Structure & Function of RNA
Although RNA, or ribonucleic acid, is structurally very similar to DNA, there are some important chemical differences that are highlighted in this animation. First, RNA contains ribose sugar, which has both 2' and 3' hydroxyl groups. RNA also uses the pyrimidine uracil rather than thymine. Finally, while double-stranded RNA does exist in viruses, animal RNA tends to be a single-stranded molecule.
The most interesting differences between DNA and RNA, however, lie in their comparative functions. DNA has only one function, the storage and transmission of genetic information. RNA, on the other hand, is not so limited.
Three types of RNA play critical roles in protein synthesis, as described earlier in this web site. Messenger RNA (mRNA) serves as the template for protein synthesis, while transfer RNA (tRNA) decodes the mRNA message and brings the correct amino acid to the ribosome. Finally, the ribosomes themselves contain ribosomal RNA (rRNA) as a critical component.
RNA can also carry out chemical reactions, like an enzyme does. Tom Cech's lab discovered this fascinating fact in 1982 while studying the mechanism for rRNA intron splicing in a protozoan called Tetrahymena. When the investigators removed every biomolecule from the splicing reaction tube, except the rRNA itself, they showed that splicing still occurred! This meant that the RNA was carrying out the splicing reaction itself.
Finally, RNA is often involved in the regulation of gene expression. For example, the Xist gene makes an RNA product that binds to DNA on the X chromosome, and thus promotes the chemical changes that are associated with X inactivation. In other words, this RNA helps turn off all of the genes on an entire chromosome.
The variety of activities that RNA can carry out has led many scientists to speculate that RNA might have been the first genetic/catalytic material on earth - predating both proteins and DNA.