We earlier imagined DNA as an instruction book. Let's even make it a reference book. When you need information about something you make a copy of the pages (genes) you're interested in, returning the book to the library. This way you don't have to risk losing or destroying the book.
In all eucaryotic cells DNA never leaves the nucleus, instead the genetic code (the genes) is copied into RNA which then in turn is decoded (translated) into proteins in the cytoplasm. Why? Wouldn't it be smarter if DNA itself was translated into proteins in the cytoplasm instead of using a RNA intermediate?
The answer, for many reasons, is no. One important reason is security. The cytoplasm is a dangerous environment for the DNA and the daily transcription of genes to proteins would be very harmful to the DNA, which has to stay intact in order to maintain life. Therefore, RNA works as a sort of throw-away version of DNA (like the copies from the reference book) - good for limited work but not for long-term storage. Another reason is to regulate the rate of protein synthesis. This will be further discussed in the section about protein-translation.
Translation is the actual synthesis of a protein under the direction of mRNA. During this process the nucleotide sequence of an mRNA (messenger RNA) is translated into the amino acid sequence of a protein. The protein synthesis requires a technical machinery of high complexity. As compared to information transfer between nucleic acid molecules, where direct copying occurs on the basis of base complementarity, the translation process involves a greater number of chemical reactions and the participation of additional nucleic acid and protein components. One of these components, the ribosome, provides the basic machinery for the translation process. The major role of the ribosome is to catalyse coupling of amino acids into protein according to the sequence specified by the mRNA. The amino acids are brought to the ribosome by tRNA (transfer RNA) molecules.
The nucleotide sequence of the mRNA is composed of four different nucleotides whereas a protein is built up from 20 amino acids. To allow the four nucleotides to specify 20 different amino acids, the nucleotide sequence is interpreted in codons, groups of three nucleotides. These codons have their corresponding anticodon in the tRNA. Furthermore each anticodon is linked to one particular amino acid. Thus, each codon specifies one amino acid. This is referred to as the genetic code.
Translation may be divided into three distinct steps. The first, initiation, results in the formation of an initiation complex in which the ribosome is bound to the specific initiation (start) site on the mRNA while the initiator tRNA is annealed to the initiator codon and bound to the ribosome. The second stage, elongation, consists of joining amino acids to the growing polypeptide chain according to the sequence specified by the message. Incorporation of each amino acid occurs by the same mechanism. Thus, the same steps are repeated over and over again until the termination codon is reached in the message. The termination codon gives the signal for the third and last stage of protein synthesis, the termination, in which the ready-made protein is released from the ribosome.