RNA is an extremely useful biological molecule, and has various essential uses in keeping a cell alive and allowing it to go about its natural business inside the body. In humans, the genetic code is made from DNA, which is short for deoxyribonucleic acid. RNA stands for ribonucleic acid, which simply means that it is quite similar to DNA, but instead of using a molecule called deoxyribose in its structure, it uses a ribose molecule instead.
A human cell generally has DNA packaged up in a nucleus, and all the types of RNA are designed to read information from DNA so a cell can make proteins from the DNA blueprint. Messenger RNA (mRNA) is found inside and outside of a nucleus, in the interior of the cell called the cytoplasm. Transfer RNA (tRNA) and ribosomal RNA (rRNA) are both present only in the cytoplasm of a cell.
mRNA is basically a backwards copy of a portion of the DNA sequence. It is like a photocopy of an important document that the cell can read when needed. tRNA molecules read this photocopy and collect the building blocks of proteins that the mRNA molecules contain code for. Finally, the rRNA completes the assembly of all the building blocks into one complete protein.
Structurally, mRNA is a single strand of little blocks called ribonucleotides. These are called uracil, adenine, cytosine and guanine. tRNA does not have as simple a structure as mRNA, because it folds itself up into a three-dimensional cloverleaf shape and then into an "L" shape. rRNA molecules do not work by themselves, but rather a few stick together with a variety of proteins to form a relatively large structure called a ribosome.
Since mRNA is a copied portion of the DNA blueprint, many different mRNA molecules can be present in a cell. These can vary in length depending on the size of the informational sequence that has to be copied. There are up to 100 different tRNAs in a human cell, as each individual building block of protein (amino acids) require at least one tRNA to be specific to itself. Many ribosomes (which contain rRNA) are also present in the cytoplasm, ready to make proteins when the cell needs them. Each ribosome contains three "small rRNAs" and one "large rRNA" molecule.