Introduction to Nucleic Acids
Nucleic Acids, organic compounds found in the chromosomes of living cells and in viruses. The structure of the nucleic acids in a cell determines the structure of the proteins produced in that cell. Since proteins are the "building blocks" of life, nucleic acids can be considered the "blueprints" of life. Nucleic acids were discovered by Friedrich Miescher, a Swiss biochemist, in 1869. He called them "nucleic" because he believed that they occurred only in the nucleus of the cell.
There are two main types of nucleic acids, which differ slightly in their chemical composition. Deoxyribonucleic acid (abbreviated DNA) ordinarily occurs only in the cell nucleus. Ribonucleic acid (abbreviated RNA) is found both in the nucleus and in the cytoplasm (the main part of the cell exclusive of the nucleus). Viruses usually contain only a single molecule of either DNA or RNA.
Genes, the basic units of heredity, consist solely of DNA (or RNA, as in some viruses). The DNA within a cell can be thought of as a master die, or template, from which proteins are built. A type of RNA called messenger RNA, or m-RNA, transmits information from DNA to the ribosomes, structures in the cytoplasm that assemble amino acids into proteins. Transfer RNA, or t-RNA, transports amino acids from the cytoplasm to the ribosomes. Another form of RNA, called r-RNA, is part of the structure of the ribosomes. Other forms of RNA act as enzymes, or catalysts, in protein production. RNA-containing viruses called retroviruses can reverse the usual cellular process and transmit genetic information from RNA to DNA instead of from DNA to RNA.
Composition and Structure
The molecular structures of DNA and RNA are very similar. Nucleic acids have a long "backbone" consisting of alternate sugar and phosphate molecules. In RNA the sugar is ribose and in DNA it is deoxyribose. Attached to each sugar molecule is a nitrogen-containing compound that is a base, a compound capable of accepting protons.
In both DNA and RNA there are four major bases. Those in DNA are called adenine, guanine, cytosine, and thymine. RNA has the first three of these plus uracil. The substitution of uracil for thymine as a base material constitutes the chief chemical difference between RNA and DNA.
Each subunit of a nucleic-acid molecule, consisting of a phosphate, sugar, and base, is known as a mononucleotide. A complete nucleic-acid molecule consists of a large number of these mononucleotides (as many as 400,000) linked together to form a polynucleotide.
The size of a molecule is indicated by its molecular weight, which is the sum of the atomic weights of all the atoms in the molecule. Nucleic acids are among the largest known molecules on earth, with molecular weights in the millions. By comparison, the molecular weight of water is 18.
Individual molecules of nucleic acids cannot be seen with optical microscopes, but they can be studied with the electron microscope and other nonoptical magnifying instruments.
In 1951-53 the British molecular biologist F. H. C. Crick and the United States biochemist James D. Watson, in association with the British physicist Maurice H. F. Wilkins, analyzed photographic images produced by X rays directed through DNA molecules. They determined that the DNA in cells is usually in the form of a double stranded helix (the shape of two stretched, intertwined coiled springs), one DNA molecule being wrapped around another. RNA, on the other hand, is usually a single strand.
The backbone, according to the Watson Crick model, forms the coil of the helix, and the bases jut inward toward the axis of the helix. The bases on the two backbones are joined to one another in pairs by relatively weak chemical bonds called hydrogen bonds. The hydrogen bonds linking the bases in DNA are easily broken, so that the two strands can be partially or completely separated. The adenine on one backbone is always paired with thymine on the other, and guanine is always paired with cytosine.
Hereditary Function of DNA
To geneticists, the arrangement of bases in a DNA molecule spells out a "language" known as the genetic code. A sequence of three specific bases forms the code for one amino acid in a protein. Since DNA contains four different bases, each of which occurs thousands of times in human cells, the number of possible different arrangements of these bases is astronomically high. This makes possible the wide diversity that exists in the genetic makeup of human beings. At conception, the characteristics of the offspring are determined by the arrangement of bases in the DNA of the parents.
A mutation (an alteration in the DNA) in the gametes of either parent is sufficient to cause a noticeable change in the offspring. Such alterations can be caused by radiation from such sources as cosmic rays and nuclear explosions.
Research and Applications
The study of nucleic acids and other living matter at the molecular level is known as molecular biology. Research in this field has proceeded at a rapid pace since the 1950's. In 1957, the American biochemist Arthur Kornberg synthesized (produced) inactive DNA in the laboratory. In 1967 Kornberg and his associates synthesized biologically active DNA, which was able to infect living cells.
By the mid-1970's, scientists had synthesized many types of DNA and were performing a variety of experiments in molecular biology. Some of these experiments led to the development of genetic engineering, a process in which some DNA is taken from one organism and combined with DNA in another organism. Through genetic engineering, the hereditary traits of an organism can be altered. Among the many uses of genetic engineering are the creation of bacteria that synthesize certain drugs and the creation of disease-resistant crops. An international project called the Human Genome Project was begun in 1990 to determine the sequence of the 3 billion nucleotides that make up the DNA in a human being; it was largely completed in 2004.
The use of genetic fingerprinting, a technique based on the individual variations of human DNA, aids in the positive identification of persons from their hair, skin, body fluids, or other body tissues.