Editing Protein (section)

===Biosynthesis===

[[File:Ribosome mRNA translation en.svg|thumb|A ribosome produces a protein using mRNA as template]]
[[File:Genetic code.svg|thumb|class=skin-invert-image|The [[DNA]] sequence of a gene [[genetic code|encodes]] the amino acid sequence of a protein]]

{{Main|Protein biosynthesis}}

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 [[codon]]s and each three-nucleotide combination designates an amino acid, for example AUG ([[adenine]]–[[uracil]]–[[guanine]]) is the code for [[methionine]]. Because [[DNA]] contains four nucleotides, the total number of possible codons is 64; hence, there is some redundancy in the genetic code, with some amino acids specified by more than one codon.<ref name = "Van_Holde_1996" />{{rp|1002–42}} Genes encoded in DNA are first [[transcription (genetics)|transcribed]] into pre-[[messenger RNA]] (mRNA) by proteins such as [[RNA polymerase]]. Most organisms then process the pre-mRNA (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 [[prokaryote]]s the mRNA may either be used as soon as it is produced, or be bound by a ribosome after having moved away from the [[nucleoid]]. In contrast, [[eukaryote]]s make mRNA in the [[cell nucleus]] and then [[Protein translocation|translocate]] it across the [[nuclear membrane]] into the [[cytoplasm]], where [[protein biosynthesis|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.<ref name=Pain2000/>

The process of synthesizing a protein from an mRNA template is known as [[translation (genetics)|translation]]. The mRNA is loaded onto the ribosome and is read three nucleotides at a time by matching each codon to its [[base pair]]ing [[anticodon]] located on a [[transfer RNA]] molecule, which carries the amino acid corresponding to the codon it recognizes. The enzyme [[aminoacyl tRNA synthetase]] "charges" the tRNA molecules with the correct amino acids. The growing polypeptide is often termed the ''nascent chain''. Proteins are always biosynthesized from [[N-terminus]] to [[C-terminus]].<ref name = "Van_Holde_1996" />{{rp|1002–42}}

The size of a synthesized protein can be measured by the number of amino acids it contains and by its total [[molecular mass]], which is normally reported in units of ''daltons'' (synonymous with [[atomic mass unit]]s), or the derivative unit kilodalton (kDa). The average size of a protein increases from Archaea to Bacteria to Eukaryote (283, 311, 438 residues and 31, 34, 49 kDa respectively) due to a bigger number of [[protein domain]]s constituting proteins in higher organisms.<ref name="Kozlowski2016">{{cite journal | vauthors = Kozlowski LP | title = Proteome-pI: proteome isoelectric point database | journal = Nucleic Acids Research | volume = 45 | issue = D1 | pages = D1112–D1116 | date = January 2017 | pmid = 27789699 | pmc = 5210655 | doi = 10.1093/nar/gkw978 }}</ref> For instance, [[yeast]] proteins are on average 466 amino acids long and 53 kDa in mass.<ref name=Lodish2004/> The largest known proteins are the [[titin]]s, a component of the [[muscle]] [[sarcomere]], with a molecular mass of almost 3,000 kDa and a total length of almost 27,000 amino acids.<ref name=Fulton1991/>