Ribosomes and protein synthesis

[Math Is Hard]

I've been looking at some explanations and pictorial illustrations of ribosomes in my biology book. Now, they say that ribosomes are the sites of protein synthesis, and I am confused because the ribosomes are situated on the outside of the rough ER, but the newly synthesized proteins seem to just "appear" (in the illustrations) on the inside of the rough ER, where they then get pinched off and sent via vesicles to the Golgi Apparatus or to wherever they are destined. What is really going on here? Are these proteins manufactured on the inside or outside of the rough ER? If they are made outside, how do they get inside?

thanks!
-mih

 

[Dr.Brain]

Ribosomes attach to m-RNA's and move from codon to codon synthesising the chain of amino acids.In this case, amino acids are brought from the cytoplasm by the clover-shaped t-RNA's . As the ribosome moves from one codon to codon , t-RNA's come and attach their bases with complementary bases on m-RNA and keep syntheissing the chain of amino acids which is then detached once the stop-codon is reached. And the chain of amino-acids (or a Protein) is left free to work for the cell.After this ribosome detaches itself from the m-RNA and starts lookign for another m-RNA to work with.
Ribosomes are not chained-organelles , they move around in the cell .

BJ

 

[iansmith]

I've been looking at some explanations and pictorial illustrations of ribosomes in my biology book. Now, they say that ribosomes are the sites of protein synthesis, and I am confused because the ribosomes are situated on the outside of the rough ER, but the newly synthesized proteins seem to just "appear" (in the illustrations) on the inside of the rough ER, where they then get pinched off and sent via vesicles to the Golgi Apparatus or to wherever they are destined. What is really going on here? Are these proteins manufactured on the inside or outside of the rough ER? If they are made outside, how do they get inside?

the proteins are manufactured outside the rough ER

First not all protein synthesis is done at the rough ER. Most protein are syntheis in the cytoplasm. So for protein synthesised in the cytoplasm, the site for a protein is dependent on a specific signal sequence on the protein. This signal will then be recognized by certain protein called chaperone. The chaperon will traffick the protein to the mitochondria or other organelles.

In the case of protein syntheised at the rough ER, an ER signal sequence will be regonized as the protein is being synthesized. This signal will cause the protein to be translocatted inside the ER (water solubule protein) or be embeded in the membrane after translocation (membrane bound proteins). the translocation is done by other proteins. These proteins form a channel that let unfolded/linear protein pass through.

 

[Math Is Hard]

Thanks, Dr. Brain. What are "codons"? I should probably have prefaced this thread with the note that I am in a beginning biology class for non-life-science majors.

Merci beaucoup, Ian. We were talking in class about "facilitated diffusion" last week, and how some molecules needed a channel protein to move through the cell membrane. Does it work the same way with proteins going into the rough ER, the translocation you mentioned? Do they diffuse through the channel protein? Or is this like "active transport"?

Also, is the ER made out of a phospholipid bilayer like the cell membrane?

Thanks for your help.

 

[selfAdjoint]

Codons are the units or "letters" of the genetic code in a particular genome. Each consists of an ordered triple of selections from the four bases. The individual bases in the codons are called nucleotides.

Google on genetic code.

 

[Math Is Hard]

Codons are the units or "letters" of the genetic code in a particular genome. Each consists of an ordered triple of selections from the four bases. The individual bases in the codons are called nucleotides.

Google on genetic code.

Thanks, SelfAdjoint. We just started studying nucleic acids and nucleotides this week, but I hadn't come across that term yet.

 

[iansmith]

Merci beaucoup, Ian. We were talking in class about "facilitated diffusion" last week, and how some molecules needed a channel protein to move through the cell membrane. Does it work the same way with proteins going into the rough ER, the translocation you mentioned? Do they diffuse through the channel protein? Or is this like "active transport"?

It is not the same but it could view as a similar mechanism. I think the mechanism may be energy-independent but it does not diffuse through the channel. The protein is pushed by the ribosome.

Look at the figures in the link below
http://nobelprize.org/medicine/laureates/1999/illpres/protein.html
http://www.cce.caltech.edu/faculty/shan/Fig1.jpg

Also, is the ER made out of a phospholipid bilayer like the cell membrane?

Yes, the ER and virtuall all organelles are made of phospholipid bilayers.

 

[Math Is Hard]

Thanks, Ian. That makes it a lot more clear. Those links are great! They don't mention "translocons" in my book (nor the SRPs and SRP receptors), and I think that's why I was having trouble making sense of the process.