Protein biosynthesis


Protein biosynthesis

Protein biosynthesis (synthesis) is the process in which cells build proteins. The term is sometimes used to refer only to protein translation but more often it refers to a multi-step process, beginning with amino acid synthesis and transcription which are then used for translation. Protein biosynthesis, although very similar, differs between prokaryotes and eukaryotes.

Amino acid synthesis

Amino acids are the monomers which are polymerized to produce proteins. Amino acid synthesis is the set of biochemical processes (metabolic pathways) which build the amino acids from carbon sources like glucose. Not all amino acids may be synthesised by every organism, for example adult humans have to obtain 8 of the 20 amino acids from their diet.

Transcription

Transcription is the process by which an mRNA template, encoding the sequence of the protein in the form of a trinucleotide code, is transcribed from the genome to provide a template for translation. Transcription copies the template from one strand of the DNA double helix, called the template strand.

Transcription can be divided into 3 stages: Initiation, Elongation and Termination, each regulated by a large number of proteins such as transcription factors and coactivators that ensure the correct gene is transcribed in response to appropriate signals.

The DNA strand is read in the 3' to 5' direction and the mRNA is transcribed in the 5' to 3' direction by the RNA polymerase.

Translation

The synthesis of proteins is known as translation. Translation occurs in the cytoplasm where the ribosomes are located. Ribosomes are made of a small and large subunit which surrounds the mRNA. In translation, messenger RNA (mRNA) is decoded to produce a specific polypeptide according to the rules specified by the genetic code. This uses an mRNA sequence as a template to guide the synthesis of a chain of amino acids that form a protein. Translation is necessarily preceded by transcription. Translation proceeds in four phases: activation, initiation, elongation and termination (all describing the growth of the amino acid chain, or polypeptide that is the product of translation).

In activation, the correct amino acid (AA) is joined to the correct transfer RNA (tRNA). While this is not technically a step in translation, it is required for translation to proceed. The AA is joined by its carboxyl group to the 3' OH of the tRNA by an ester bond. When the tRNA has an amino acid linked to it, it is termed "charged".Initiation involves the small subunit of the ribosome binding to 5' end of mRNA with the help of initiation factors (IF), other proteins that assist the process.Elongation occurs when the next aminoacyl-tRNA (charged tRNA) in line binds to the ribosome along with GTP and an elongation factor.Termination of the polypeptide happens when the A site of the ribosome faces a stop codon (UAA, UAG, or UGA). When this happens, no tRNA can recognize it, but releasing factor can recognize nonsense codons and causes the release of the polypeptide chain.The capacity of disabling or inhibiting translation in protein biosynthesis is used by antibiotics such as: anisomycin, cycloheximide, chloramphenicol, tetracycline, streptomycin, erythromycin, puromycin etc.

Events following protein translation

The events following biosynthesis include post-translational modification and protein folding.During and after synthesis, polypeptide chains often fold to assume, so called, native secondary and tertiary structures. This is known as "protein folding".

Many proteins undergo "post-translational modification". This may include the formation of disulfide bridges or attachment of any of a number of biochemical functional groups, such as acetate, phosphate, various lipids and carbohydrates. Enzymes may also remove one or more amino acids from the leading (amino) end of the polypeptide chain, leaving a protein consisting of two polypeptide chains connected by disulfide bonds.

ee also

*Genetic code
*Cistron
*Operon
*"lac" operon

External links

* [http://www.ncbi.nlm.nih.gov/books/bv.fcgi?highlight=translation&rid=cooper.section.1167#1178 Translation of mRNA] Section of "The Cell: A Molecular Approach" by Geoffrey M. Cooper
* [http://www.scienceaid.co.uk/biology/genetics2/proteinsynthesis.html Science aid:Protein synthesis] For high school
* [http://www.accessexcellence.org/AB/GG/protein_synthesis.html Protein Synthesis]
* [http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/T/Transcription.html Transcription]
*: [http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/T/Translation.html Translation]
* [http://learningobjects.wesleyan.edu/proteinsynthesis/ Protein Synthesis Animation] Wesleyan University Learning Objects animation of protein synthesis.
* [http://cmol.nbi.dk/models/dynamtrans/dynamtrans.html Interactive Java simulation of transcription initiation.] From [http://cmol.nbi.dk/ Center for Models of Life] at the Niels Bohr Institute.
* From RNA to Protein Synthesis [http://www.asterpix.com/console/?avi=7698391 hypervideo] .


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