Where & How Do Ribosomes Make Proteins? Let’s Know!

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Where do ribosomes make proteins?

In prokaryotes, there is no nucleus in the cell so the ribosomes catalyze the formation of proteins by working on the mRNA in the cytoplasm with the help of tRNA.

In eukaryotes, there is a definite nucleus where the ribosome and mRNA production takes place. The ribosomes and mRNA come out of the nucleus into the cytoplasm where the ribosomes work upon and catalyze the formation of proteins. Meaning that in eukaryotic cell, ribosomes make proteins in the cytoplasm.

We all know that the production of mRNA from DNA is transcription, and the production of proteins from mRNA is translation.

Ribosomes are those macromolecules (can also say cell organelles) that catalyze the production of proteins during translation, thus overall helping in the production or sythesis of proteins.

So, in the eukaryotes, translation takes place in the cytoplasm i.e. outside the nucleus and so it will be correct to say that ribosome make proteins in the cytoplasm.

Whereas, in the case of prokaryotes, there is no nucleus in the cell, and so it is obvious that ribosomes make proteins in the cytoplasm.

So, in eukaryotes, the various mature mRNA molecules exit the nucleus and enter the cytoplasm where the ribosomes are located.

On the other hand, in prokaryotic organisms, ribosomes can attach to mRNA while it is still being transcribed all itself in the cytoplasm.

It has been seen that ribosomes when not active, split into two separate subunits, one large and one small subunit, and float freely in the cytoplasm.

And, as the protein synthesis begins, one small and one large subunit come together to form an active ribosome and catalyze the translation of mRNA to proteins.

How do ribosomes make proteins?

Ribosomes remain free as two subunits (one large subunit and one small subunit) by floating freely in the cytoplasm when protein production (translation) is not going on.

A eukaryotic ribosome also called the 80S ribosome, is made up of two subunits – the large 60S subunit and a small 40S subunit.

A prokaryotic ribosome also called the 70S ribosome, is made up of two subunits – the large 50S subunit and a small 30S subunit.

But when, it’s time for translation, the two subunits assemble together to form an active ribosome and catalyze the translation of mRNA to proteins.

During assembling together, the large subunit sits on top of the small subunit, with the mRNA template sandwiched in between the two subunits in the Start Codon region of the mRNA. All these happen at particular sites within the cytoplasm.

Meaning that Translation occurs with one large subunit sitting on top of the Start Codon of mRNA, and the other small subunit sitting at the bottom at the same Start Codon position of mRNA.

tRNA also gets attached to the mRNA. Each tRNA molecule has two distinct ends, one of which binds to a specific amino acid, and the other which binds to the corresponding mRNA codon.

In this way, it covers the mRNA strand from top and bottom forming the ribosomal structure. As the ribosome is formed it starts translation that is the synthesis of proteins from the Start Codon end of mRNA to the Stop Codon end of mRNA.

The smaller subunit reads the coding of mRNA, and the larger subunit functions to link the amino acids with peptide bonds to form various protein chains.

The function of tRNAs here is to carry the various amino acids to the ribosome and join with their complementary codons.

In prokaryotes, translation can happen at the same time when the mRNA is being formed during transcription, as no processing of mRNA is required.

However, in eukaryotes, translation only begins when the processing of the mRNA strand has been fully completed.

Translation has 3 steps viz. Initiation, Elongation, and Termination.

  1. During Initiation, the ribosome assembles around the mRNA to be read and the first tRNA carrying the amino acid methionine matches the amino acid chain with the start codon, AUG.
  2. During Elongation, the amino acid chain gets longer due to the addition of more and more amino acids one after the other over time. To do this, the mRNA is read at one codon at a time, and the matching amino acid for that codon is added to the growing protein chain.
  3. During Termination, the growing protein chain of amino acid is released after reaching any of the stop codons (UAG, UAA, or UGA). This is the final step of translation leading to the production of protein.

In such a way, during the translation process with the help of ribosomes, proteins are produced by adding together long chains of amino acids by encoding the mRNA strand that was produced during transcription.

Structure of Ribosome
Structure of Ribosome (Image credit: OpenStax Biology)

Free Ribosomes vs. Bound Ribosomes: Which type of ribosomes make proteins?

In eukaryotes, there are two types of ribosomes that are formed: Free Ribosomes and Bound Ribosomes.

In prokaryotes, there are only the Free Ribosomes present inside the cell.  Since there are no membrane-bound organelles in prokaryotes, all ribosomes are free ribosomes.

Free Ribosomes are located freely in the cytoplasm and create proteins that can be used within the cell.

Whereas on the other hand, Bound Ribosomes are the membrane-bound ribosomes that remain attached to the outer membrane of the Rough Endoplasmic Reticulum and create proteins that leave out of the cell.

The free ribosomes are able to move throughout the cell and can continuously synthesize proteins by translation of mRNA in the cytoplasm and used within the cell.

Free ribosomes remain excluded from the cell nucleus and other organelles, and can only work in the cytoplasm strictly.

On the other hand, Bound ribosomes not only create proteins that can leave out of the cell immediately but can also create proteins that will be stored in units called vesicles for future delivery outside of the cell.

So, Which type of ribosomes make proteins? Both Free and Bound Ribosomes can make proteins. Both ribosomes can produce different types of proteins. The prominent difference is that Bound Ribosomes made proteins are exported from the cell to be used outside the cell, whereas, free ribosomes made proteins will be used inside the cell.

For example, you can say that the proteins like insulin, calcitonin, ACTH, and other peptide hormones are made by the Bound Ribosomes of the peptide hormone-secreting cells of the endocrine glands.

Another example, the proteins like LRPPRC, CHCHD3, etc. that are used in mitochondrial transcription so utilized by the mitochondria of the cell and so are made by using free ribosomes.

Where do the proteins go after formation by ribosomes?

If the proteins are produced by free ribosomes, then those proteins will remain inside the cell and will be eventually utilized by the cell itself for its proper functioning, growth, and division.

In simple words, anything that you can imagine of is made up of proteins inside the cell is due to the involvement of free ribosomes. Without free ribosomes, the various components of the cell could not function.

If the proteins are produced by bound ribosomes, then those proteins will be secreted outside the cell and will be utilized by the target cells or target regions of the body.

Bound Ribosomes are actively involved in the production and secretion of numerous proteins in the form of various enzymes, hormones, etc. that are secreted outside the cell.

In both cases, when the ribosomes make polypeptide chains, it is necessary for the various linear protein chains to fold into the correct three-dimensional (3-D) shapes. The 3-D shaped proteins are biologically functional and so can be targeted to the correct part of the cell.

Now, once the liner protein chain attains its correct three-dimensional (3-D) shape, there occurs a chemically modified amino acid at the amino-end or the carboxyl-end of the 3-D protein.

This modified amino acid will direct and help transport the 3-D shaped protein from the cytoplasm to its correct compartment or location within the cell.

For example: Just like the peptide hormone insulin that is produced by Bound ribosomes in the islets β-cells of the pancreas will be transported and stored in the various secretory granules within the cell so that it can be secreted outside the cell upon stimulation by granular exocytosis.

For example: Just like the mitochondrial proteins like UCP2, UCP3, including 1100 to 1400 other distinct mitochondrial proteins are produced by the respective free ribosomes and are then transported and stored within the mitochondria. They are generally involved in the mitochondrial function like carrying out reactions of the electron transport chain (ETC).

What cells have lots of ribosomes?

It’s true that protein requirement is necessary for each and every cell of the body, and so protein production is essential for each and every cell of the body.

And, ribosomes are those macromolecules without which protein production cannot be possible. And so, ribosomes are found in practically every cell type of multicellular organism, as well as in prokaryotes.

Those cells that are actively involved in protein production all of the time will have more amount of ribosomes as compared to those cells which are less actively involved in protein production.

For example: Hormone producing cells like pancreatic endocrine cells (Islets of Langerhans) will have more ribosomes than the bone cells (osteoblasts, osteocytes, osteoclasts, and other bone lining cells).

Each ribosome consists of about approximately 60% ribosomal RNA (rRNA) and 40% proteins. Almost more than half of the cell’s energy (almost 60%) is utilized in the production of ribosomes.

Just for instance, a human brain cell can have as many as 8 to 16 million ribosomes per cell, and so it can devote up to 60% of its energy to constructing them from RNA and 80 different types of proteins.

Small cells, take for example in a bacteria like E. coli, there can be almost 10,000 to 15,000 ribosomes per bacteria cell. It’s because the cell metabolism isn’t so diverse when compared to those of eukaryotic organisms like human brain cells.

Ribosomes are better known as the protein factory of the cell as they synthesize the proteins by joining amino acids into protein chains.

So, it won’t be wrong to conclude that there are millions of protein factories called ribosomes in every cell of higher organisms like mammals, etc.

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