Nucelic Acids
Nucleic acids are long chain molecules such as DNA (deoxyribonucleic acid) and RNA (ribonucleic acid) that link together individual nucleotides that are composed of a pentose sugar, a nitrogenous base, and one or more phosphate groups.
The most common nucleotides, the building blocks of nucleic acids found in DNA are deoxyadenylic acid, deoxyguanylic acid, deoxythymidylic acid, and deoxycytidylic acid. Each of these carries a nitrogenous base: deoxyadenylic acid contains adenine (A), deoxyguanylic acid contains gutamine (G), deoxythymidylic acid contains thymine (T), and deoxycytidylic acid contains cytosine (C). The nucleotides subunits of ribonucleic acid are adenylic acid, cytidylic acid, guanylic acid, and uridylic acid. Each of the RNA subunit nucleotides carries a nitrogenous base: adenylic acid contains adenine (A), cytidylic acid contains cytosine (C), guanylic acid contains guanine (G), and uridylic acid contains uracil.
Nucleic acids are complex molecules that contain a cell's genetic information and the instructions for carrying out cellular processes. In eukaryotic cells, the two nucleic acids, ribonucleic acid (RNA) and deoxyribonucleic acid (DNA), work together to direct protein synthesis.
A DNA molecule is made of phosphate-base-sugar nucleotide chains, and its three-dimensional shape affects its genetic function. In humans and other higher organisms, DNA is shaped in a two-stranded spiral helix organized into structures called chromosomes. DNA or RNA in some bacteria is circular. Most RNA molecules are single-stranded and take various shapes.
Nucleic acids were first identified by the Swiss biochemist Johann Miescher (1844-1895). Miescher isolated a cellular substance containing nitrogen and phosphorus. Thinking it was a phosphorus-rich nuclear protein, Miescher named it nuclein.
The substance identified by Miescher was actually a protein plus nucleic acid, as the German biochemist Albrecht Kossel discovered in the 1880s. Kossel also isolated nucleic acids' two purines (adenine and guanine) and three pyrimidines (thymine, cytosine, and uracil), as well as carbohydrates.
The American biochemist Phoebus Levene, who had once studied with Kossel, identified two nucleic acid sugars. Levene identified ribose in 1909 and deoxyribose (a molecule with less oxygen than ribose) in 1929. Levene also defined a nucleic acid's main unit as a phosphate-base-sugar nucleotide. The nucleotides' exact connection into a linear polymer chain was discovered in the 1940s by the British organic chemist Alexander Todd.
In 1951 American molecular biologist James Watson and the British molecular biologists Francis Crick and Maurice Wilkins developed a model of DNA that proposed its now accepted two-stranded helical shape in which adenine is always paired with thymine and guanine is always paired with the cytosine. In RNA, uracil replaces thymine.
During the 1960s scientists discovered that three consecutive DNA or RNA bases (a codon) comprise the genetic code or instruction for production of a protein. A gene is transcribed into messenger RNA (mRNA), which moves from the nucleus to structures in the cytoplasm called ribosomes. Codons on the mRNA order the insertion of a specific amino acid into the chain of amino acids that are part of every protein. Codons can also order the translation process to stop. Transfer RNA (tRNA) molecules already in the cytoplasm read the codon instructions and bring the required amino acids to a ribosome for assembly.
Some proteins carry out cell functions while others control the operation of other genes. DNA is replicated (completely copied) when a cell prepares to divide, one copy (strand) ultimately goes to each of the two new cells in a process called semiconservative replication. Until the 1970s cellular RNA was thought to be only a passive carrier of DNA instructions. It is now known to perform several enzymatic functions within cells, including transcribing DNA into messenger RNA and making protein. In certain viruses called retroviruses, RNA itself is the genetic information. This, and the increasing knowledge of RNA's dynamic role in DNA cells, has led some scientists to believe that RNA was the basis for the Earth's earliest life forms, an environment called the RNA World.
Since the 1970s nucleic acids' cellular processes have become the basis for genetic engineering, in which scientists add or remove genes in order to alter the characteristics or behavior of cells. Such techniques are used in agriculture, pharmaceutical and other chemical manufacturing, and medical treatments for cancer and other diseases.
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