RNA

=RNA=


 * RNA ** (also **ribonucleic acid **) is a nucleic acid closely related to DNA . RNA performs a variety of functions in living cells, some listed below, and constitutes the genome of some viruses: notably RNA viruses and retroviruses.

 Unlike DNA, the monomers of RNA ( **ribonucleotides **) contain ribose sugar rather than deoxyribose sugar. Ribose is similar in structure to deoxyribose, except that it has OH groups on both its 2' and 3' carbons (while deoxyribose has only a H atom on its 2' carbon). Another difference is that in RNA, the nucleobase thymine is substituted for uracil. Uracil is still able to base-pair with adenine using two hydrogen bonds.

 RNA is usually, though not always, found in single-stranded form in the cellular environment. When in the cytoplasm, it is usually bound to chaperone proteins to form **ribonucleoprotein particles** (or **RNPs**).


 * TYPES OF RNA AND THEIR CELLULAR ROLES: **


 * **Messenger RNA **  (also **mRNA) is the molecule formed by DNA transcription. It is complementary to the template (non-coding) strand of DNA and carries this information out through the nuclear envelope to the cytoplasm, where protein biosynthesis occurs at the ribosomes. mRNA is made up of a series of base triplets (codons) that are complementary to the base triplets (anticodons) of various tRNAs (see below) **


 * **Transfer RNA  (also  tRNA) is responsible for carrying amino acids to the ribosomes in order for them to be assembled into a polypeptide. When charged with amino acid, the molecule become s aminoacyl-tRNA **  in a process catalysed by **<span style="font-family: Arial,Helvetica,sans-serif; font-size: 90%;">aminoacyl-tRNA synthetase ** <span style="font-family: Arial,Helvetica,sans-serif; font-size: 90%; font-weight: normal;">. Aminoacyl-tRNA has the amino acid attached at its 3' end by means of an ester bond


 * **<span style="font-family: Arial,Helvetica,sans-serif; font-size: 90%;">Ribosomal RNA **<span style="font-family: Arial,Helvetica,sans-serif; font-size: 90%;"> (also **<span style="font-family: Arial,Helvetica,sans-serif; font-size: 90%;">rRNA **<span style="font-family: Arial,Helvetica,sans-serif; font-size: 90%;">) forms the catalytic subunits of both prokaryotic and eukaryotic ribosomes, with the remaining ribosomal structure being formed of protein. rRNA can be seen as the mediator between mRNA and tRNA, as it decodes the message in mRNA and receives amino acids from tRNA by means of a **<span style="font-family: Arial,Helvetica,sans-serif; font-size: 90%;">peptidyl transferase **<span style="font-family: Arial,Helvetica,sans-serif; font-size: 90%;">. The size of rRNAs may be measured by sedimentation rate (in Sveltberg units, S). Eukaryotic ribosomes are 70S with 50S and 30S subunits, while prokaryotic ribosomes are 80S with 60S and 40S subunits. The Sveltberg units are not summative, because sedimentation rate isn't purely about mass, but also about the shape of the subunits