All modern life on Earth uses three different types of biological molecules that each serve critical functions in the cell. Proteins are the workhorse of the cell and carry out diverse catalytic and structural roles, while the nucleic acids, DNA and RNA, carry the genetic information that can be inherited from one generation to the next.
RNA, which stands for ribonucleic acid, is a polymeric molecule made up of one or more nucleotides. A strand of RNA can be thought of as a chain with a nucleotide at each chain link. Each nucleotide is made up of a base (adenine, cytosine, guanine, and uracil, typically abbreviated as A, C, G and U), a ribose sugar, and a phosphate.
The structure of RNA nucleotides is very similar to that of DNA nucleotides, with the main difference being that the ribose sugar backbone in RNA has a hydroxyl (-OH) group that DNA does not. This gives DNA its name: DNA stands for deoxyribonucleic acid. Another minor difference is that DNA uses the base thymine (T) in place of uracil (U). Despite great structural similarities, DNA and RNA play very different roles from one another in modern cells.
RNA plays a central role in the pathway from DNA to proteins, known as the "Central Dogma" of molecular biology. An organism's genetic information is encoded as a linear sequence of bases in the cell's DNA. During the process known as transcription, a RNA copy of a segment of DNA, or messenger RNA (mRNA), is made. This strand of RNA can then be read by a ribosome to form a protein. RNAs also play important roles in protein synthesis, as will be discussed in the ribozyme section, as well as in gene regulation.
Another major difference between DNA and RNA is that DNA is usually found in a double-stranded form in cells, while RNA is typically found in a single-stranded form, as shown in the illustration above. The lack of a paired strand allows RNA to fold into complex, three-dimensional structures. RNA folding is typically mediated by the same type of base-base interactions that are found in DNA, with the difference being that bonds are formed within a single strand in the case of RNA, rather than between two strands, in the case of DNA.
Next: Exploring the RNA World.