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Polypeptide synthesis and processing

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Proteins are made up of amino acids linked by peptide bonds, therefore a protein may be referred to as a polypeptide (of course, some proteins such as haemoglobin have extra bits to them). All are encoded for by the information stored in DNA. Let's see how exactly this happens.


Transcription: DNA to mRNA

In a process called transcription, mRNA is formed based on DNA. The bases on the coding strand of DNA are transcribed into a new molecule, mRNA, which is synthesised by the enzyme RNA polymerase



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As you can see, the DNA double helix unwinds, RNA polymerase anneals to the coding strand and recruits freely available bases (A, U, C, G) to build an mRNA strand.


Splicing: pre-mRNA to mRNA

In eukaryotes, genes contain non-coding sequences which must be removed before mRNA is used to produce proteins. These are called introns as opposed to exons which are coding sequences. Splicing therefore is the process of excising (cutting out) introns to be left with mRNA containing purely coding sequence



Translation: mRNA to tRNA

The resulting mRNA finally leaves the nucleus where the above business had been taking place, and arrives in the cytoplasm where the final step takes place. More specifically, in ribosomes. Each mRNA codon is matched against an anticodon on tRNA, which is matched to its respective amino acid. This binds to the next amino acid and so forth, until a polypeptide is made.



Polypeptides often undergo further modification and combination to form the fully-fledged protein itself. For example, haemoglobin is made of 4 polypeptide chains. Some common post-translational modifications of polypeptides include phosphorylation and glycosylation, the respective addition of phosphate groups and sugars to proteins.



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