Nucleic Acids | Research & Encyclopedia Articles

Nucleic Acids

Nucleic acids are complex molecules that contain a cell's genetic information and the instructions for carrying out cellular processes. The two nucleic acids, ribonucleic acid (RNA) and deoxyribonucleic acid (DNA), work together.

A molecule is made of phosphate-base-sugar nucleotide chains; its three-dimensional shape affects its genetic function. In humans and other higher organisms, DNA is shaped in a two-stranded helix (spiral) and further organized on structures called chromosomes. DNA in some bacteria is circular. Most RNA molecules are single-stranded and take various shapes, such as a cloverleaf.

Nucleic acids were discovered by the Swiss biochemist Johann Miescher (1844-1895). Born in Basel where his father was a physician, Miescher was an assistant to Ernst Hoppe-Seyler at the University of Strasbourg (then in France, but soon to become part of Germany) in 1859 when he isolated a cellular substance containing nitrogen and phosphorus. Thinking it was a phosphorus-rich nuclear protein, Miescher named it nuclein.

It was actually a protein plus nucleic acid, as the German biochemist Albrecht Kossel discovered in the 1880s. Kossel, another former assistant of Hoppe-Selyer, 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-- ribose in 1909 and deoxyribose (meaning that it had less oxygen than ribose) in 1929. This meant that there were two nucleic acids, one named for each type of sugar. 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 the American James Watson and the British Maurice Wilkins and Francis Crick determined DNA's two-stranded helical shape. Adenine is always paired with thymine and guanine is always paired with the cytosine. In RNA, uracil replaces thymine. In the 1960s scientists discovered that three consecutive DNA or RNA bases (a codon) comprise the genetic code or instruction for production of a protein. The codons were matched to specific amino acids in the 1960s, mainly by the American Marshall Nirenberg who found that a gene may be one codon or many.

A gene is transcribed into messenger RNA (mRNA), which moves from the nucleus to structures in the cytoplasm called ribosomes. The American biochemist Mahlon Bush Hoagland discovered by accident in the 1950s that transfer RNA (tRNA) molecules already in the cytoplasm read the 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 "parent" strand and the " child" of the other strand going to each of the two new cells. The process is 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. In the 1990s scientists are determining the precise sequence of DNA nucleotides for humans and other species. The information available when these genome projects are completed is expected to allow further advances in genetics-based medicine.