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Lecture 6

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Department
Biological Sciences
Course
55-213
Professor
Hubberstey
Semester
Winter

Description
Lecture 6 2013-01-24 9:59 PM Protein)production:)TRANSLATION+ • How+genes+are+actually+expressed+in+the+cell+ • Genes+code+for+RNA+molecule,+which+is+then+translated+to+protein+ • Only+1+strand+of+DNA+serves+to+produce+RNA+template+via+transcription+ • mRNA+has+a+code+on+it,+of+3+nucleotides+that+are+read.+Each+of+those+3+nucleotides+ corresponds+to+a+particular+amino+acid+determined+by+the+sequence.++ • Genetic+code+is+read+in+groups+of+3+nucleotides+(codon)+ • Gene+includes+series+of+codons+that+are+read+sequentially+(by+the+ribosome)+from+ starting+point+to+a+termination+point+in+5’+to+3’+direction+ • Starts+at+AUG+and+terminates+at+stop+codon+ • At+stop+codon,+ribosome+falls+off+of+mRNA+and+is+released+as+protein+in+the+cell+ • Codons+are+non$overlapping+(in+most+cases)+ o AUG+CAU+GGG+AAG+ o Met+++His+++Gly+++Lys+ • There+are+multiple+codons+that+code+for+the+same+amino+acid++ • Focus+of+translation+in+this+course:+Evolution+of+gene+structure+!+has+to+do+with+ introns+and+exons,+how+they+make+up+the+gene+and+their+sequence+ + Translational)Mutations:+ • Insertions+and+deletions+into+the+DNA+can+lead+to+amino+acid+changes+in+the+protein+ • Mutations+are+passed+on+from+transcription+to+translation,+and+then+screw+up+the+ protein+structure+ • Frameshift+mutations:+changes+the+reading+frame+of+the+codons,+usually+causes+ severe+effects+ • The+more+amino+acids+that+make+up+a+protein,+the+less+effect+the+mutation+will+have+ (1+or+2+mutations+in+1000+won’t+make+a+big+difference,+for+example)+ • Acridines:+chemicals+(dyes)+that+induce+structural+distortion+in+DNA+!+leads+to+ frameshift+mutations+ • If+insertions/deletions+are+multiples+of+3+nucleotides+less+severe+consequences+result+ • Because+of+the+codon+triplet,+the+number+3+is+very+important.+Any+multiple+of+3+ nucleotides+(3,+6,+9,+12)+that+are+either+inserted+or+deleted+isn’t+as+potentially+severe+ on+the+protein+as+a+non+multiple+of+3+(1,+5,+8,+etc).+Why?+When+you+take+out+a+ multiple+of+3,+you+lose+1+amino+acid.+When+you+take+out+a+non+multiple+of+3,+you+lose+ the+amino+acid+and+you+shift+the+reading+frame+for+the+rest+of+the+sequence+!+ severe+consequence!+ • See+figure+2.10+!+the+genetic+code+is+triplet+ o Ala+codons:+GCU,+GCC,+GCA,+GCG+ o Alanine+is+degenerate+as+it+has+4+codons+ coding+for+it+ o Insertion+and+deletion+(double+mutant+ example)+isn’t+as+bad+ o Triple+insertion+(triple+mutant+example)+also+ isn’t+as+bad+ o However,+1+insertion+or+deletion+affect+ several+amino+acids+and+disrupt+the+protein+ • In+the+biotechnology+industry,+a+big+thing+is+protein+ engineering.+Biologists+will+make+mutations+on+ purpose+that+may+make+the+protein+better+(i.e.+more+ stable+at+higher+temperatures,+etc).++ • Although+mutations+happen+naturally+in+genes,+ molecular+biologists+can+you+use+that+to+their+ advantage+in+order+to+make+proteins+in+the+lab+via+ their+induced+mutations+ + Gene)Reading)Frames:+ • Three+possible+ways+of+reading+an+RNA+sequence+ o Ex:+ACGTTACGTCCCATGTTT+ • Frame+1:+ACG+TTA+CGT+CCC+ATG+TTT+ • Frame+2:+++CGT+TAC+GTC+CCA+TGT+TT+ • Frame+3:+++++GTT+ACG+TCC+CAT+GFF+T+ • Open+reading+frame+(ORF):+reading+frame+used+to+produce+the+protein+from+a+ particular+gene+(*important)definition*)+ o ORFs+start+with+an+AUG+initiator+codon+and+end+with+a+termination/STOP+ codon+(UAA,+UGA,+UAG)+ o Any+mutations+within+the+ORFs+can+alter+accurate+protein+translation+ (blocked+reading+frame)+ o See+figure+2.11+ o Generally,+the+longest+reading+frame+is+the+one+that’s+referred+to+as+the+ORF+ • After+the+human+genome+was+finished,+peopled+looked+at+genes+and+identified+them.+ However,+there+are+still+some+that+haven’t+been+identified;+they’re+called+URFs.+ • URF:+unidentified+reading+frame+ o An+ORF+for+which+no+protein+has+been+identified+ o E.g.+FLJ10560+!+there’s+an+unidentified+protein+on+chromosome+3+which+we+ don’t+know+what+it+does+or+its+identification+ o Because+they’re+not+identified,+they’re+given+a+bunch+of+letters+and+numbers+ o We+know+we+have+22+000+genes,+but+we+don’t+have+an+identification+for+ every+single+one+yet+ + Gene)and)protein)sequences)are)collinear:) • Sequence+of+nucleotides+is+same+as+sequence+of+amino+acids+ • 3n+bp+=+n+amino+acids+ + Transcription:+ • One+strand+of+DNA+serves+as+template+for+production+of+messenger+RNA+(mRNA)+ • See+figure+2.13+!+RNA+is+complementary+to+1+strand+of+DNA+ + + + + + + + + + + + + • For+a+very+short+time,+you+get+a+RNA+hybrid+ • Some+genes+are+being+read+by+RNA+polymerase+from+left+to+right,+and+others+are+ being+read+from+right+to+left+on+the+opposite+strand+ o All+genes+are+not+expressed+in+the+same+direction+ o Sometimes+the+template+strand+is+on+the+opposite+strand+ o This+is+due+to+recombination++ o RNA+polymerase+and+ribosome+always+do+their+work+5’+to+3’+ o But+sometimes,+the+gene+is+actually+made+in+the+opposite+direction+on+the+ chromosome+ o You+could+predict+which+one+is+the+template+strand+of+the+DNA,+because+it’ll+ be+complimentary+to+the+RNA+ o Don’t+always+assume+the+coding+strand+is+the+template+ • So+is+the+entire+mRNA+molecule+translated?+ o No.+the+polyadenylation,+introns+and+exons,+5’+to+3’+cap,+and+the+untranslated+ region+may+be+transcribed,+but+ultimately+won’t+be+translated.++ • Complete+mRNA+contains+5’+untranslated+region+(UTR)+called+leader+ • It+also+contains+3’+UTR+called+trailer+ • Why+do+we+have+this+leader+and+trailer?++ *very)important)slide!*+ o They+provide+information+to+the+ translational+machinery+about+where+the+ gene+is+going+to+start+and+stop+ o See+figure+2.14+!+a+gene+corresponds+to+a+ transcript+of+RNA!+ o The+gene+includes+the+leader+and+trailer+in+it+ o The+ribosome+initially+binds+to+the+leader+ (which+has+no+sequence)+and+ends+at+the+ trailer+(which+also+has+no+sequence)+ o They+provide+information+to+the+ribosome+ about+where+the+translation+is+going+to+start+ and+stop+ o What+else+do+they+do?++ " Protect+the+RNA+from+degradation.+There+are+sequences+in+these+ UTRs+that+bind+specific+proteins+(binding+proteins)+that+actually+ physically+protect+the+RNA+molecule+from+getting+chewed+up.+RNA+is+ much+more+sensitive+to+nuclease+attack+than+DNA+is.++ " When+the+RNA+is+transported,+the+signals+the+UTR+provide+can+
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