1 year, 3 months ago-
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Proteins are the building blocks of life is an established fact. DNA stores the code for protein synthesis, and this information is translated into proteins.
That sounds pretty fascinating and we wonder, where do all these processes occur?
DNA is found in the nucleus, and we may naturally think that in the nucleus, protein synthesis occurs. Well, that’s not the case. Permeability of the synthesized polypeptides across the nuclear membrane is technically not feasible; hence, protein synthesis in the nucleus is completely impossible.
So Where are proteins synthesized? What is the site of protein synthesis?.
Generally speaking, protein synthesis takes place in the cell cytoplasm. However, is this the correct answer as well? Does protein synthesis universally take place only in the cell cytoplasm?
Arguably, it may be counter questioned that prokaryotic and eukaryotic cells are structurally different, then how protein synthesis occurs at the same place in both the cells.To solve this jigsaw, let’s understand the fundamental- How and where does protein building begin?
Protein synthesis in cells is a two-step process- transcription and translation.
Firstly, in transcription, the genetic information from DNA is copied to the complementary single-stranded mRNA. Transcription occurs in the cell’s nucleus. Transcription is followed by translation; wherein protein synthesis is completed. Hence, sometimes protein synthesis is also known as translation. After transcription, mRNA moves out of the nucleus and carries out the translation process. In translation, the formation of the polypeptide from the encoded genetic code on mRNA occurs with the help of ribosomes. Ribosomes are the spherical organelles that read the code for amino acids and combine them to form a polypeptide. Each ribosome is made up of two subunits- a smaller subunit and a larger subunit. These two units are found freely in the cytoplasm of the cell. However, during the translation process, these two units come together and get latched to the mRNA strand for carrying out the protein synthesis (Fig 1). Rightly so for this reason, ribosomes are better known as the protein factory of the cell. This is because protein synthesis occurs in the ribosomes.
So this leads us to conclude that ribosomes are the site of the translation step in protein synthesis, and their location will determine the site of protein synthesis in a cell.
That brings us to the question- where are ribosomes located?
Prokaryotic cells and eukaryotic cells differ in their ribosomal structure and location inside the cell. So let us understand them separately.
Figure 1: Ribosomes translating along with mRNA to form a polypeptide (Image source: Dr. Amita Joshi)Interesting fact: Protein synthesis seems to be such a lengthy procedure, and we wonder how long does protein synthesis take? Isn’t it! Surprisingly, this is a pretty fast process and is accomplished in few seconds up to a maximum of a minute.
Where Does Protein Synthesis Take Place In Prokaryotes?
Prokaryotes have 70S ribosomes having 50S and 30S subunits. The ribosomes are freely found in abundance in the cytoplasm of the prokaryotic cell. With the above discussion, we now know that ribosomes are the protein factory of the cell. Based on the site of ribosomes in a prokaryotic cell, we can safely say, prokaryotic protein synthesis occurs in the cytoplasm of the cell
Where Does Protein Synthesis Take Place In Eukaryotes?
From the above discussion, it is clear that the availability/location of the ribosomes eventually determines the place of protein synthesis in a cell. Eukaryotic ribosome is a large 80S type and is made up of 60S and 40S subunits. The nucleolus is the site of ribosome synthesis in a eukaryotic cell.
Unlike prokaryotes, ribosomes are not limited only to the cytoplasm in eukaryotes; they are also found in mitochondria and endoplasmic reticulum (ER).
This brings us to the answer to a common question- Do mitochondria have ribosomes?. The answer to this is- Yes, mitochondria possess ribosomes.
Ribosomes are available freely in the cytoplasm as well as attached to the ER in a eukaryotic cell. ER is a mash of tubules or sacs that extend from the nuclear membrane to the whole of the cell cytoplasm. Depending upon the cell type and its function, the size of the ER varies from cell to cell.
In simple words, I would conclude that in eukaryotic cells, the processes of protein synthesis occur in different cellular locations. What’s your take on it?Does Protein Synthesis Take Place In Rough ER?
ER is of two types- Smooth ER and Rough ER. The ER to which the ribosome gets attached is known as rough ER, while the ER without ribosomes forms the smooth ER.
Figure 2: Rough and Smooth endoplasmic reticulum (ER) (Image sources: Dr. Amita Joshi)
Predictably, only rough ER is involved in synthesizing the protein, as they have ribosomes attached to their surface.
This brings us to another query, how is protein synthesis at rough ER different from the freely available cytoplasmic ribosomes?.
Well, functionally, both ribosomes are the same and can not be distinguished. Interestingly, all the protein synthesis is initiated by the freely available cytoplasmic ribosome. However, the proteins that are destined for secretion are diverted to the rough ER with the help of a signal sequence. So mRNA with signal sequence gets latched to the rough ER’s ribosomes and synthesizes the complete polypeptide. Once protein synthesis is completed, such secretory proteins are transferred to the ER lumen canal for distribution in the cell.Does Protein Synthesis Take Place In Mitochondria?
Mitochondria possess their own genes containing genetic information which is translated into proteins with the help of mitochondrial mRNA and ribosomes. Interestingly, the mitochondrial ribosomes are 70S type that carry out the protein synthesis along with mitochondrial mRNA.
The majority of the polypeptides synthesized by the mitochondria are hydrophobic in nature and form the functional center of the electron transport chain. Mitochondria is a highly preserved cell organelle, however, the complete mechanism of mitochondrial protein synthesis is being explored.
This leads us to the conclusion that, in a eukaryotic cell, protein synthesis occurs in the cytoplasm, ER, and mitochondria.Figure 3: Location of mitochondrial ribosomes (image source: Dr. Amita Joshi)
[At this stage, can you answer- What organelle synthesizes protein?
The protein synthesis organelles are cytoplasm, mitochondria and ER.]Finally, putting all the jigsaw pieces together, based on the present knowledge, it can be safely concluded that the location of the protein synthesis is determined by the site of the ribosomes in the cells. Accordingly, organelles involved in protein synthesis are cytoplasm, endoplasmic reticulum, and mitochondria.
In my view, the place of protein synthesis is not universally cytoplasm. Instead, the site of ribosomes governs it. Do you agree? What are your thoughts on it?
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