 Proteins are assembled from amino acids using information encoded in genes . Each protein has its own unique amino acid sequence that is specified by the nucleotide sequence of the gene encoding this protein . The genetic code is a set of three @-@ nucleotide sets called codons and each three @-@ nucleotide combination <unk> an amino acid , for example <unk> ( <unk> @-@ <unk> @-@ <unk> ) is the code for <unk> . Because DNA contains four <unk> , the total number of possible codons is 64 ; hence , there is some <unk> in the genetic code , with some amino acids specified by more than one codon . <unk> encoded in DNA are first transcribed into pre @-@ messenger RNA ( mRNA ) by proteins such as RNA polymerase . Most organisms then process the pre @-@ mRNA ( also known as a primary transcript ) using various forms of Post @-@ transcriptional modification to form the mature mRNA , which is then used as a template for protein synthesis by the ribosome . In prokaryotes the mRNA may either be used as soon as it is produced , or be bound by a ribosome after having moved away from the <unk> . In contrast , eukaryotes make mRNA in the cell nucleus and then <unk> it across the nuclear membrane into the cytoplasm , where protein synthesis then takes place . The rate of protein synthesis is higher in prokaryotes than eukaryotes and can reach up to 20 amino acids per second . 
