Editing Protein biosynthesis (section)

==Translation==
{{main|Translation (biology)}}

[[File:Translation - cycle.png|upright=1.4|thumb|alt=Five strands of mRNA with all with a ribosome attached at different stages of translation. The first strand has a ribosome and one tRNA carrying its amino acid base pairing with the mRNA, the second strand has a ribosome and a second tRNA carrying an amino acid base pairing with the mRNA, the third strand has the ribosome catalysing a peptide bond between the two amino acids on the two tRNA's. The fourth strand has the first tRNA leaving the ribosome and a third tRNA with its amino acid arriving. The fifth strand has the ribosome catalysing a peptide bond between the amino acids on the second and third tRNA's with an arrowing indicating the cycle continues| Illustrates the translation process showing the cycle of tRNA codon-anti-codon pairing and amino acid incorporation into the growing polypeptide chain by the ribosome.]]
[[Image:Protein translation.gif|thumb|upright=1.2|right|A ribosome on a strand of mRNA with tRNA's arriving, performing codon-anti-codon base pairing, delivering their amino acid to the growing polypeptide chain and leaving. Demonstrates the action of the ribosome as a [[biological machine]] which functions on a [[Nanoscopic scale|nanoscale]] to perform translation. The ribosome moves along the mature mRNA molecule incorporating tRNA and producing a polypeptide chain.]]

During translation, ribosomes synthesize polypeptide chains from mRNA template molecules. In eukaryotes, translation occurs in the cytoplasm of the cell, where the ribosomes are located either free floating or attached to the [[endoplasmic reticulum]]. In prokaryotes, which lack a nucleus, the processes of both transcription and translation occur in the cytoplasm.<ref name="Khan Academy 2020">{{Cite web |title=Stages of translation (article) |url=https://www.khanacademy.org/science/biology/gene-expression-central-dogma/translation-polypeptides/a/the-stages-of-translation |access-date=10 March 2020 |website=Khan Academy |language=en}}</ref>

[[Ribosome]]s are complex [[molecular machine]]s, made of a mixture of protein and [[ribosomal RNA]], arranged into two subunits (a large and a small subunit), which surround the mRNA molecule. The ribosome reads the mRNA molecule in a 5'-3' direction and uses it as a template to determine the order of amino acids in the polypeptide chain.<ref name="Khan Academy 2020 - 2">{{Cite web |title=Nucleus and ribosomes (article) |url=https://www.khanacademy.org/science/biology/structure-of-a-cell/prokaryotic-and-eukaryotic-cells/a/nucleus-and-ribosomes |access-date=10 March 2020 |website=Khan Academy |language=en}}</ref> To translate the mRNA molecule, the ribosome uses small molecules, known as [[transfer RNA]]s (tRNA), to deliver the correct amino acids to the ribosome. Each tRNA is composed of 70-80 nucleotides and adopts a characteristic cloverleaf structure due to the formation of hydrogen bonds between the nucleotides within the molecule. There are around 60 different types of tRNAs, each tRNA binds to a specific sequence of three nucleotides (known as a [[codon]]) within the mRNA molecule and delivers a specific amino acid.<ref name="Cooper 2000">{{Cite book |title=The cell : a molecular approach |vauthors=Cooper GM |date=2000 |publisher=Sinauer Associates |isbn=9780878931064 |edition=2nd |location=Sunderland (MA)}}</ref>

The ribosome initially attaches to the mRNA at the [[start codon]] (AUG) and begins to translate the molecule. The mRNA nucleotide sequence is read in [[genetic code|triplets]]; three adjacent nucleotides in the mRNA molecule correspond to a single codon. Each tRNA has an exposed sequence of three nucleotides, known as the anticodon, which are complementary in sequence to a specific codon that may be present in mRNA. For example, the first codon encountered is the start codon composed of the nucleotides AUG. The correct tRNA with the anticodon (complementary 3 nucleotide sequence UAC) binds to the mRNA using the ribosome. This tRNA delivers the correct amino acid corresponding to the mRNA codon, in the case of the start codon, this is the amino acid methionine. The next codon (adjacent to the start codon) is then bound by the correct tRNA with complementary anticodon, delivering the next amino acid to ribosome. The ribosome then uses its [[peptidyl transferase]] enzymatic activity to catalyze the formation of the covalent peptide bond between the two adjacent amino acids.<ref name="Toole2015" />

The ribosome then moves along the mRNA molecule to the third codon. The ribosome then releases the first tRNA molecule, as only two tRNA molecules can be brought together by a single ribosome at one time. The next complementary tRNA with the correct anticodon complementary to the third codon is selected, delivering the next amino acid to the ribosome which is covalently joined to the growing polypeptide chain. This process continues with the ribosome moving along the mRNA molecule adding up to 15 amino acids per second to the polypeptide chain. Behind the first ribosome, up to 50 additional ribosomes can bind to the mRNA molecule forming a [[polysome]], this enables simultaneous synthesis of multiple identical polypeptide chains.<ref name="Toole2015" /> Termination of the growing polypeptide chain occurs when the ribosome encounters a stop codon (UAA, UAG, or UGA) in the mRNA molecule. When this occurs, no tRNA can recognise it and a [[release factor]] induces the release of the complete polypeptide chain from the ribosome.<ref name="Cooper 2000" /> Dr. [[Har Gobind Khorana]], a scientist originating from India, decoded the RNA sequences for about 20 amino acids.{{Citation needed|date=January 2021|reason=List exact rather than approximate number and provide source.}} He was awarded the [[Nobel Prize]] in 1968, along with two other scientists, for his work.