Nucleotides can polymerize into chains of ribonucleic acid (RNA). RNA is very similar to deoxyribonucleic acid (DNA), with two crucial differences. Firstly the pentose sugar in RNA is ribose, which unlike the deoxyribose found in DNA, has five oxygen atoms as opposed to the four found in deoxyribose. Additionally, RNA does not contain thymine, rather it contains uracil, which serves a similar function to thymine, forming complementary base pairing with the adenine nucleobase (Figure 1).

Figure 1: Chemically, DNA and RNA differ in two ways. Firstly, DNA molecules contain deoxyribose as their pentose sugar, while RNA molecules contain ribose sugars. Secondly DNA molecules contain thymine molecules that form complementary base pairing with adenine molecules.

Ribonucleic acids are generally single stranded, though they can form complementary base pairings which leads to a double stranded structure. Like DNA, they also have directionality and are read in a 5' (5 prime) to 3' (3 prime) manner, which means from the end of the RNA strand that has the phosphate group attached, to the 5 carbon of the pentose molecule.

RNA is more diverse in its function than DNA is. There are three main types of RNA, there is messenger RNA (or mRNA), transmission or transfer RNA (tRNA) and ribosomal RNA (rRNA). Furthermore the RNA can be the genetic material for some organisms, notably viruses called retroviruses, where the instructions for how to reconstruct the organism are stored as RNA as opposed to DNA. Additionally, some RNA molecules have catalytic properties, similar to the protein based enzymes, allowing them to catalyze, or speed up certain reactions. It is believed that prior to the emergence of proteins that RNA molecules were the main enzymes and storage molecules in early life forms, before proteins and DNA emerged as specific molecules for these purposes.

mRNA is a molecule that is produced by DNA transcription. mRNA is a single stranded nucleic acid that is complementary to the DNA sequence it was transcribed from. mRNA is made in the nucleus of a cell, and is sent to the ribosome, where it is translated into protein, thus the name messenger RNA. mRNA is degraded by RNases, so mRNA is a short term messaging molecule, produced when transcription of a particular gene product is required. mRNA has also been used in new vaccines, by delivering an mRNA molecule that has the instructions to produce a protein found in the organism being vaccinated against.

tRNA molecules are involved in protein synthesis, where the mRNA gets translated into the primary sequence of amino acids. tRNA molecules bind to mRNA in the ribosome using base complementarity, and in doing so bring amino acids into proximity with the elongating peptide chain. Specific tRNA molecules bind to specific codons (three base) sequences in the mRNA, and bring specific amino acids into the protein sequence, which is the basis behind the genetic code.

rRNA or ribosomal RNA molecules are RNA molecules found in the ribosomes. The ribosomes are the parts of the cell that produce protein. The rRNA molecules catalyze the formation of the protein chains during protein translation, so they are ribozymes, or enzymes formed by RNA, as opposed to the more typical protein based enzymes. This is an example of the diversity of the functions of RNA.

There are also other forms of RNA that are used in research contexts, undertake particularly niche biochemical reactions, or have been explored as novel compounds, called biologics, to be used in the treatment of diseases. These include iRNA (interfering RNA), siRNA (small interfering RNA) or small nucleolar RNA (snoRNA). Furthermore, some viruses, notably retroviruses including HIV, store their genetic material as RNA, and when they infect cells convert their RNA into DNA as part of their infection process. This reflects the range and diversity of RNA functions in biological systems.