Naked Science Forum
Non Life Sciences => Chemistry => Topic started by: Hannah on 07/01/2003 22:27:04
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Hi, i am a AS level biology student and i have my exam next week and i understand most things its just that i dont understand protein synthesis at all, is there someone out there who can help a stuck AS level student pass her exams. Thanks so much is u can help that would be great.
Hannah
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Dear Hannah
sorry to hear that you are struggling - fire away and we'll see if we can't iron out your biology troubles !
TNS
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Hannah - you titled your message 'protein synthesis', but then didn't go on to say exactly what you are having problems with - put down a list below and I'll see if I can help !
Best wishes !
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Dear Hannah
I'm assuming from the title of your post that your worry is with how genes encode proteins ?
Here is a simple overview. If there is any part upon which you need elaboration, reply with your queries and I or one of the other guys above will try to iron it out !
PROTEIN SYNTHESIS.
Proteins contribute structural integrity to cells by forming the cytoskeleton, regulate the cellular biochemical environment through their roles as enzymes, ion channels and transporters, they function as chemical messengers, or hormones, capable of influencing the behaviour of local and distant cells by interacting with cell-surface receptors (which are themselves proteins), and they are also crucial for defence, as in antibodies, and in generating movement, as in muscles.
Proteins consist of one or more polypeptide chains, each of which is a linear polymer of amino acid residues. Twenty types of amino acid occur naturally in proteins. A polypeptide can be defined simply by its sequence of amino acids. This sequence is referred to as the primary structure.
The amino acids are linked together by a 'peptide bond' formed by a condensation reaction (removal of water) when the COOH group of one amino acid bonds with the NH2 of the second.
Protein synthesis begins in the nucleus, at the level of the gene which is the segment of DNA containing the genetic message dictating the primary structure of the protein. RNA Polymerase binds at the promoter site and transcribes the gene, producing a messenger RNA with a sequence complementary to the DNA sequence, and of course containing uracil in place of thymine.
Because most genes also contain non-coding sequences of DNA (so called introns) the primary RNA strand then undergoes post-transcriptional modification - splicing - where the introns are removed and the adjacent exons (the coding sequences) are all linked together. The mRNA is then further modified by adding a cap and tail to the respective 5' and 3' ends before it is exported out of the nucleus through a nuclear pore to the cytoplasm.
The genetic code comprises a 4 letter alphabet A C T G, and words 3 letters long, using various combinations of the 3 letters - AUG for instance, UUU, CAG etc. Because there are 4 DNA letters, which can be used in any order in groups of 3, there are 4x4x4 = 64 different possible words that can be spelled out. Each word codes for a different amino acid, but since there are only 20 naturally occurring amino acids used in our proteins, in some instances there are some amino acids that are coded for by several words.
In the cell cytoplasm the mRNA strand interacts with a ribosome, or more often ribosomes arranged in groups - polyribosomes - which read the sequence and insert the correct amino acids in the correct order to make the protein. The 3 letter words on the mRNA (called the CODONs) bind with complementary sequences (called ANTICODONS) on specialised RNAs called transfer RNAs(tRNAs), whose job it is to ferry the correct amino acids to the ribosome. Because a single tRNA has only a single 3 letter ANTICODON and only ferries one type of amino acid, this ensures that only the correct amino acids are inserted. The ribosome catalyses the formation of the peptide bonds between each amino acid added to the chain, until the mature protein is complete.
The protein is then subjected to post-translational modification via the golgi apparatus where it undergoes glycosylation (addition of sugar groups) and phosphorylation before it is churned out into the approprtiate cellular compartment where it does its job.
Chris
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Thanks chris. i know this is a bit stale but still was helpful for my purposes. cheers.
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Pleasure - amazing to see work I contributed a decade ago resurfacing and being read!