What is rRNA, mRNA, tRNA?

rRNA

Ribosomal RNA (rRNA) along with ribosomal proteins make up ribosomes, the "workbenches" on which polypeptides (proteins) are synthesized. It turns out that it is actually rRNA, and not a protein, in the large subunit of the ribosome that performs the peptidyl transferase function of linking amino acids together via peptide bonds. In eukaryotes, the genes coding for rRNAs are located in the nucleolus of the nucleus.

mRNA

The message carrying the information needed to make a particular polypeptide exists in the mRNA molecule. It binds with a ribosome and the ribosome starts reading it one codon - 3 consecutive mRNA bases - at a time. Each of the possible 64 codons codes for a particular amino acid, or for a release factor (in which case it is a STOP codon). So the order of bases in the mRNA specifies the order in which amino acids are linked together to form a polypeptide. mRNA is often said to be linear, but actually the two ends can link up and form a loop.

tRNA

A tRNA molecule has 2 main sites. At one end it has an amino-acid attachment site and on the opposite end it has a 3-base anticodon. An enzyme (an aminoacyl-tRNA synthetase) recognizes the type of tRNA and attaches the appropriate amino acid to it, at which point the tRNA is said to be charged. Charged tRNA molecules "bump into" the empty ribosome A site, but only if there is a complementary match between the mRNA codon associated with that site and the anticodon on the tRNA does the charged tRNA dock. tRNA is shaped like an uppercase L.

All of the RNAs are found in the nucleus because that is where they are synthesized (and processed), although none of them actyally function in the nucleus.

rRNA remains in the nucleus (nucleolus) to combine with certain proteins to form the ribosomal subunits - the subunits are then exported from the nucleus and they form a ribosome in the cytosol when mRNA binds to it.

tRNA and mRNA are exported from the nucleus and play their roles in the cytosol.