The nucleus of every one of your cells contains the DNA that is the software for how your body works. When the nucleus needs to direct something, it uncompresses the appropriate part of the DNA strand, and then an RNA copy is made, called "messenger RNA" (mRNA), and exported from the nucleus into the outer part of the cell.
The instructions on the mRNA specify what proteins should be manufactured in the cell. Some are structural or functional; others are for controlling the effects of other processes. Every protein is different.
How does the cell build the protein from the instructions in the mRNA? That's accomplished by a little device called a ribosome, two pieces of RNA coiled up into exactly the right shape to perform this function.
The ribosome latches onto the mRNA strand, like the head of a Turing machine, and then waits around for the right amino acid (“peptide”) to come by, carried by another specially-shaped piece of RNA called a “transfer RNA” (tRNA). Only the right kind of tRNA, carrying the right peptide, can connect to the ribosome while it's sitting there scanning the first three regions of the mRNA.The peptide is held in place by the ribosome and the tRNA detaches and goes off to find another peptide.
The ribosome moves down to the next three sites on the mRNA and the process repeats. As the ribosome moves along, it builds a chain of connected peptides, not just any chain but exactly the chain specified by the mRNA it's scanning.
Eventually the ribosome reaches a special code that tells the ribosome to let the completed protein float away.
(Of course, it's vastly more complicated than that.)
