Translation

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Translation

• mRNA exits the nucleus through the nuclear pores
• In the cytoplasm, it joins with the other key
  players to assemble a polypeptide.
• The other parts of the machinery are
• t-RNAs and ribosomes

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T-RNA small,  80n, single strand with secondary
structure (folding). Anticodon at end of
loop. function picks up aa transports it to
ribosome
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Ribosomes composed of rRNA and proteins
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• Sequence of 4 Steps in Translation...             
                    
• ACTIVATION Add an amino acid to tRNA
• Requires enzyme and ATP Creates an aa-tRNA 
•             

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2. INITIATION assemble players
-ribosome, -mRNA, -aa-tRNA -Small ribosomal
sub units binds to mRNA. -Initiator tRNA binds to
P site -Large ribosomal unit binds to complex
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3. ELONGATION adding new aas - peptidyl
transferase
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4. TERMINATION stopping the process
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Components of translation
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Another view of translation
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PUTTING IT ALL TOGETHER!
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• REGULATION OF GENE EXPRESSION
• The control in the DNA transcription process is
  very tight.
• Cells are able to "turn on" or "turn off" genes
  when their products are not required in cell
  metabolism or control.
• Regulation of gene expression is now only being
  to be fully understood and is a major area of
  research today.

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GENE REGULATION IN PROKARYOTES

• Genes for a particular metabolic pathway are
  usually in clusters, under the control of one
  promoter and one operator.
• This cluster of genes, with its regulatory sites
  is called an operon

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The lac operon

• E. coli can use lactose as a source of energy
• To use lactose, it must split the lactose into
  glucose and galactose. This means producing
  enzymes.
• Only produces enzymes when lactose is present in
  the environment. WHY?

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• Three genes are involved
• 1. LacZ codes for B-galactosidase degrades
  lactose
• 2. LacY codes for B-galactosidase permease
  causes lactose to enter the cell
• 3. LacA codes for a transacetylase function
  unknown!

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• The lacl gene codes for the lacl protein
• This protein is always produced.
• When lactose is NOT present in the environment,
  the lacl protein binds to the operator site.
• This blocks RNA polymerase from attaching to the
  promoter site. Transcription is turned OFF

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operator   -  binds repressor protein
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• When lactose is PRESENT in the environment, The
  lactose binds to the lacl protein, and changes
  its shape
• The lacl protein falls off the operator site
  and RNA polymerase can now attach to the promoter
  site and transcription of the lac genes proceeds.
• Lactose is an inducer molecule. Its presence
  activates transcription of the genes that degrade
  it.

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• Often, both glucose and lactose are present in
  the environment.
• E. coli uses glucose first, since the enzymes for
  its use are always present.
• There is a mechanism to slow down the use of
  lactose even if it is present.
• When all glucose is used, then transcription of
  lac genes will speed up.

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• CATABOLIC REPRESSION
• glucose prevents the action of the LAC
  operon through another regulator-like protein,
  the cAMP receptor protein (CRP)
• CRP binds to DNA at the CRP gene
• CRP is aka as CAP ( Catabolite Activator protein)
• This involves the use of cAMP as an intermediate
  messenger

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CRP gene
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• CRP (aka CAP) is an allosteric protein, regulated
  by cAMP                
• when glucose is high - lots of ATP little cAMP
• CRP-alone conformation doesn't bind to CRP DNA
  region - favors slow transcription of lac genes
• when glucose is low - all the ATP is hydrolyzed
  favoring high cAMP amounts
• cAMP-CRP conformation can bind to CRP DNA
  region -favors rapid transcription of lac genes

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The TRP operon

• This cluster of genes is responsible for the
  enzymes that synthesize Tryptophan, an essential
  amino acid.
• They are always turned on EXCEPT when tryptophan
  is present in the environment.
• The operon consist of five genes, an operator, a
  promoter and the trp repressor protein.

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• Tryptophan is needed to inactivate the trp
  operonit is a corepressor.
• This type of regulation is by repression because
  the effector molecule interacts with the
  repressor protein so that it can bind to the
  operator

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