Download Translation

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
T-RNA: small, 80n, single strand with secondary
structure (folding). Anticodon at end of loop.
function = picks up aa & transports it to ribosome
Ribosomes: composed of rRNA and proteins
Sequence of 4 Steps in
Translation...
1. ACTIVATION : Add an
amino acid to tRNA
Requires enzyme and
ATP
Creates an aa-tRNA
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
3. ELONGATION: adding new aa’s - peptidyl transferase
4. TERMINATION: stopping the process
Components
of translation
Another view of
translation
PUTTING IT ALL
TOGETHER!
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.
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
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?
• 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!
• 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
operator - binds repressor protein
• 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.
• 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.
•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
CRP gene
• 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
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.
• Tryptophan is needed to inactivate the
trp operon—it 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