A much smaller molecule and a fraction of the size of DNA, the newly formed molecule is small enough to exit and escape the nucleus via the nuclear pores on the periphery of a nucleus. Something the DNA molecule cannot do, besides the DNA is far too important a molecule to be lost from the nucleus, hence the need for the process of a messenger RNA molecule and for transcription.
Once a strand of mRNA has been completed it peels away from it DNA template, the genetic code has now been transcribed on to the smaller molecule (mRNA), it moves out of the nucleus through a nuclear pore and seeks out a ribosome residing in the cytoplasm (ribosomes can be considered the protein production factories of the cell), where the second stage of protein synthesis can begin, though it is this transcribed molecule which is all important for the second stage to occur, for without instructions the second stage, translation wouldn’t occur.
Once the mRNA has vacated the once exposed strand of DNA polynucleotides, and has left the nucleus in search of a ribosome, the unwound DNA strands (template) begins to re-coil, re-form and the hydrogen bonds between the two uncoiled sections of a strand are re-formed allowing the strands to reunite into its double helical structure once again, this happen with the assistance of the enzyme DNA Polymerase.
One newly transcribed mRNA molecule will possess a sequence of coded bases which possess an identical sequence to that of the one of the strands which didn’t act as the template, with the exception as mentioned earlier where uracil replaces thymine in the newly formed strand of mRNA.
Once attached to a ribosome an mRNA molecule transcribed with the relevant information can instruct the ribosome and another form of RNA, transfer RNA as to what order the assembly of a new polypeptide chain should be.
The diagram below outline the process.
DNA can be seen to unwind, allowing RNA nucleotides to pair with one strand of complementary exposed bases which is acting as a template.
(Picture Source:- http://meyerbio1b.wikispaces.com/Transcription+and+Translation).
The translation stage of protein synthesis.
Protein synthesis is a continuous and vital operation within a cell. The synthesis of proteins for certain functions is fundamental to the continued health of a cell and an organism as a whole.
The stage of protein synthesis termed translation occurs after the first stage of protein synthesis has been completed, which is termed as just considered transcription.
Translation is therefore the second stage in the process of synthesis of proteins. In a nutshell translation can be described as the process by which the information attained and encoded into mRNA molecules during the transcription phase is converted and used to make a specific particular polypeptide chain, and thus the primary structure of a protein.
After the genetic code of the DNA within the nucleus is transcribed onto an mRNA molecule it leaves the nucleus via the pores contained in the nucleus’s envelope, these pores are large enough to allow RNA passage but not large enough for DNA to pass through- which is just as well because DNA is vital to the continued functioning of a cell/organism and is too valuable to lose from the nucleus.
Once in the cytoplasm the mRNA molecule seeks out a ribosome (the protein factories), where the conversion of the information replicated during the transcription stage can occur, (this information will then be used in the production of peptide chain building from the ribosome).
As considered the process of translation occurs at the site of a ribosome, which reside in the cytoplasm of a cell. We know that from our basic biology knowledge gained previously that a ribosome is composed of two units, a large subunit and a small subunit and it is the small subunit of a ribosome with which the strand of mRNA binds. mRNA because of its structure (made from one strand of complementary bases in the replication process of transcription), carries specific information in the form of codons (a triplet of nitrogenous bases), which have a corresponding and complementary relationship with amino acids to be used in the protein synthesis.
The translation process occurs with the assistance of another form of RNA in the flavour of transfer RNA (tRNA). Perhaps the most crucial property of tRNA is its ability to bind to an amino acid at one of it ends of its molecule and to an mRNA strand at its other end, it is therefore said to possess two functional sites.
Once connected with the ribosome the strand of mRNA lines up in a linear form where a ribosome is free to then move along the strand and ‘read’ the coded instructions- the codons triplets. As this happens, as ribosomes move across a triplet on the mRNA strand, a tRNA molecule with complementary bases to the first triplet of bases on the mRNA goes off and finds and brings the particular corresponding amino acid to the ribosome, when it returns it attaches to the mRNA via specific base pairing and starts to build the primary sequence of a polypeptide chain.
As the ribosome move on to the next triplet on the mRNA strand a second tRNA molecule repeats this process and attaches itself to the next triplet of bases on the mRNA strand in the same way. Now there are two amino acids present, and this results in a peptide bond occurring linking the two together, the result of this means the tRNA molecule which was holding the initial amino acid is released and is free to be re-used in the process if necessary. The now joined amino acids are held on to in position by the second tRNA molecule which is still attached to the mRNA strand.
A third tRNA molecule then binds to the next triplet on the mRNA, this amino acid binds to the amino acid chain again through a peptide bond, and now the second tRNA molecule is released and becomes free to be reused. This chain of three amino acids are held together by peptide bonds and are attached to the strand of mRNA by the complementary base pair of the third tRNA molecules anti-codon.
As is demonstrated this process happens one triplet, one amino acids at a time, and the growing peptide chain moves onto and binds to a protein on the latest tRNA molecule, allowing previous tRNA, ‘to detach themselves from the mRNA and return to the pool of tRNA in the cytoplasm from which they can be drawn upon again when required.’ (Roberts et al 2000 p612).
As the ribosome moves along the mRNA strand it actually facilitates two tRNA molecules with their amino acids at any one time, still the addition of amino acids to the peptide chain is done one by one. As amino acids link and tRNA detach from the mRNA the ribosome is able to move on to the next set of triplets/codons on the mRNA, one triplet further along the strand moving in one direction. Moving one triplet at a time following a prescribed sequence. It is a process which sees 15 amino acids a second added to a peptide chain. (Adds 1996 p64).
tRNA attaches to a specific free amino acid in the cytoplasm at its binding site (refer to tRNA picture), with the assistance of an specific enzyme. Once combined this form of RNA ‘transfers’ the accompanying bound amino acid to the strand of mRNA on the ribosome where the three bases situated at the messenger RNA binding site pair up via hydrogen bonding with the appropriate corresponding bases on the mRNA molecule. It is at this point where the codons of the mRNA and the anit-codons in the tRNA meet.
It is the sequence within the strand of mRNA base triplets that dictates the way in which amino acids are linked up, they will link up in an order corresponding to the sequence of base triplets in the mRNA strand.
The anti-codon at the base of each tRNA must make a perfect complementary match with the codon on the mRNA strand before the amino acid is released, obeying complementary base pairing rules amino acids are released one by one and when released it joins the peptide chain of amino acids.
Where the process of translation begins with a start code, a AUG codon, the process of translation is continuous until a stop code is read on the mRNA strand, when the codon UGA or UAA or UAG is read, then the process is complete for that particular protein. The stop codon does not code for an amino acid but terminated translation. It is feasible for a mRNA strand to be read repeatedly creating many number of the exact same polypeptide chains. This can happen either one chain at a time for each mRNA or it is possible for several ribosomes to attach to a strand of mRNA concurrently, speeding up polypeptide formation, where polysomes are formed. (Adapted from Adds 1996 p65). In essence a strand of mRNA can be read by many ribosomes at the same time, Roberts et al (2000 p612) states that a polyribosome is seen to consist of five to 50 individual ribosome.
This process works by, taking the example of the process of translation from above where once a triplet is read and an amino acid delivered the ribosome moves onto the next triplet code, once both tRNA molecules housed in a ribosome have move on, the strand may be read again by subsequent ribosomes queued up behind the previous ribosome. The result of course will be the same polypeptide chain being synthesised seeing as the same coded template is being translated, it is seen as an advantageous process allowing large number of polypeptides to be made from a single mRNA strands in a short space of time.
Once a strand has been read it and is finished with it will fragment and breakup allowing for the recycling of the constituent nucleotides. The newly created synthesised polypeptide chain isn’t yet a protein at this stage, as it is subject to folding and coiling and the influences of differing forms of bonding, depending upon the particular protein coded for of course will dictate its protein structure and ultimate shape and function. The stage of translation is complete the moment the primary structure, the polypeptide chain detaches from the mRNA strand and the ribosome. The mRNA coded instruction has thus been translated into a primary structure of a protein.
Protein synthesis is a continuous ongoing process within the life of a cell.
Bibliography
Adds. J et al (1996 ) Cell Biology and Genetics
Thomas Nelson and Sons Ltd, Surrey, UK
Allan. R & Greenwood. T (1999) Advanced Biology 1 2000
Biozone International Publishers, Glasgow, UK
Barnett. G et al (2008) AS Level Biology The Revision Guide
Coordination Group Publications Ltd. UK
Boyle. M & Senior. K (200) Collins Advanced Science- Biology 3rd Edition
Harper Collins Press, UK
Evers & Starr (2007) Biology Concepts and applications 7th Edition
Thomson Brooks/Cole Publishers, USA
Fullick. A (1998) Heinemann Advanced Science Biology
Heinemann Publisher, UK
Indge. B (2004) The Complete A-Z Biology Handbook
Hodder & Stoughton, Oxon UK
Jones. M (2B008) Biology 1 for OCR
Cambridge University Press, UK
Kennedy. P & Sochacki. F (2008) OCR Biology
Heinermann, Glasgow, UK
Kent. M (2000) Advanced Biology
Oxford University Press, UK
Philips. W & Chilton T (1993) A Level Biology
Oxford university Press, UK
Sutton. J (1998) Biology: Cell biology and Genetics
Macmillan Press Ltd London UK
Toole. G & Toole. S (2008) AQA Bioilogy AS level Student Book
Nelson Thornes Ltd, Cheltenham UK
