Structures of nucleic acids

Both RNA and DNA are made up of recurring units called nucleotides. These consist 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.

The organic bases of DNA include two compounds and two molecules. The two py-rimidine compounds are cytosine (C) and thy­mine (T). The two purine molecules are ade­nine (A) and guanine (G). In RNA, uracil (U) is substituted for thymine. The sugar molecule is attached to both the phosphoric acid and the base. The phosphoric acid is linked to the sugar of the next nucleotide. Hence, any sugar molecule in the middle of the chain is linked to one base and two phosphoric acid residues.

Although the basic parts of nucleic acids had been known for many years, it was not until 1953 that Francis Crick and James Watson of Cambridge University 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. This structure is similar to that of a ladder. The bases con­nected by the hydrogen bonds form the rungs. The sugars and phosphates form the sides of the ladder. 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 are always found in complementary pairs. Ade­nine links with thymine. Guanine is in combi­nation with cytosine.

The DNA double helix stores all the infor­mation about the structural proteins and en­zymes that make up the organism. A few vi­ruses contain only RNA and do not possess DNA. But in all other species, the purpose of the RNA is to transcribe the information stored in the 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 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 obviously does not give sufficient information to specify what is needed to make the protein. Three bases to­gether give a choice of 64 (4X4X4) combina-

Biochemistry: Nucleic acids 115

tions. Of these combinations of the four bases—A, C, G, and T— a total of 61 code for 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. The three-base unit is referred to as acodon.

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.

Protein synthesis

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, along the appropriate part of the mole­cule. A molecule of messenger RNA (m-RNA) is then formed from free nucleotides. 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.

Mutations

DNA is a huge, extremely complicated mole­cule. It is inevitable that 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. Ionizing radiation (like atomic fallout) and some chemicals in­crease the rate at which these mistakes occur. They probably inhibit the natural repair mech­anisms. Some mutations produce inheritable diseases. This usually happens where the change causes the production of the wrong amino acid. This renders an enzyme ineffective by altering its shape.

Mutations are now also known to cause cancer, which is an abnormal growth of partic­ular cells. But not all mutations are damaging. Some cause beneficial variation in a species, which is an important mechanism in evolution.

NH

OH

OH

+

Water

OH

+

Phosphate ion

+

Energy

Date: 2015-12-11; view: 1897