Deoxyribonucleic acid and ribonucleic acid -- DNA and RNA -- are closely related molecules that participate in transmitting and expressing genetic information. Both consist of molecular chains containing alternating units of sugar and phosphate. Nitrogen-containing molecules, called nucleotide bases, hang off each sugar unit. The different sugar units in DNA and RNA are responsible for the differences between the two biochemicals.
Ribose, the sugar of RNA, has a ring structure arranged as five carbon atoms and one oxygen atom. Each carbon binds to a hydrogen atom and a hydroxyl group, which is a molecule of one oxygen and one hydrogen atom. Deoxyribose is identical to RNA except that one carbon binds to a hydrogen atom instead of a hydroxyl group. This one difference means that two strands of DNA can form a double-helix structure while RNA remains as a single strand. DNA’s double-helix structure is very stable, giving it the ability to encode information for a long time. The cell creates RNA as needed during the process of transcription, but DNA is self-replicating.
Each sugar unit in DNA and RNA binds to one of four nucleotide bases. Both DNA and RNA use the bases A, C and G. However, DNA uses the base T while RNA uses the base U instead. The sequence of bases along the strands of DNA and RNA is the genetic code that tells the cell how to make proteins. In DNA, the bases of each strand bind to the bases on the other strand, forming the double-helix structure. In DNA, A’s can only bind to T’s and C’s can only bind to G’s. The structure of a DNA helix is preserved in a protein-RNA cocoon called a chromosome.
The cell makes protein by transcribing DNA to RNA and then translating the RNA into proteins. During transcription, a portion of the DNA molecule, called a gene, is exposed to enzymes that assemble RNA strands according to the nucleotide-base binding rules. The one difference is that DNA A bases bind to RNA U bases. The enzyme RNA polymerase reads each DNA base in a gene and adds the complementary RNA base to the growing RNA strand. In this way, DNA’s genetic information is transmitted to RNA.
The cell also uses a second type of RNA to make ribosomes, which are tiny protein-making factories. A third type of RNA helps transfer amino acids to growing protein strands. DNA plays no role in translation. RNA’s extra hydroxyl groups make it a more reactive molecule that is less stable in alkaline conditions than DNA. The tight structure of a DNA double helix makes it less vulnerable to enzyme action, but RNA is more resistant to ultraviolet rays.
