端木·宇 2008-6-19 22:21
Protein Synthesis
Now that we’ve described DNA and RNA, it’s time to take a look at theprocess of protein synthesis. The synthesis of proteins takes twosteps: transcription and translation. Transcription takes theinformation encoded in DNA and encodes it into mRNA, which heads out ofthe cell’s nucleus and into the cytoplasm. During translation, the mRNAworks with a ribosome and tRNA to synthesize proteins. [align=center][img]http://www.24en.com/d/file/sat/sat2/biology/2008-01-24/70e41c60df80ef4f0dc6298b4c1247ec.gif[/img][/align][b] Transcription[/b]
The first step in transcription is the partialunwinding of the DNA molecule so that the portion of DNA that codes forthe needed protein can be transcribed. Once the DNA molecule is unwoundat the correct location, an enzyme called RNA polymerase helps line upnucleotides to create a [b]complementary strand [/b]of mRNA. SincemRNA is a single-stranded molecule, only one of the two strands of DNAis used as a template for the new RNA strand.
[align=center][img]http://www.24en.com/d/file/sat/sat2/biology/2008-01-24/a081d6cead463ed51936a6f82d1f6bbd.gif[/img][/align] The new strand of RNA is made according to the rules of base pairing:
[list][*]DNA cytosine pairs with RNA guanine[*]DNA guanine pairs with RNA cytosine[*]DNA thymine pairs with RNA adenine[*]DNA adenine pairs with RNA uracil[/list]
For example, the mRNA complement to the DNAsequence TTGCAC is AACGUG. The SAT II Biology frequently asks about thesequence of mRNA that will be produced from a given sequence of DNA.For these questions, don’t forget that RNA uses uracil in place ofthymine.
After transcription, the new RNA strand isreleased and the two unzipped DNA strands bind together again to formthe double helix. Because the DNA template remains unchanged aftertranscription, it is possible to transcribe another identical moleculeof RNA immediately after the first one is complete. A single gene on aDNA strand can produce enough RNA to make thousands of copies of thesame protein in a very short time.
[b] Translation[/b]
In translation, mRNA is sent to thecytoplasm, where it bonds with ribosomes, the sites of proteinsynthesis. Ribosomes have three important binding sites: one for mRNAand two for tRNA. The two tRNA sites are labeled the A site and P site.
[align=center][img]http://www.24en.com/d/file/sat/sat2/biology/2008-01-24/c7be1b6d3ddf23a919a4f3d431054eb1.gif[/img][/align] Once the mRNA is in place, tRNA molecules,each associated with specific amino acids, bind to the ribosome in asequence defined by the mRNA code. tRNA molecules can perform thisfunction because of their special structure. tRNA is made up of manynucleotides that bend into the shape of a cloverleaf. At its tail end,tRNA has an acceptor stem that attaches to a specific amino acid. Atits head, tRNA has three nucleotides that make up an [b]anticodon[/b].
[align=center][img]http://www.24en.com/d/file/sat/sat2/biology/2008-01-24/99c832c1b4ed093de89b488f34f10925.gif[/img][/align] An anticodon pairs complementary nitrogenousbases with mRNA. For example if mRNA has a codon AUC, it will pair withtRNA’s anticodon sequence UAG. tRNA molecules with the same anticodonsequence will always carry the same amino acids, ensuring theconsistency of the proteins coded for in DNA.
[i] The Process of Translation[/i]
Translation begins with the binding of themRNA chain to the ribosome. The first codon, which is always the startcodon methionine, fills the P site and the second codon fills the Asite. The tRNA molecule whose anticodon is complementary to the mRNAforms a temporary base pair with the mRNA in the A site. A peptide bondis formed between the amino acid attached to the tRNA in the A site andthe methionine in the P site.
[align=center][img]http://www.24en.com/d/file/sat/sat2/biology/2008-01-24/819df721df82ab52fa5589de5e12a6ce.gif[/img][/align] The ribosome now slides down the mRNA, sothat the tRNA in the A site moves over to the P site, and a new codonfills the A site. (One way to remember this is that the A site bringsnew amino acids to the growing polypeptide at the P site.) Theappropriate tRNA carrying the appropriate amino acid pairs bases withthis new codon in the A site. A peptide bond is formed between the twoadjacent amino acids held by tRNA molecules, forming the first twolinks of a chain.
[align=center][img]http://www.24en.com/d/file/sat/sat2/biology/2008-01-24/9bc7d6e583715ebf23608ef318424e24.gif[/img][/align] The ribosome slides again. The tRNA that wasin the P site is let go into the cytoplasm, where it will eventuallybind with another amino acid. Another tRNA comes to bind with the newcodon in the A site, and a peptide bond is formed between the new aminoacid to the growing peptide chain.
[align=center][img]http://www.24en.com/d/file/sat/sat2/biology/2008-01-24/0d05761855b4f33a53f5b1ccb0e4a214.gif[/img][/align]
The process continues until one of the threestop codons enters the A site. At that point, the protein chainconnected to the tRNA in the P site is released. Translation iscomplete.
jianbanjiq 2008-11-4 20:12
楼主的文章实在太棒了
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