bc project protein synthesis
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INTRODUCTION
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• Protein synthesis is the process in
which cells build proteins. The term is sometimes used to
refer only to protein translation but more often it refers to a
multi-step process, beginning with amino acidsynthesis and transcription of nuclear DNA into messenger
RNA, which is then used as input to
• The cistron DNA is transcribed into a variety of RNA
intermediates. The last version is used as a template insynthesis of a polypeptide chain. Proteins can often besynthesized directly from genes by translating mRNA.When a protein must be available on short notice or in largequantities, a protein precursor is produced.
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• Aproprotein is an inactive protein containing one or more inhibitory peptides that can be activated when theinhibitory sequence is removed by proteolysis during posttranslational modification. A preprotein is a form thatcontains a signal sequence (an N-terminal signal peptide)that specifies its insertion into or through membranes, i.e.,targets them for secretion. The signal peptide is cleaved off
in the endoplasmic reticulum.Preproproteins have bothsequences (inhibitory and signal) still present.
• For synthesis of protein, a succession of tRNA moleculescharged with appropriate amino acids have to be broughttogether with an mRNA molecule and matched up by base-pairing through their anti-codons with each of its successivecodons. The amino acids then have to be linked together toextend the growing protein chain, and the TRNAs, relievedof their burdens, have to be released. This whole complexof processes is carried out by a giant multimolecular machine, the ribosome, formed of two main chains of RNA,
called ribosomal RNA (rRNA), and more than 50 differentproteins. This molecular juggernaut latches onto the end of an mRNA molecule and then trundles along it, capturingloaded tRNA molecules and stitching together the aminoacids they carry to form a new protein chain.
Protein synthesis
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1.DNA unwinds
2. mRNA copy is made of one of the DNA strands.
3. mRNA copy moves out of nucleus into cytoplasm.
4. tRNA molecules are activated as their complementary
amino acids are attached to them.
5.mRNA copy attaches to the small subunit of the
ribosomes in cytoplasm. 6 of the bases in the mRNA
are exposed in the ribosome.
6.A tRNA bonds complementarily with the mRNA via its
anticodon.
7.A second tRNA bonds with the next three bases of the
mRNA, the amino acid joins onto the amino acid of the
first tRNA via a peptide bond.
8.The ribosome moves along. The first tRNA leaves the
ribosome.
9.A third tRNA brings a third amino acid
10.Eventually a stop codon is reached on the mRNA.
The newly synthesised polypeptide leaves the
ribosome.
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OVERVIEW
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Tran s c r(mak i n g
• The part of the DNA molecule (the gene) that the
cell wants the information from to make a proteinunwinds to expose the bases.
• Free mRNA nucleotides in the nucleus base pair with
one strand of the unwound DNA molecule.
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Transcri
• The mRNA copy is mode with the help of RNA
polymerase. This enzyme joins up the mRNA
nucleotide to make a mRNA strand.
• This mRNA strand is a complementary copy of theDNA (gene).
• The mRNA molecule leaves the nucleus via a
nuclear pore into the cytoplasm.
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t R N A–p ic k u p
f r o m t
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Translatio
mRNA use(protein)
1.
• First the mRNA attaches itself to a ribosome(to
small subunit).
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• Six bases of mRNA are exposed.
• A complementary tRNA molecule with its attached
amino acid base pairs via its anticodon UAC with the
AUG on the mRNA in the first position P.
• Another tRNA base pairs with the other three mRNA
bases in the ribosome at position A.
• The emzyme peptidal transferase forms a peptde
bond between the two amino acids.
• The first tRNA(without its amino acid) leaves the
ribosome.
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Translat
• The ribosome moves along the mRNA to the next
codon.
• The second tRNA molecule moves into position P.
• Another tRNA molecule pairs with the mRNA in
position A bringing its amino acid.
• A growing polypeptide is formed in this way until a
stop codon is reached.
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End of
•
A stop codon on the mRNA is reached and thissignals the ribosome to leave the mRNA.
• A newly synthesized protein is now
complete.
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T r a n sm R N
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Protein Synthesis InhibitorsProtein Synthesis Inhibitors
• Many of the antibiotics utilized for the treatment ofbacterial infections as well as certain toxins
function through the inhibition of translation.Inhibition can be effected at all stages oftranslation from initiation to elongation totermination.
Several Antibiotic and Toxin inhibitors of Translation
Inhibitor Comments
Chloramphenicol
inhibits prokaryotic peptidyl transferase
Streptomycin inhibits prokaryotic peptide chain initiation, also induces mRNA misreading
Tetracycline
inhibits prokaryotic aminoacyl-tRNA binding to the ribosome small subunit
Neomycin similar in activity to streptomycin
Erythromycin inhibits prokaryotic translocation through the ribosome large subunit
Fusidic acidsimilar to erythromycin only by preventing EFG from dissociating from thelarge subunit
Puromycinresembles an aminoacyl-tRNA, interferes with peptide transfer resulting inpremature termination in both prokaryotes and eukaryotes
Diphtheria(diptheria) toxin
protein from Corynebacterium diphtheriae which which causes diphtheria(diptheria); catalyzes ADP-ribosylation and inactivation of eEF-2; eEF-2
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contains a modified His residue known asdiphthamide (dipthamide), it is thisresudue that is the target of the toxin
ADP-ribosylated diphthamide (dipthamide) residue
Ricinfound in castor beans, catalyzes cleavage of the eukaryotic large subunitRrna
Cycloheximide
inhibits eukaryotic peptidyltransferase
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