RNA is similar to DNA but differs in several respects. 1. It is shorter 2. It is single stranded (with few exception: few virus) 3. It is nuclear and cytoplasmic 4. It has ribose 5. It has uracil rather than thymine. The other bases are the same. There are three basic types of RNA: 1. Messenger RNA (mRNA): relatively long strands that encode the information from a single gene (DNA). It is the template for protein synthesis. This is the product of transcription. An mRNA is an RNA that is translated into protein. mRNAs are very short-lived compared to DNA. In prokaryotic cells a primary transcript is used directly as an mRNA (often times before it is even completely transcribed). In eukaryotic cells a primary transcript is processed before being exported from the nucleus as an mRNA: • A 5'CAP of 7-methyl guanosine is added. • A poly (A) tail is added to the 3' end of the transcript. • Introns (intervening sequences) must be cut from the transcript by a process known as RNA splicing. In prokaryotes - They are only around for a few minutes. - Continuous synthesis of protein requires a continuous synthesis of mRNA. This helps the prokaryotic cell respond quickly to a fluctuating environment and fluctuating needs. - The mRNA of prokaryotic cells is polycistronic (one transcript can code for several different proteins). In eukaryotic cells - The mRNA are stable for 4-24 hrs. - The mRNA of eukaryotic cells is monocistronic (each transcript only encodes a single protein) 2. ribosomal RNA (rRNA): Ribosomes are composed of rRNA and protein. The rRNA forms base pairs with the nucleotides of mRNA during translation (protein synthesis). 3. transfer RNA (tRNA): short (90 nucleotides) RNA molecules responsible for translating nucleic acid language to protein language. In other words the "adapter" molecule that converts nucleic acid sequence to protein sequence.