Chapter 27 Protein Metabolism
Multiple Choice Questions 1. The The gen genet etic ic code code Page: 1038 i!!icult": 2 #ns: C A certain bacterial mRNA is known to r epresent only one gene and to contain about 800 nucleotides. If you assume that the average amino acid residue contributes 110 to the peptide molecular weight the largest polypeptide that this mRNA could code for would have a molecular weight of about!
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800. $000. &0000. 8000 000. An upper upper limit limit cannot cannot be determ determined ined from from the data given given..
2. The The gen genet etic ic code code Page: 1038 i!!icult": 2 #ns: C Assuming that the average amino acid residue contributes 110 to the peptide molecular weight what will be the minimum length of the mRNA encoding a protein of molecular weight $0000)
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1&& 1&& nucl nucleo eoti tide dess *+0 *+0 nuc nucle leot otid ides es 1*00 1*00 nucl nucleo eoti tide dess $000 $000 nucl nucleo eoti tide dess A minimum minimum length length cannot cannot be determ determined ined from from the data given given..
3. The The gen genet etic ic code code Pages: Pages: 103$%1 103$%10&& 0&& i!!ic i!!icult ult": ": 3 #ns: #ns: ,hich of the following are features of the wobble hypothesis)
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A naturally occurring tRNA e-ists in yeast that that can read both arginine arginine and lysine codons. codons. A tRNA tRNA can reco recogni gnie e only only one one codon. codon. /ome tRNAs tRNAs can recognie recognie codons codons that specify specify two differen differentt amino acids acids if both are nonpolar. nonpolar. he wobble wobble22 occurs occurs only in in the first first base base of the anticod anticodon. on. he third third base base in a codon codon always always forms forms a normal normal ,atson3%r ,atson3%rick ick base base pair. pair.
&. The The gen genet etic ic code code Page: 103$ i!!icult": 2 #ns: C ,hich one of the following is true about the genetic code)
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All codons codons recogni recognied ed by a given tRNA tRNA encode encode different different amino amino acids. acids. It is absolutel absolutely y identic identical al in all living living things. things. /everal /everal different different codon codonss may encode encode the the same same amino amino acid. acid. he base in in the middle middle position position of the tRNA anticodon anticodon sometimes sometimes permits wobble2 base pairing with 4 or & different codons. (" he first first positio position n of the tRNA tRNA anticodo anticodon n is always always adenosi adenosine. ne.
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Chapter 27 Protein Metabolism
'. Protein s"nthesis Page: 10&' i!!icult": 1 #ns: ,hich one of the following statements about ribosomes is true)
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he large subunit contains rRNA molecules the small subunit does not. he RNA in ribosomes plays a structural not catalytic role. here are about 4$ of them in an E. coli cell. here are two ma5or subunits each with multiple proteins. hey are relatively small with molecular weights less than 10000.
(. Protein s"nthesis Page: 10&$ i!!icult": 2 #ns: # ,hich of the following statements about tRNA molecules is false)
A" A % 6 and 7 are the only bases present in the molecule. #" Although composed of a single strand of RNA each molecule contains several short double3 helical regions. %" Any given tRNA will accept only one specific amino acid. '" he amino acid attachment is always to an A nucleotide at the & ' end of the molecule. (" here is at least one tRNA for each of the 40 amino acids. 7. Protein s"nthesis Page: 10'0 i!!icult": 2 #ns: ) ,hich of the following statements about the tRNA that normally accepts phenylalanine is false) mRNA codons for phenylalanine are 777 and 77%."
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It interacts specificially with the 9he synthetase. It will accept only the amino acid phenylalanine. Its molecular weight is about 4$000. 9henylalanine can be specifically attached to an :;< group at the & ' end. he tRNA must contain the se=uence 777.
8. Protein s"nthesis Page: 10'1 i!!icult": 2 #ns: ) ,hich of the following is not true of tRNA molecules)
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he &' 3terminal se=uence is :%%A. heir anticodons are complementary to the triplet codon in the mRNA. hey contain more than four different bases. hey contain several short regions of double heli-. ,ith the right enyme any given tRNA molecule will accept any of the 40 amino acids.
$. Protein s"nthesis Page: 10'1 i!!icult": 2 #ns: # Aminoacyl3tRNA synthetases amino acid activating enymes"!
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recognie2 specific tRNA molecules and specific amino acids. in con5unction with another enyme attach the amino acid to the tRNA. interact directly with free ribosomes. occur in multiple forms for each amino acid.
Chapter 27 Protein Metabolism
(" re=uire 69 to activate the amino acid. 10. Protein s"nthesis Page: 10'1 i!!icult": 2 #ns: * In E. coli aminoacyl3tRNA synthetases! A" #" %" '" ("
activate amino acids in 14 steps. are amino acid>specific? there is at least one enyme specific for each amino acid. fall into two classes each of which attaches amino acids to different ends of the tRNA. have no proofreading activities. re=uire a tRNA an amino acid and 69 as substrates.
11. Protein s"nthesis Pages: 10'1%10'& i!!icult": 2 #ns: ,hich of the following statements about aminoacyl3tRNA synthetases is false)
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/ome of the enymes have an editing@proofreading capability. he enyme attaches an amino acid to the &' end of a tRNA. he enyme splits A9 to A9 B 99i. he enyme will use any tRNA species but is highly specific for a given amino acid. here is a different synthetase for every amino acid.
12. Protein s"nthesis Page: 10'1 i!!icult": 2 #ns: he enyme that attaches an amino acid to a tRNA aminoacyl3tRNA synthetase"!
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always recognies only one specific tRNA. attaches a specific amino acid to any available tRNA species. attaches the amino acid at the $' end of the tRNA. catalyes formation of an ester bond. splits A9 to A'9 B 9i.
13. Protein s"nthesis Page: 10'1 i!!icult": 2 #ns: In the activation2 of an amino acid for protein synthesis!
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leucine can be attached to tRNA 9he by the aminoacyl3tRNA synthetase specific for leucine. methionine is first formylated then attached to a specific tRNA. the amino acid is attached to the $' end of the tRNA through a phosphodiester bond. there is at least one specific activating enyme and one specific tRNA for each amino acid. two separate enymes are re=uired one to form the aminoacyl adenylate the other to attach the amino acid to the tRNA.
1&. Protein s"nthesis Page: 10'( i!!icult": 2 #ns: # Cormation of the ribosomal initiation comple- for bacterial protein synthesis does not re=uire!
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(C3u. formylmethionyl tRNAfet. 69. initiation factor 4 IC34". mRNA.
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Chapter 27 Protein Metabolism
1'. Protein s"nthesis Page: 10'8 i!!icult": 2 #ns: In bacteria the elongation stage of protein synthesis does not involve!
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aminoacyl3tRNAs. (C3u. 69. IC34. peptidyl transferase.
1(. Protein s"nthesis Page: 10'8 i!!icult": 2 #ns: ) ,hich one of the following statements about the elongation phase of protein synthesis is true)
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At least five high3energy phosphoryl groups are e-pended for each peptide bond formed. 'uring elongation incoming aminoacylated tRNAs are first bound in the 9 site. (longation factor (C3u facilitates translocation. 9eptidyl transferase catalyes the attack of the carbo-yl group of the incoming amino acid on an ester linkage in the nascent polypeptide. (" 9eptidyl transferase is a riboyme. 17. Protein s"nthesis Page: 10'$ i!!icult": 2 #ns: C ,hich of the following statements about bacterial mRNA is true)
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A ribosome usually initiates translation near the end of the mRNA that is synthesied last. An mRNA is never degraded but is passed on to the daughter cells at cell division. 'uring polypeptide synthesis ribosomes move along the mRNA in the direction $ ' → &' . Ribosomes cannot initiate internally in a polycistronic transcript. he codon signaling peptide termination is located in the mRNA near its $ ' end.
18. Protein s"nthesis Page: 10'$ i!!icult": 2 #acterial ribosomes!
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#ns: *
bind tightly to specific regions of 'NA forming polysomes. contain at least one catalytic RNA molecule riboyme". contain three species of RNA and five different proteins. have specific different binding sites for each of the 40 tRNAs. re=uire puromycin for normal function.
1$. Protein s"nthesis Page: 10(2 i!!icult": 1 #ns: * he large structure consisting of a mRNA molecule being translated by multiple copies of the macromolecular comple-es that carry out protein synthesis is called a!
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lysosome. polysome. proteosome. ribosome.
Chapter 27 Protein Metabolism
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(" synthosome. 20. Protein s"nthesis Page: 10(1 i!!icult": 3 #ns: C It is possible to convert the %ys that is a part of %ys3tRNA %ys to Ala by a catalytic reduction. If the resulting Ala3tRNA %ys were added to a mi-ture of 1" ribosomes 4" all the other tRNAs and amino acids &" all of the cofactors and enymes needed to make protein in vitro and *" mRNA for hemoglobin where in the newly synthesied hemoglobin would the Ala from Ala3tRNA %ys be incorporated) A" #" %" '" ("
Nowhere? this is the e=uivalent of a nonsense mutation ,herever Ala normally occurs ,herever %ys normally occurs ,herever either Ala or %ys normally occurs ,herever the dipeptide Ala3%ys normally occurs
21. Protein s"nthesis Pages: 10((%10(7 i!!icult": 2 #ns: C ,hich one of the following antibiotics does not function by interfering with the translational process)
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%hloramphenicol %yclohe-imide 9enicillin 9uromycin /treptomycin
22. Protein targeting and degradation Page: 10($ i!!icult": 2 #ns: ,hich of the following is true about the sorting pathway for proteins destined for incorporation into lysosomes or the plasma membrane of eukaryotic cells)
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#inding of /R9 to the signal peptide and the ribosome temporarily accelerates protein synthesis. he newly synthesied polypeptides include a signal peptide at their carbo-yl termini. he signal peptide is cleaved off inside the mitochondria by signal peptidase. he signal recognition particle /R9" binds to the signal peptide soon after it appears outside the ribosome. (" he signal se=uence is added to the polypeptide in a posttranslational modification reaction. 23. Protein targeting and degradation Pages: 10($%1070 i!!icult": 2 #ns: # 6lycosylation of proteins inside the endoplasmic reticulum does not involve!
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a
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Chapter 27 Protein Metabolism
2&. Protein targeting and degradation Page: 1070 i!!icult": 2 #ns: ) 9osttranslational glycosylation of proteins is inhibited specifically by!
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chloramphenicol. cyclohe-imide. puromycin. streptomycin. tunicamycin.
2'. Protein targeting and degradation Page: 1071 i!!icult": 2 #ns: he signal se=uences that direct proteins to the nucleus are!
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always at the amino terminus of the targeted protein. cleaved after the protein arrives in the nucleus. glycosyl moieties containing mannose +3phosphate residues. not located at the ends of the peptide but in its interior. the same as those that direct certain proteins to lysosomes.
2(. Protein targeting and degradation Pages: 1072%107& i!!icult": 3 #ns: C he pathway for polypeptides e-ported from E. coli includes the following steps which occur in what order for correct e-port)
1. 4. &. *. A" #" %" '" ("
A chaperone /ecA binds to the polypeptide. A chaperone /ec# binds to the polypeptide. A9 is hydrolyed by /ec A. /ecA pushes 40 amino acids of the polypeptide into the translocation comple-.
1 4 & * 1 4 * & 4 1 * & 4 & 1 * & 1 * 4
27. Protein targeting and degradation Page: 107( i!!icult": 3 #ns: 7bi=uitin3mediated protein degradation is a comple- process and many of the signals remain unknown. ;ne known signal involves recognition of amino acids in a processed protein that are either stabiliing Ala 6ly et /er etc." or destabiliing Arg Asp Deu Dys 9he etc." and are located at!
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a heli-3turn3heli- motif in the protein. a lysine3containing target se=uence in the protein. a inc finger structure in the protein. the amino3terminus of the protein. the carbo-y3terminus of the protein.
Chapter 27 Protein Metabolism
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+hort #ns,er Questions 28. The genetic code Pages: 103'%103( i!!icult": 2 ;utline one of the e-perimental methods providing evidence that the genetic code was a triplet code. #ns: ,hen one or two nucleotides were added to or deleted from a gene the resulting mRNA produced a protein with a different amino acid se=uence after the deletion or insertion. ,hen three nucleotides were added or deleted the resulting protein had a normal se=uence e-cept for the insertion or deletion of a single amino acid residue. 2$. The genetic code Page: 103( i!!icult": 3 'escribe succinctly two ways in which synthetic polynucleotides were used in solving the genetic code you need not describe how the synthetic polynucleotides were made". #ns: 1" ,hen synthetic polymers of only one nucleotide were used as mRNA in vitro only one of the 40 amino acids was converted into protein. Cor e-ample poly7" containing only the codon 777" directed the synthesis of polyphenylalanine showing that 777 encodes 9he. 4" rinucleotides of known se=uence were used to stimulate aminoacyl3tRNA binding to ribosomes. #ecause only that aminoacyl3tRNA whose anticodon matched the trinucleotide mRNA2 was bound the coding specificity of each se=uence of three bases could be determined by determining which of the 40 aminoacyl3tRNAs bound. &" Random polymers of RNA containing known ratios of nucleotides e.g. E0F A and &0F " generate only certain codons in predictable ratios. he identities and ratios of the amino acids specified by such polymers provided important clues that helped solve the genetic code. *" Additional assignments were made possible using synthetic oligonucleotides containing repeats of specific two three or four base pair se=uences. 30. The genetic code Page: 103( i!!icult": 3 Gou have isolated a fragment of viral 'NA that totally encodes at least two proteins 140 and 80 amino acids long. he 'NA fragment is *00 base pairs long. a" ,hy might you consider this unusual) b" Gou se=uence the two proteins and find no se=uence homology. 9ropose a model to account for these findings. #ns: a" wo distinct proteins of these sies should re=uire mRNAs of &+0 and 4*0 base pairs because each amino acid residue re=uires & base pairs to code for it. b" No homology means that the smaller protein cannot be derived from the larger by proteolysis? if it were there would be 80 amino acid residues of identical se=uence in the two proteins. ;ne possible e-planation is that the two genes coding for these proteins overlap and are read in different reading frames. 31. The genetic code Page: 1038 i!!icult": 2 he template strand of a segment of double3stranded 'NA contains the se=uence! $' "% 6A AA 66A A6 %%% % a" ,hat is the base se=uence of the mRNA that can be transcribed from this strand) b" ,hat amino acid se=uence could be coded by the mRNA base se=uence in a" using only the first reading frame starting at the $' end) Refer to Cig. 4E3E p. 10&8." c" /uppose the other complementary" strand is used as a template for transcription. ,hat is the amino acid se=uence of the resulting peptide again starting from the $' end and using only the first reading frame)
(&
Chapter 27 Protein Metabolism
#ns: a" $' "6AA 666 %7A 7%% 77A 7%A AA6&H" b" 6lu36ly3Deu3/er3Deu3/er3Dys c" he codons translate to Deu3/top3/top. No peptide would be produced because of the stop codons. /ee also Cig. 4E3+ p. 10&8." 32. The genetic code Page: 1038
i!!icult": 3
'escribe the possible outcomes that could occur because of a single base change in an mRNA #ns: he most likely result is a single amino acid change in the encoded protein when a codon is altered to one of another amino acid.
utant 1 4 & * $
(3/er3Ile3Arg Deu3R93Ile3Arg Deu3/er3AR63Arg Deu3/er3Ile39R; Deu3/er3Ile3R9
,hat was the nucleotide se=uence of the region of mRNA that coded for the amino acid se=uence in the wild3type organism) Refer to Cig. 4E3+ p. 10&8." #ns: $' "% or $' "7 76 7%6 A7A %66 3&. The genetic code Pages: 103$%10&& i!!icult": 2 In protein synthesis +1 codons specify the 40 amino acids. #ase pairing between the codon and the tRNA anticodon assures that the correct amino acid will be inserted into the nascent polypeptide chain. ,hy then does the cell re=uire only &4 different tRNAs to recognie +1 different codons) #ns: %ertain tRNAs have the unusual nucleotide inosinate in the first anticodon position. #ecause inosinate can base pair with A 7 or % a tRNA containing hypo-anthine can recognie three different codons. In each recognied codon there is a standard anticodon3codon base pair with the first two bases of the codon? wobble2 in the third base pair allows one tRNA to read three different codons. /imilarly tRNAs with 7 or 6 in the first anticodon position also e-hibit a wobble effect that
Chapter 27 Protein Metabolism
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permits pairing with two different codons. 3'. Protein s"nthesis Pages: 10&'%10&$ i!!icult": 1 Indicate whether the following statements are true " or false C". A ribosome is the comple- within which protein synthesis occurs. Ribosomes contain many separate proteins. he three ribosomal RNAs in a bacterial ribosome are distributed in three separate large ribosomal subunits. here are four binding sites for aminoacyl3tRNAs on a ribosome. #ns: ? ? C? C 3(. Protein s"nthesis Pages: 10'1%10'3 i!!icult": 2 he process of charging tRNAs with their cognate amino acids involves multiple proofreading steps to increase the overall fidelity. #riefly describe these steps. #ns: here are two main stages of selection! 1" the synthetase strongly favors activation of the correct amino acid to become aminoacyl3A9 incorrect amino acids are very poorly activated" and 4" when the correct uncharged tRNA is bound to the enyme only the correct aminoacyl3A9 is tolerated in the proofreading2 active site incorrect aminoacyl3A9s though they may become bound are rapidly hydrolyed". In addition for most synthetases if their tRNA does manage to become acylated by the wrong amino acid that product is also rapidly hydrolyed. 37. Protein s"nthesis Pages: 10'3%10'& i!!icult": 2 he recognition of an amino acid by its cognate aminoacyl3tRNA synthetase is said to involve a second genetic code2. ,hat is meant by this) #ns: (ach of the 40 amino acids has a uni=ue synthetase but many have multiple cognate tRNAs that must be charged aminoacylated". he second code2 refers to features common to all tRNAs that carry the same amino acid making them specifically recogniable by the correct synthetase. hese features occur at different places in different tRNA classes and may be as simple as a single 637 basepair or re=uire ten or more specific nucleotides throughout the se=uence. /ee Cig. 4E31+ p. 10$&." 38. Protein s"nthesis Page: 10'' i!!icult": 3 In 1J+1
((
Chapter 27 Protein Metabolism
3$. Protein s"nthesis Page: 10'(%10'8 i!!icult": 3 A given mRNA se=uence might be translated in any of three reading frames. 'escribe how prokaryotes and eukaryotes determine the correct reading frame. #ns: In prokaryotes the /hine3'algarno se=uence in the mRNA base pairs with a complementary se=uence in the 1+/ RNA of the ribosome? this positions the correct start codon A76" on the &0/ ribosomal subunit. hus the initiating A76 is distinguished by its pro-imity to the /hine3'algarno se=uence. In eukaryotes the initiating A76 codon is the first A76 that the ribosome encounters as it scans the mRNA from its $' end. In both cases the initiating A76 also sets the correct reading frame. &0. Protein s"nthesis Pages: 10'(%10(1 i!!icult": 1 atch the factor or enyme at the right with the stages" of protein synthesis at which it acts. If a factor or enyme participates in two stages of protein synthesis indicate both of them.
Amino acid activation Initiation (longation ermination
a" RC 1 b" (C3u c" aminoacyl3tRNA d" /hine3'algarno se=uence
#ns: c? d? b? a &1. Protein s"nthesis Pages: 10'(%10(1 i!!icult": 2 Indicate whether each of the following statements is true " or false C".
Assembly of a complete ribosome onto an mRNA re=uires A9 hydrolysis. Aminoacylation or charging2 of tRNA re=uires the formation of an aminoacyl3A9 intermediate. Aminoacyl3tRNA binding to the A site of the ribosome re=uires the accessory factor (C36 and 69 hydrolysis. ranslocation of a growing polypeptide from the A to the 9 site on the ribosome re=uires (C3 6 and 69 hydrolysis. ermination of translation re=uires release factors but no N9 hydrolysis. #ns: C? ? C? ? &2. Protein s"nthesis Pages: 10'(%10(1 i!!icult": 3 #riefly describe the role of the following components in bacterial protein synthesis.
a" Initiation factor 4 IC34" b" 1+/ RNA c" 9eptidyl transferase d" Release factors e" (longation factor 6 (C36" f" N 103formyltetrahydrofolate g" A9 h" tRNAfet
Chapter 27 Protein Metabolism
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#ns: a" IC34 is a protein factor that when bound to 69 brings the fet3tRNA fet to the initiation comple-. b" 1+/ RNA is a component of the small &0/" subunit. It contains a se=uence complementary to the /hine3'algarno se=uence in the mRNA and helps to line up the mRNA initiation A76 codon on the ribosome. c" 9eptidyl transferase is a riboyme in the $0/ ribosomal subunit. It catalyes formation of each peptide bond as the ribosome moves along the mRNA. d" Release factors are proteins that bring about the release of the finished polypeptide when the ribosome encounters a termination codon in the mRNA. e" (C36 participates in the translocation of the ribosome down the mRNA by one codon after each peptide bond is formed. f" N 103formyl3 tetrahydrofolate is the cofactor that donates a methyl group in the conversion of tRNA3bound et to fet. g" A9 is the substrate for aminoacyl3tRNA synthetases? it donates an A9 residue in the formation of aminoacyl adenylate which donates the aminoacyl group to tRNA. h" tRNA fet is the transfer RNA that initiates protein synthesis by inserting the first amino acid fet" in every prokaryotic protein. &3. Protein s"nthesis Pages: 10'(%10(1 i!!icult": 2 Number the following steps in the proper order with regard to protein synthesis.
Aminoacyl3tRNA binds to the A site. 'eacylated tRNA is released from ribosome. 9eptide bond formation shifts the growing peptide from the 9 to the A site. he $0/ subunit binds to the initiation comple- of the &0/ subunit and mRNA. #ns: 4? *? &? 1 &&. Protein s"nthesis Pages: 10'(%10(2 i!!icult": 2 Indicate whether each of the following statements is true " or false C".
#acterial mRNA is broken down within a few minutes of its formation in E. coli. #acterial mRNA consists only of the bases that code for amino acids. 9olysomes do not necessarily contain mRNA. #acterial mRNA normally occurs as a double3stranded structure with one strand containing codons the other containing anticodons. #acterial mRNA can be translated while it is still being synthesied. #ns: ? C? C? C? &'. Protein s"nthesis Pages: 10'7%10'8 i!!icult": 2 #ns: * Regarding translation in eukaryotes versus that in prokaryotes bacteria" indicate whether each of the following statements is true " or false C".
In eukaryotes the &' end of the mRNA is associated with the $ ' end during initiation whereas in prokaryotes it is not. In prokaryotes it is initiated at an A76 near a /hine3'algarno se=uence in the mRNA whereas in eukaryotes it is initiated at an A76 near the & ' end of the mRNA. In prokaryotes it is initiated with et whereas in eukaryotes it is initiated with fet. In prokaryotes translation and transcription are coupled whereas in eukaryotes they are not.
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Chapter 27 Protein Metabolism
#ns: C C &(. Protein s"nthesis Pages: 10'8%10(1 i!!icult": 2 9olypeptide chain elongation in E. coli occurs by the cyclical repetition of three steps. ,hat are these steps and what cellular components are necessary for each of them to occur) #ns: he three steps are! 1" An aminoacyl3tRNA is brought to the A site by (C3u with bound 69? 4" peptidyl transferase a riboyme" catalyes peptide3bond formation? &" the ribosome translocates three nucleotides down the mRNA helped by (C36 translocase". his shifts the peptidyl3tRNA to the A site and the deacylated tRNA to the ( site. &7. Protein s"nthesis Page: 10(0 i!!icult": 2 A new antibiotic was recently discovered that inhibits prokaryotic protein synthesis. In the presence of the antibiotic protein synthesis can be initiated but only dipeptides that remain bound to the ribosome are formed. ,hat specific step of protein synthesis is likely to be blocked by this antibiotic) #ns: he antibiotic probably blocks translocation. &8. Protein s"nthesis Page: 10(2 i!!icult": 1 #ns: * he large structure consisting of a mRNA molecule being translated by multiple copies of the macromolecular comple-es that carry out protein synthesis is called a!
A" #" %" '" ("
lysosome. polysome. proteosome. ribosome. synthosome.
&$. Protein s"nthesis Pages: 10(2%10(' i!!icult": 2 Collowing the synthesis of their polypeptide chain many proteins re=uire further posttranslational modifications before they attain their full biological activity or function. Dist and describe briefly at least four possible types of modification that can occur. #ns: 1" Amino3terminal modification N3acetylation or deacetylation" 4" removal of signal se=uences used for targeting &" side3chain modification phosphorylation carbo-ylation methylation etc." *" attachment of N 3linked to Asn" or O3linked to /er or hr" oligosaccharide moieties $" isoprenylation of %ys side3chains +" incorporation of prosthetic groups heme biotin etc." E" processing of proenymes or ymogens and 8" formation of disulfide crosslinks. '0. Protein s"nthesis Page: 10(' i!!icult": 3 In no more than three sentences describe a nonsense suppressor tRNA and how it differs from a normal tRNA. #ns: A suppressor tRNA has one or more altered bases in its anticodon which allows it to base pair with one of the stop codons. his allows the insertion of an amino acid instead of the chain termination that would normally have occurred at the point of a nonsense stop" codon.
Chapter 27 Protein Metabolism
($
'1. Protein targeting and degradation Page: 10(8%10($ i!!icult": 2 ,hen first synthesied proinsulin has an additional leader or signal peptide at its amino terminus. his complete molecule is called preproinsulin and the signal peptide is cleaved off to give proinsulin. #riefly what is the likely function of the signal peptide) #ns: he leader peptide in proinsulin is a signal se=uence directing it to the endoplasmic reticulum from which it enters the 6olgi comple- and is packaged into a secretory vesicle for secretion by e-ocytosis. '2. Protein targeting and degradation Page: 10($ i!!icult": 2 'escribe the se=uence of events between the transcription of an mRNA for a secreted protein and the arrival of that protein in the lumen of the endoplasmic reticulum. #ns: 1" he mRNA forms an initiation comple- with a cytoplasmic ribosome and transcription begins. 4" he amino3terminal portion of the nascent chain containing a signal se=uence binds to an /R9 signal recognition particle" interrupting polypeptide elongation. &" he /R93ribosome3 nascent chain comple- binds to the ribosome and /R9 receptors on the cytosolic face of the (R? the /R9 then dissociates. *" %hain elongation continues with the newly synthesied polypeptide crossing into the (R lumen as it grows. $" he signal se=uence is cleaved. '3. Protein targeting and degradation Pages: 1071%1073 i!!icult": 2 ,hat are the stages in targeting of nuclear proteins and why are the targeting se=uences not removed upon arrival of the protein in the nucleus) #ns: In the cytoplasm where eukaryotic protein synthesis occurs proteins carrying nuclear localiation signal ND/" se=uences are bound by a comple- of i mportin α and β which is then bound to a nuclear pore. Ran 69ase mediates translocation of this comple- into the nucleus where importin β dissociates from importin α and importin α releases the nuclear protein. Importin α and β are then e-ported from the nucleus and available for another cycle of import. he nuclear envelope of higher eukaryotes breaks down at each cell division distributing the nuclear contents throughout the cell. ,hen the nuclear envelope is reestablished ND/3carrying proteins can be reimported to fulfill their function. '&. Protein targeting and degradation Page: 107' i!!icult": 3 'escribe the role of ubi=uitin in mediating intracellular protein breakdown. #ns: 7bi=uitin a protein found in all eukaryotic cells is covalently 5oined to proteins targeted for degradation. he carbo-yl3terminal residue of ubi=uitin is first activated by the formation of a thioester with the first enyme in the pathway in an A93dependent reaction. After displacement of the first enyme by a second also yielding a thioester link ubi=uitin is finally 5oined through its carbo-yl terminus to a lysine ε3amino group in the protein to be degraded. he presence of ubi=uitin targets the protein for degradation by cellular proteases.
