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University of California - Irvine
Biological Sciences
Rahul Warrior

Translation •  The  synthesis  of  every  protein  molecule  in  a  cell  is  directed   by  an  mRNA  originally  copied  from  DNA     •  Protein  produc▯on  includes  two  kinds  of  processes:   –  informa▯on-­‐transfer  processes,  in  which  the  RNA  base   sequence  determines  an  amino  acid  sequence   –  chemical  processes,  in  which  the  amino  acids  are  linked   together.     •  The  complete  series  of  events  is  called  transla▯on   2   Polypeptides •  Polypep▯de  chains  are  linear  polymers  of  amino  acids   •  There  are  twenty  naturally  occurring  amino  acids,  the   fundamental  building  blocks  of  proteins   •  Pep▯de  bonds  link  the  carboxyl  group  of  one  amino  acid  to   the  amino  group  of  the  next  amino  acid   •  The  sequence  of  amino  acids  in  proteins  is  specified  by  the   coding  informa▯on  in  specific  genes   3   4   5   9.1  Ribosomes  Are  Transla▯on  Machines   •  Polypep▯des  are  strings  of  amino  acids  that  are   assembled  by  ribosomes   •  Ribosomes  are  lmachinesz  that  contain  mul▯ple   ribosomal  RNAs  (rRNAs)  and  proteins   •  Ribosomes  translate  mRNA  in  the  5ʹ′–3ʹ′  direc▯on,   reading  each  triplet  codon  and  assembling  the   amino  acids  in  the  order  specified  by  the  codons   6 Messenger  RNA   •  mRNA  sequence  dictates  the  resul▯ng  amino  acid   sequence   •  Boundaries  of  transla▯on  are  defined  by  a  start   codon  that  corresponds  to  the  N-­‐terminus  of  the   protein  and  a  stop  codon  that  corresponds  to  the     C-­‐terminus   •  The  5ʹ′  untranslated  region  (5ʹ′  UTR)  and  3ʹ′  UTR   are  segments  of  the  mRNA  outside  of  the   translated  regions   7 8 Bacterial  and  Eukaryo▯c  Ribosome  Structures   •  Ribosomes  in  bacteria  and  eukaryotes  perform     three  tasks   1. Bind  mRNA  and  iden▯fy  the  start  codon,  where   transla▯on  begins   2. Facilitate  complementary  base  pairing  of  mRNA  codons   and  the  corresponding  tRNA  an▯codons   3. Catalyze  forma▯on  of  pep▯de  bonds  between  amino  acids   on  the  growing  polypep▯de  chain   9 The  transla▯on  system  consists  of  five   major  components: •  Messenger RNA: mRNA is needed to provide the coding sequence of bases that determines the amino acid sequence in the resulting polypeptide chain •  Ribosomes are particles on which protein synthesis takes place •  Transfer RNA: tRNA is a small adaptor molecule that translates codons into amino acid •  Aminoacyl-tRNA synthetases: set of molecules catalyzes the attachment of a particular amino acid to its corresponding tRNA molecule •  Initiation, elongation, and termination factors 10   9.2  Transla▯on  Occurs  in  Three  Phases   •  Transla▯on  can  be  divided  into  three  phases:   ini▯a▯on,  elonga▯on,  and  termina▯on   •  The  phases  are  similar  in  bacteria  and  eukaryotes,   though  there  are  several  differences   11 Eukaryo▯c  Transla▯onal  Ini▯a▯on   •  The  eukaryo▯c  40S  ribosomal  subunit  complexes   with  eukaryo▯c  ini▯a▯on  factor  (eIF)  proteins   •   eIF1A,  eIF3,  and  a  charged  tRNAt  bind  the  small   subunit  to  form  the  preini▯a▯on  complex   •  This  complex  is  recruited  to  the  5ʹ′  cap  region  of   mRNA   12 Later  Steps  of  Eukaryo▯c  Ini▯a▯on   •  The  preini▯a▯on  complex  joins  a  group  of  at  least   four  eIF4  proteins  that  assembles  at  the  5ʹ′  cap  of     the  mRNA   •  Together  all  of  these  components  comprise  the   ini▯a▯on  complex   •  Once  the  ini▯a▯on  complex  is  formed,  it  uses   scanning  to  move  the  small  subunit  along  the  5ʹ′   UTR  in  search  of  the  start  codon   13 The  Final  Steps  of  Eukaryo▯c  Ini▯a▯on     •  The  correct  start  codon  (AUG)  can  be  located   because  it  is  embedded  in  a  consensus   sequence:     5ʹ′-­‐ACCAUGG-­‐3ʹ′  (this  sequence  is  called  the   Kozak  sequence)   •  Loca▯on  of  the  start  codon  leads  to   recruitment  of  the  60S  subunit  to  the  complex   15 Polypep▯de  Elonga▯on   •  Elonga▯on  begins  with  recruitment  of  elonga▯on   factor  (EF)  proteins  that  use  energy  of  GTP   hydrolysis  to   1.  Recruit  charged  tRNAs  to  the  A-­‐site   2.  Form  pep▯de  bonds  between  sequen▯al  amino  acids   3.  Translocate  the  ribosome  in  the  3ʹ′  direc▯on  along     the  mRNA   17 Translation: Elongation •  In  the  first  step  of  elonga▯on,  the  40S  subunit  moves  one   codon  farther  along  the  mRNA,  and  the  charged  tRNA   corresponding  to  the  new  codon  is  brought  into  the  A  site  on   the  60S  subunit   •  A  pep▯dyl  transferase  ac▯vity  catalyzes  a  coupled  reac▯on  in Met which  the  bond  connec▯ng  the  methionine  to  the  tRNA  is   transferred  to  the  amino  group  of  the  next  amino  acid,   forming  the  first  pep▯de  bond     •  In  the  next  step,  the  60S  subunit  swings  forward  to  catch  up   with  the  40S,  and  at  the  same  ▯me  the  tRNAs  in  the  P  and  A   sites  of  the  large  subunit  are  shi▯ed  to  the  E  and  P  sites,   respec▯vely   18   19 20 Transla▯on  Termina▯on   •  The  elonga▯on  cycle  con▯nues  un▯l  one  of  the   three  stop  codons  (UAA,  UAG,  UGA)  enters  the  A-­‐ site  of  the  ribosome   •  Bacteria  and  eukaryotes  both  use  release  factors   (RF)  to  bind  a  stop  codon  in  the  A-­‐site   •  The  polypep▯de  bound  to  the  tRNA  at  the  P-­‐site  is   then  released  when  the  GTP  complexed  to  the  RF   is  hydrolyzed   21 22 23 Translation •  The  mRNA  is  translated  in  the  5'  -­‐to-­‐3'  direc▯on.  The   polypep▯de  is  synthesized  from  the  amino  end  toward   the  carboxyl  end     24   9.4  The  Gene▯c  Code  Translates  Messenger   RNA  into  Polypep▯de   •  correspondence  between  nucleo▯de  sequences   of  mRNAs  and  the  amino  acid  sequences  of  the   resul▯ng  polypep▯des   •  Transfer  RNAs  are  adaptor  molecules  that   interpret  and  then  act  on  informa▯on
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