Information about the structure and function of a living organism is stored in nucleic acids so that it can be passed on to the next generation. Nucleic acids consist of only two types of molecules: deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). They are found in the cells of all living organisms, from viruses to humans.
Structures of nucleic acids
Both RNA and DNA are made up of recurring units called nucleotides, consisting of complexes of three different molecules: a five-carbon monosaccharide (sugar), an organic base, and phosphoric acid. In RNA, the sugar is ri-bose; in DNA, it is deoxyribose, a ribose derivative. There are four different organic bases in DNA. Cytosine (C) and thymine (T) are called pyrimidines, and adenine (A) and guanine (C) are called purines. In RNA, uracil (U) is substituted for thymine. Each sugar molecule is attached to one of the DNA bases, and linked to the next sugar molecule by bonds to the phosphoric acid. Hence, any sugar molecule in the middle of the chain is linked to one base and two phosphoric acid residues. Each sugar, base, and phosphoric acid unit is called a nucleotide.
Although the basic parts of nucleic acids had been known for many years, it was not until 1953 that Francis Crick of Britain and James Watson of the United States worked out the three-dimensional structure of the DNA molecule. They suggested that the bases of two nucleotide chains are connected together by hydrogen bonds. The sugar and the phosphate run alternately along each side, in a ladderlike structure. The bases connected by the hydrogen bonds form the ladder’s rungs, and the sugars and phosphates form the sides. The ladder is twisted into a regular helical formation, the famous DNA double helix. (A helix has a spiral, coiled form, like a spring.) The purine and pyrimidine bases always form complementary pairs. Adenine links with thymine, and guanine links with cytosine.
The DNA double helix stores all the information about the structural proteins and enzymes that make up an organism. A few viruses contain only RNA and no DNA. But in all other species, RNA’s function is to transcribe the information stored in DNA. The information is then transferred to sites in the cell, called ribosomes, where it is translated into the making of protein.
The genetic code
The sequence of the four organic bases in a DNA molecule forms what amounts to a four-letter code. This code must provide the words in an enormous encyclopedia of possible protein types. There are at least 20 words or amino acids in proteins. A single base cannot supply enough information to specify what is needed to make the protein. Each sequence of three bases, referred to as a codon, specifies a single amino acid in protein synthesis. There are 64 (4 x 4 x 4) different ways of combining the four amino acids in sequences of three. Of these combinations of the four bases—A, C, G, and T—a total of 61 code for one of the 20 specific amino acids. Several different combinations therefore code for the same amino acid. The remaining three compounds perform the same function as the period at the end of a sentence. They show that the last amino acid in the protein has been reached. This theory that three bases code for a particular amino acid is supported by experimental evidence.
DNA is found mainly in the nucleus of plant and animal cells. Proteins are manufactured in the cell, but outside the nucleus. They are manufactured by ribosomes within the cytoplasm, fluid that fills the inside of a cell. A complex chain of events links the DNA with the actual manufacture of protein.
The double helix of DNA is the largest molecule in the cell. RNA exists as much smaller molecules and in several different types. To relay information to the ribosome (which manufactures the protein), the two strands of the DNA double helix must first split apart, like a zipper. A molecule of messenger RNA !m-RNA is then formed from free nucleotides according to the code carried by DNA. The nucleotides pair with the bases of the section of DNA coded for the required protein. The RNA bases pair only with the complementary bases of the DNA. Thus, the information is coded “in negative.” The sequence of the RNA must be transcribed back into its original form. This is done after the m-RNA has moved out of the cell and taken up a position on the ribosome. Another RNA molecule called transfer RNA (t-RNA) picks up a free amino acid and takes it to the ribosome. The enzymes that control this attachment are highly specific. Each molecule of t-RNA carries only one type of amino acid.
The t-RNA molecule is smaller than m-RNA.
It consists of a single nucleotide chain twisted back on itself into a cloverleaf shape. At one end is a sequence of three bases that attach to the appropriate complementary codon (three-base unit) on the m-RNA. The amino acid at the other end is enzymatically joined to the polypeptide chain as the m-RNA slides along the ribosome. Several protein molecules may be formed simultaneously from the same m-RNA molecule.
The same genetic material is found in all the cells of an organism. But not all the cells produce the same proteins. There are also differences in the rates of production between cells. The mechanism by which the function of a gene (the section of DNA that codes for a particular protein) is controlled is not completely understood. Production of a protein can be stopped or slowed down in three ways: The DNA can stop making m-RNA, attachment on the ribosome can be prevented, or the rate at which m-RNA is destroyed can be increased.
DNA is a huge molecule—extremely complicated, but precisely ordered. Mistakes in its duplication occasionally occur; these are called mutations. They happen when the wrong base is coded or when sections of DNA are removed or put in the wrong place. The rate at which these mistakes occur is accelerated by certain chemicals and by ionizing radiation such as radiation from atomic fallout or accidents at nuclear power plants like Chernobyl), probably by inhibiting natural repair mechanisms. Many mutations are harmless, but some mutations produce inheritable diseases, usually where the change causes the production of the wrong amino acid. This renders an enzyme ineffective by alterinq its shape.
Mutations are known to cause cancer, which is an abnormal growth of particular cells. But not all mutations are harmful. From some, beneficial variations may arise, an important mechanism in the evolution of new species.