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Transcript
F. PROTEIN SYNTHESIS [or translating the message] 1. The code mRNA a. the message on mRNA is written in 3-letter triplets called codons there are 20 different amino acids, so how many codons do we need? given the words are 3 letters long and we have a choice of 4 letters, how many words do we make? b. there are 43 or 64 possible codons c. pg 248 shows the codons – you will have to learn the patterns but NOT memorize this with the exception of the START and the STOP codons 2nd BASE 1st BASE U C 3rd BASE A G U UUU UUC UUA UUG Phe Phe Leu Leu UCU UCC UCA UCG Ser Ser Ser Ser UAU UAC UAA UAG Tyr Tyr stop stop UGU UGC UGA UGG Cys Cys stop Trp U C A G C CUU CUC CUA CUG Leu Leu Leu Leu CCU CCC CCA CCG Pro Pro Pro Pro CAU CAC CAA CAG His His Gln Gln CGU CGC CGA CGG Arg Arg Arg Arg U C A G A AUU AUC AUA AUG Ile Ile Ile Met ACU ACC ACA ACG Thr Thr Thr Thr AAU AAC AAA AAG Asn Asn Lys Lys AGU AGC AGA AGG Ser Ser Arg Arg U C A G G GUU GUC GUA GUG Val Val Val Val GCU GCC GCA GCG Ala Ala Ala Ala GAU GAC GAA GAG Asp Asp Glu Glu GGU GGC GGA GGG Gly Gly Gly Gly U C A G d. notice that UUU & UUC both code for phe and CUU, CUC, CUA & CUG all code for leu this indicates that the 3rd letter is the least important in the code e. only two amino acids have a unique code UGG trp and AUG met f. all proteins must start with met as AUG is the start code; this may be removed later g. there is no amino acid that is coded by UAA, UAG or UGA and so the protein breaks here and these are called STOP codes 2. The ribosomes [rRNA] a. the rRNA holds together the large subunit [60s] and the small subunit [40s] to form the active ribosome [80s] b. the 5’ [CAP region] adheres to the active ribosome c. the ribosome will move the mRNA through it starting at the 5’ end and going toward the 3’ end 3. The tRNA a. each tRNA has a cross-like structure [pg 251] b. at the base there is an anticodon which is the complement of the codon on themRNA eg the mRNA has the codon AUG for met the tRNA—met has the anticodon UAC c. at the 3’ end of the tRNA is the acceptor site that holds the amino acid d. it requires 1 ATP to attach each amino acid to the correct tRNA to make an amino-acyl tRNA e. there are 61 possible tRNA for the 20 amino acids, therefore a cell needs 61 enzymes for this show animation, then complete notes STEPS IN PROTEIN SYNTHESIS 1. The mRNA, rRNA and tRNA are transcribed from DNA in the nucleus. They randomly drift around in the cytoplasm where the large and small subunits of the ribosome are also drifting. CC GAG CUC AUG CCA AUC GGC CAG GUA UAG GGG ACC mRNA UAC tRNA large subunit of ribosome rRNA small subunit of ribosome 2. The mRNA leaves the nucleus and encounters the small subunit of the ribosome. CC GAG CUC AUG CCA AUC GGC CAG GUA UAG GGG ACC 3. The rRNA attaches the large ribosome subunit to the small subunit that is attached to the mRNA. The tRNA’s are activated by the attachment of an amino acid, by the use of an ATP. Gly Gln Met Ile CCG Pro UAC GUC Val UAG GGU CC GAG CUC AUG CCA AUC GGC CAG GUA UAG GGG ACC P-site A-site CAU 4. The tRNA with Met attached, randomly enters the P-site on the ribosome. Because its anti-codon matches the codon in the P-site, this tRNA is held in place by hydrogen bonds. Gly Met Gln Ile CCG UAC Met Pro GUC Val UAG GGU CAU UAC CC GAG CUC AUG CCA AUC GGC CAG GUA UAG GGG ACC P-site A-site 5. The tRNA for the next amino acid randomly moves into the A-site. Pro Gly Gln GGU Ile CCG Met Pro GUC Val UAG UAC GGU CC GAG CUC AUG CCA AUC GGC CAG GUA UAG GGG ACC P-site A-site CAU 6. The tRNA that brought Met now is released after Met is bonded to the Pro. The mRNA is now moved through the ribosome until the tRNA with Met-Pro is in the P-site. The tRNA for the next code now fits into the A-site Gly Ile Gln UAC UAG CCG Met- Pro Ile GUC GGU UAG CC GAG CUC AUG CCA AUC GGC CAG GUA UAG GGG ACC P-site A-site Val CAU 7. The processes of step 5 & step 6 are repeated until one of the STOP codes is in the A-site. At this point all of the amino acids have been bonded into a polypeptide chain connected to the last tRNA in the P-site. UAG UAC GGU CCG Met-Pro-Ile-Gly-Gln-Val CAU CC GAG CUC AUG CCA AUC GGC CAG GUA UAG GGG ACC P-site A-site GUC 8. Because there was no tRNA to match UAG, the polypeptide chain is released. and the ribosome falls apart. The mRNA will be broken down. C AUG CCA AUC GGC C CC GAG CU AG GUA UAG GGG ACC mRNA Met-Pro-Ile-Gly-Gln-Val large subunit of ribosome polypeptide chain small subunit of ribosome Now attempt the assignment at the bottom of the page From the piece of DNA below, make the complimentary strand of DNA, transcribe the appropriate strand, package it for the cytoplasm and then translate it as described in the steps below. In row A; make the complimentary DNA strand; In row C, transcribe the DNA into RNA; In column 1 and 28, place the appropriate 3’ or 5’; In column 2 & 27, ‘package’ the RNA in row C; In row D, underline the ‘words’ in the RNA; In row E, translate the RNA into amino acids using the 3letter codes for them 1 2 3 4 5 6 7 8 9 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 2 7 2 8 A 5 C C A T G C G A T G C G C G T T C C T G A G A G C 3 B 3 G G T A C G C T A C C G C A A G G A C T C T C G 5 C 53 CAP C C A U G C G A U G G C G U U C C U G A G A G C polyA 3 D E met arg trp arg ser