General Summary of Protein Synthesis:
1. Transcription (Is this a prokaryote?)
2. RNA leaves the nucleus after modification, through nuclear pore.
Although not evident in this drawing, the ER membrane is contiguous
with the nuclear membrane and surrounds the nucleus. Depending on the fate of the mRNA, cytosolic proteins will be
synthesized on free ribosomes (3) while the mRNA for those proteins
destined for secretion or to be included on a membrane go to the
ribosomes on the ER (rough ER, 4).
Once the mRNA has left the nucleus, it will be bound by the small
and large Ribosomal subunits! Translation begins and
a) the polypeptide that is being made has a specific sequence of amino
acids called a Signal Sequence, the whole complex (mRNA,
ribosome, initial polypeptide) will be brought to the Rough ER to be
further translated and translocated across the ER membrane into the
ER lumen OR into the lumen and the ER membrane.
b) OR – if there is no Signal sequence the polypeptide is translated on
free ribosomes I the cytosol
Q? On which kind of ribosome complex do you think a Ribosomal protein
would be translated?? Why?
5. Inside the ER lumen, the proteins are folded with the help of
chaperone proteins (ie. BiP). This means, the proper bonds
(Hydrogen, ionic, Disulphide) are made between the PROPER amino
acids on the pooypeptide chain. This will make the proper
TERTIARY (3 ) structure.
Also, on the proteins that require sugar modification (mainly cell
membrane proteins) the basic sugar (oligosaccharide) unit will be
attached to the polypeptide in the ER. 6. The folded proteins leave the ER in a vesicle and head to the Golgi.
Q1? How did the membrane proteins get in the plasma