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'''Protein biosynthesis''' (or '''protein synthesis''') is a core biological process, occurring inside [[Cell (biology)|cells]], [[homeostasis|balancing]] the loss of cellular [[protein]]s (via [[Proteolysis|degradation]] or [[Protein targeting|export]]) through the production of new proteins. Proteins perform a number of critical functions as [[enzyme]]s, structural proteins or [[hormone]]s. Protein synthesis is a very similar process for both [[prokaryote]]s and [[eukaryote]]s but there are some distinct differences.<ref name="Alberts 2015">{{cite book | vauthors = Alberts B |title=Molecular biology of the cell |date=2015 |publisher=Garland Science, Taylor and Francis Group |location=Abingdon, UK |isbn=978-0815344643 |edition= Sixth}}</ref>
Protein synthesis can be divided broadly into two phases: [[Transcription (biology)|transcription]] and [[Translation (biology)|translation]]. During transcription, a section of [[DNA]] encoding a protein, known as a [[gene]], is converted into a template molecule called [[messenger RNA]] (mRNA). This conversion is carried out by enzymes, known as [[RNA polymerases]], in the [[cell nucleus|nucleus of the cell]].<ref name="O'Connor 2010">{{cite book | vauthors = O'Connor C |title=Essentials of Cell Biology |date=2010 |publisher=Cambridge, MA |location=NPG Education |url=https://www.nature.com/scitable/ebooks/essentials-of-cell-biology-14749010/ |access-date=3 March 2020}}</ref> In eukaryotes, this mRNA is initially produced in a premature form ([[Primary transcript|pre-mRNA]]) which undergoes [[post-transcriptional modification]]s to produce [[Mature messenger RNA|mature mRNA]]. The mature mRNA is exported from the cell nucleus via [[nuclear pore]]s to the [[cytoplasm]] of the cell for translation to occur. During translation, the mRNA is read by [[ribosome]]s which use the [[nucleotide]] sequence of the mRNA to determine the sequence of [[amino acid]]s. The ribosomes catalyze the formation of [[covalent bond|covalent]] [[peptide bond]]s between the encoded amino acids to form a [[polypeptide chain]].{{cn|date=March 2024}}
Following translation the polypeptide chain must fold to form a functional protein; for example, to function as an enzyme the polypeptide chain must fold correctly to produce a functional [[active site]]. To adopt a functional three-dimensional shape, the polypeptide chain must first form a series of smaller underlying structures called [[protein secondary structure|secondary structures]]. The polypeptide chain in these secondary structures then folds to produce the overall 3D [[protein tertiary structure|tertiary structure]]. Once correctly folded, the protein can undergo further maturation through different [[post-translational modification]]s, which can alter the protein's ability to function, its location within the cell (e.g. cytoplasm or nucleus) and its ability to [[protein–protein interaction|interact with other proteins]].<ref name="Wang 2013">{{cite journal | vauthors = Wang YC, Peterson SE, Loring JF | title = Protein post-translational modifications and regulation of pluripotency in human stem cells | journal = Cell Research | volume = 24 | issue = 2 | pages = 143–160 | date = February 2014 | pmid = 24217768 | pmc = 3915910 | doi = 10.1038/cr.2013.151 | doi-access = free }}</ref>
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Histone-based regulation of DNA transcription is also modified by acetylation. Acetylation is the reversible covalent addition of an [[acetyl group]] onto a lysine amino acid by the enzyme [[acetyltransferase]]. The acetyl group is removed from a donor molecule known as [[Acetyl-CoA|acetyl coenzyme A]] and transferred onto the target protein.<ref name="Drazic 2016">{{cite journal | vauthors = Drazic A, Myklebust LM, Ree R, Arnesen T | title = The world of protein acetylation | journal = Biochimica et Biophysica Acta (BBA) - Proteins and Proteomics | volume = 1864 | issue = 10 | pages = 1372–1401 | date = October 2016 | pmid = 27296530 | doi = 10.1016/j.bbapap.2016.06.007 | doi-access = free }}</ref> [[Histone acetylation and deacetylation|Histones undergo acetylation]] on their lysine residues by enzymes known as [[histone acetyltransferase]]. The effect of acetylation is to weaken the charge interactions between the histone and DNA, thereby making more genes in the DNA accessible for transcription.<ref name="Bannister 2011">{{cite journal | vauthors = Bannister AJ, Kouzarides T | title = Regulation of chromatin by histone modifications | journal = Cell Research | volume = 21 | issue = 3 | pages = 381–395 | date = March 2011 | pmid = 21321607 | pmc = 3193420 | doi = 10.1038/cr.2011.22 | doi-access = free }}</ref>
The final, prevalent post-translational chemical group modification is phosphorylation. Phosphorylation is the reversible, covalent addition of a [[phosphate]] group to specific amino acids ([[serine]], [[threonine]] and [[tyrosine]]) within the protein. The phosphate group is removed from the donor molecule [[Adenosine triphosphate|ATP]] by a protein [[kinase]] and transferred onto the [[hydroxyl]] group of the target amino acid, this produces [[adenosine diphosphate]] as a
===Addition of complex molecules===
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