Polynucleotides

James Richard Fromm


A polymer of mononucleotides is called a polynucleotide. In polynucleotides, only one phosphoric acid is present on each ribose sugar so hydrolysis of polynucleotides produces equimolar solutions of nitrogenous base, ribose sugar, and phosphate. The phosphoric acid component of polynucleotides readily loses a proton and so polynucleotides are also called nucleic acids. Nucleic acids include either ribose sugars (ribonucleic acids, often abbreviated RNA), or 2'-deoxyribose sugars (deoxyribonucleic acids, often abbreviated DNA). Only one type of sugar is found to be included in each polymer.

Polynucleotides, both DNA and RNA, are the information carriers of living organisms and play the central role in reproduction. The simplest form of life may be the virus. A virus is a packet of infectious polynucleotides, which may be either RNA or DNA and either single-stranded or double-stranded, surrounded by a protective protein coat. One of the most well-understood small viruses is the tobacco mosaic virus (TMV virus, which infects the leaves of tobacco plants.

The tobacco mosaic virus particle, or virion, contains one single-stranded RNA molecule which is about 6400 nucleotides long. Its coat is a protein which contains 336,540 amino acid residues, made up of 2130 identical subunits each 158 amino acid residues long.

The structure of the TMV virion is very stable and the virions can remain infective for decades, although in the absence of the protein coat the TMV RNA is easily attacked by enzymes. In 1955, Heinz Fraenkel-Conrat and Robley Williams showed that under suitable conditions of pH and salt content the separate coat subunits and RNA of TMV spontaneously self-assemble into virions which are indistinguishable in structure and infectivity from the original TMV which contained them. This stable and infective form of the virion has a protein coat which is just sufficient to cover the RNA. Each virion is a rod about 30 micrometers long and 1.8 micrometers in diameter which has a mass of about 30 million daltons.

The segment of a DNA or RNA molecule which carries the genetic information required for the synthesis of a single polypeptide chain is called a gene. Each TMV virion is believed to contain six genes. More complex viruses, and all higher forms of life, contain far more genes.

The T4 bacteriophage virus, which attacks the common Escherichia coli human intestinal bacteria, is a much more complex virus than the tobacco mosaic virus. Its genetic information, about 165 genes, is contained in a double-stranded DNA molecule. The T4 bacteriophage virion consists of this DNA molecule inside a complex structured protein coat. The protein coat consists of a hollow head which contains the DNA molecule, a tail, and a baseplate with six short spikes and six long fibers. The tail can contract, and the entire assembly serves much as a hypodermic syringe to inject the DNA of the virion into the host bacterial cell.

Viruses, whether simple or complex, cannot generate metabolic energy nor can they synthesize proteins. They reproduce the proteins necessary to make up their protective coats by taking over control of the protein-synthesizing mechanism of the cells they infect, and use the metabolic energy generated by the host cell. The effect of this is usually to destroy the host cell. For example, the T4 bacteriophage virion ruptures the bacterial cell wall, destroying the cell, when the assembly of its daughter virions is complete. The entire process from infection to release of about two hundred new virions takes about twenty minutes.

Higher organisms are made up of cells which are surrounded by a cell wall or cell membrane. The simplest cells are the prokaryotic cells or prokaryotes which are found as some of the simpler one-celled algae and bacteria. The common human intestinal bacterium Escherichia coli is a typical eucaryote; its cells are about 200 micrometers in diameter. Prokaryotic cells contain only one chromosome, consisting of a single molecule of closely coiled double-stranded DNA. Different lengths of this chromosome each correspond to a different gene of the procaryotic cell. A bacterium may contain several thousand genes.

Human cells, like those of all higher animals and plants, are eukaryotic cells. Eucaryotic cells are 100 to 10,000 times as large as prokaryotes. Eukaryotic cells contain a nucleus, which is separated from the rest of the cell contents by a nuclear membrane. This nuclear membrane is in addition to the cell membrane. The nucleus contains the genetic material of the cell, which consists of several to many chromosomes. Each chromosome consists of closely coiled double-stranded DNA. Human cells contain 21 chromosomes, and like other eukaryotic cells contain millions of different genes.

The transcription of genetic information into molecular and ultimately cellular structure is the subject of molecular genetics. The central canon of molecular genetics is that information flows from DNA to RNA to proteins. This means that the order of bases in DNA, which is the actual genetic information transferred in reproduction in more complex living organisms, is first expressed in the order of bases within smaller RNA polynucleotides. The sequence or order of bases within the RNA molecules then is expressed as the sequence of the amino acids which form a protein.

The order of amino acids in a protein is its primary structure. Most proteins will under physiological conditions spontaneously assume the most stable configurations, forming sulfide bonds and hydrogen bonds, and so establish unique secondary and tertiary structures. The information coded in the base sequence of DNA is thus expressed in the actual physical structure and nature of the protein. The mechanism by which this transfer of information occurs, and the actual assembly of proteins by living cells, is probably the single most active area of research in biochemistry and molecular genetics. Only a few of its highlights are mentioned in the following sections.


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Copyright 1997 James R. Fromm