RNA

1. DNA has 2'-deoxyribose for its sugar base; RNA has ribose. Both are pentose sugars, but the lack of oxygen in 2'-deoxyribose prevents that carbon from being used in bond formation (both hydrogen bonds and covalent bonds -- the removal of introns from RNA transcripts involves 2'-5' bonds made between parts of the strand).

2. RNA pairs adenine with uracil; DNA uses thymine, which is 5-methyl-uracil. The methyl group on thymine allows for more stable hydrogen bonding to make the characteristic double helix of DNA, while RNA is usually found single-stranded (with some exceptions). Also, the methyl group on thymine allows for a method of correcting errors in DNA sequence; there is an enzyme that reacts with 2'-deoxyuridine and replaces it with 2'-deoxythymidine in DNA.

Also, as January mentioned, it is thought that RNA may have been the start of life on this planet. It has been shown that RNA can spontaneously evolve from free nucleotides under the theoretical conditions of the planet 4 billion years ago. Given its ability to carry information as well as its ability to act as an enzyme (ribozyme), it seems a likely candidate.

RNA stands for ribonucleaic acid and is a polynucleotide similar to DNA. It is composed of many nucleotides, each of which is made up of three different parts: a phosphate group, a ribose sugar, and one of four nitrogenous bases (adenine, guanine, cytosine and uracil). This is similar to DNA but has two main differences: DNA uses a deoxyribose sugar instead of a ribose sugar, and DNA uses thymine instead of uracil.

RNA performs three main functions in the body, all associated with protein synthesis. The genes for specific proteins, encoded in sets of DNA triplets, are copied into RNA form in a process called transcription in the nucleus. The single strand of RNA formed by this is called messenger RNA (mRNA). The mRNA leaves the nucleus and travels to one of many organelles called ribosomes, which assemble proteins from basic amino acids according to the information on the mRNA strand. Ribosomes are made out of ribosomal RNA (rRNA) and proteins. When protein synthesis occurs at the ribosomes, the amino acids are found bonded to molecules of transport RNA (tRNA) which forms a complex shape so that it bonds to a specific amino acid and has an exposed anticodon, which is the reverse of the RNA triplet for that amino acid. In the ribosome, each mRNA triplet is matched to the opposite tRNA anticodon, one triplet at time as each amino acid bonds to the one before it, creating a polypeptide.

The fact RNA is so important in protein synthesis, as well as its appearence in the place of DNA in some viruses, lead many people to believe that RNA was used as a form of information storage before DNA and DNA storage gradually evolved as it is much more stable than RNA.