BC Project Protein Synthesis

8/6/2019 BC Project Protein Synthesis

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INTRODUCTION



• 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.



• 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



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.



OVERVIEW



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.



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.





t R N A–p ic k u p

f r o m t







Translatio

mRNA use(protein)

1.

• First the mRNA attaches itself to a ribosome(to

small subunit).



• 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.





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.





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.



T r a n sm R N









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



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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BC Project Protein Synthesis