Two kinds of molecules participate in protein synthesis. Both are based on a similar building block, the nucleotide, giving them their name—nucleic acids. One of these molecules, deoxyribonucleic acid or DNA, is the genetic material, and the other, ribonucleic acid or RNA, is produced in the nucleus and moves to the cytoplasm, where it participates in protein synthesis. The study of how the information stored in DNA codes for RNA and protein is molecular genetics.
DNA and RNA are large molecules made up of sub-units called nucleotides. A nucleotide consists of a nitrogen-containing organic base, either in the form of a double ring (purine) or a single ring (pyrimidine). Nucleotides also contain a pentose (five-carbon) sugar and a phosphate (— P04) group. DNA and RNA molecules, however, differ in several ways. Both DNA and RNA contain the purine bases adenine and guanine, and the pyrimidine base cytosine. The second pyrimidine in DNA, however, is thymine, whereas in RNA it is
uracil. A second difference between DNA and RNA involves the sugar present in the nucleotides. The pentose of DNA is de-oxyribose, and in RNA it is ribose. A third important difference between DNA and RNA is that DNA is a double-stranded molecule and RNA is single stranded, although it may fold back on itself and coil.
The key to understanding the function of DNA is knowing how nucleotides link into a three-dimensional structure. The DNA molecule is ladder-like, with the rails of the ladder consisting of alternating sugar-phosphate groups. The phosphate of a nucleotide attaches at the fifth (5′) carbon of deoxyribose. Adjacent nucleotides attach to one another by a covalent bond between the phosphate of one nucleotide and the third (3′) carbon of deoxyribose. The pairing of nitrogenous bases between strands holds the two strands together. Adenine (a purine) is hydrogen bonded to its complement, thymine (a pyrimidine), and guanine (a purine) is hydrogen bonded to its complement, cytosine (a pyrimidine). Each strand of DNA is oriented such that the 3′ carbons of deoxyribose in one strand are oriented in the opposite directions from the 3′ carbons in the other strand.
The strands’ terminal phosphates are, therefore, at opposite ends, and the DNA molecule is thus said to be antiparallel (Gr. anti, against +para, beside + allelon, of one another). The entire molecule is twisted into a right-handed helix, with one complete spiral every 10 base pairs.