RNA is a macromolecule found in the nucleus and cytoplasm of cells which plays an important role in protein synthesis and other chemical activities of the cell. The structure of RNA is similar to that of DNA, except for it is usually single stranded unlike the DNA which is usually double stranded.

It composed of repeating nucleotide units of adenine, guanine, cytosine and uracil(thymine in DNA), whose ribose components are linked by phospho-diester bonds. It is also the genetic material in some viruses.

More generally, a molecule derived from DNA by transcription is the RNA molecule. Most common RNA types are messenger(m)-RNA, transfer(t)-RNA and ribosomal(r)-RNA.

Among these three varieties of RNA common in the mammalian cell, the most abundant is ribosomal RNA (rRNA), which occurs in two sizes, 28S (approximately 4600 nucleotides) and 18S (approximately 1800 nucleotides); together they form the basic core of the eukaryotic ribosome.

Messenger RNA (mRNA) is the term used to describe the mature form of the primary RNA transcript of the individual gene once it has been processed to eliminate introns and to contain a polyadenylated tail. mRNA links the coding sequence present in the gene to the ribosome, where it is translated into a polypeptide sequence.

Transfer RNA (tRNA) is the form of RNA used to shuttle successive amino acids to the growing polypeptide chain. A tRNA molecule contains an anti-codon, a three-nucleotide sequence by which the tRNA molecule recognizes the codon contained in the mRNA template, and an adapter onto which the amino acid is attached.

Some RNAs facilitate the biochemical modification of itself or other RNA molecules.(miRNA, siRNA,snRNA, hnRNA etc.)

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