the difference between DNA and RNA?
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DNA, organic chemical of complex molecular structure that is found in all prokaryotic and eukaryotic cells and in many viruses. DNA codes genetic information for the transmission of inherited traits.
A brief treatment of DNA follows. For full treatment, see genetics: DNA and the genetic code.
Discovery
- DNA was first isolated in 1869 by Swiss chemist Friedrich Miescher, who extracted a previously unknown substance from the nuclei of white blood cells and called it “nuclein”; its significance was not yet understood.
- In 1944, bacteriologist Oswald Avery and colleagues Maclyn McCarty and Colin MacLeod demonstrated that DNA, not protein, was the substance responsible for transferring genetic traits in bacteria.
- In 1952, scientist Rosalind Franklin, using X-ray diffraction, captured her famous “Photo 51,” which revealed the helical shape of DNA.
- In 1953, using experimental data from several researchers, including Franklin and Maurice Wilkins, biochemist James Watson and biophysicist Francis Crick described DNA as two intertwined strands held together by complementary base pairing.
The chemical DNA was first discovered in 1869, but its role in genetic inheritance was not demonstrated until 1943. In 1953 James Watson and Francis Crick, aided by the work of biophysicists Rosalind Franklin and Maurice Wilkins, determined that the structure of DNA is a double-helix polymer, a spiral consisting of two DNA strands wound around each other in opposite (antiparallel) directions. The breakthrough led to significant advances in scientists’ understanding of DNA replication and hereditary control of cellular activities.
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Structure
Each strand of a DNA molecule is composed of a long chain of monomer nucleotides. The nucleotides of DNA consist of a deoxyribose sugar molecule to which is attached a phosphate group and one of four nitrogenous bases: two purines (adenine and guanine) and two pyrimidines (cytosine and thymine). The nucleotides are joined together by covalent bonds between the phosphate of one nucleotide and the sugar of the next, forming a phosphate-sugar backbone linked by phosphodiester bonds, from which the nitrogenous bases protrude. One strand is held to another by hydrogen bonds between the bases; the sequencing of this bonding is specific—i.e., adenine bonds only with thymine, and cytosine only with guanine.
The configuration of the DNA molecule is highly stable, allowing it to act as a template for the replication of new DNA molecules, as well as for the production (transcription) of the related RNA (ribonucleic acid) molecule, which differs from DNA by containing ribose sugar and the nitrogenous base uracil instead of thymine. A segment of DNA that codes for the cell’s synthesis of a specific protein is called a gene.
Replication and organization
DNA replicates by separating into two single strands, each of which serves as a template for a new strand. The new strands are copied by the same principle of hydrogen-bond pairing between bases that exists in the double helix. Two new double-stranded molecules of DNA are produced, each containing one of the original strands and one new strand. This “semiconservative” replication is the key to the stable inheritance of genetic traits.
Within a cell, DNA is organized into dense protein-DNA complexes called chromosomes. In eukaryotes, DNA is wrapped around proteins called histones to form chromatin, and the chromosomes are located in the nucleus, although DNA also is found in mitochondria and chloroplasts. In prokaryotes, which do not have a membrane-bound nucleus, the DNA is found as a single circular chromosome in the cytoplasm.
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Alternative DNA structures and forms
Some prokaryotes, such as bacteria, and a few eukaryotes contain small, circular molecules of extrachromosomal DNA known as plasmids. Unlike chromosomal DNA, plasmids replicate independently within the cell and often carry genes that provide selective advantages, such as antibiotic resistance or the ability to metabolize unusual substances. Because plasmids can be transferred between cells, they play an important role in horizontal gene transfer (the movement of genetic material between distinct organisms), particularly among bacteria. In biotechnology and molecular biology, plasmids have been widely used as vectors in recombinant DNA technology to insert, replicate, and study genes, as well as to produce proteins such as insulin and other medically important substances.
In cellular organisms, alternative DNA structures can also occur under certain conditions. Among these are G-quadruplexes, four-stranded DNA configurations that form in guanine-rich areas of the genome. These structures have been identified in human DNA and are thought to play roles in regulating gene expression, DNA replication, and chromosome stability. Researchers are also investigating their possible involvement in cancer and other diseases, since G-quadruplex formation can influence how genes are activated or suppressed.
Viral genomes vary widely in size, structure, and replication strategy. The genetic material of viruses, for example, may be single- or double-stranded DNA or RNA. Retroviruses carry their genetic material as single-stranded RNA and use an enzyme known as reverse transcriptase to synthesize DNA from the RNA template after infecting a host cell. The newly formed viral DNA can then integrate into the host genome, allowing the virus to replicate using the host’s cellular machinery.