What are ribosomes made of?
Ribosomes are made up of the combination of ribosomal proteins and ribosomal RNA (rRNA). That’s why ribosomes are also called riboprotein complexes.
Each ribosome consists of about approximately 60% ribosomal RNA (rRNA) and 40% proteins.
Most of the ribosomal proteins fall into various energy-consuming enzyme families including ATP-dependent RNA helicases, AAA-ATPases, GTPases, and Kinases.
A complete ribosome is made up of a large subunit and a small subunit combined together, with the large subunit sitting on top of the small subunit with an RNA template sandwiched between the two.
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.
Each subunit consists of one or more ribosomal RNA (rRNA) molecules and many ribosomal proteins (r-proteins).
Ribosomes are tiny molecular machines inside the cells and their main function is the production of proteins by the translation of mRNA.
That is ribosomes have two main functions — decoding the message of the mRNA and the formation of peptide bonds of proteins by joining the amino acid chains together.
So, it has been seen that ribosomes when not active, split into the 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 are ribosomes made in prokaryotic cells?
In prokaryotic cells, we all know that there isn’t any membrane-bound nucleus and so all the genetic materials that are included in the single circular chromosome floats freely in the center of the cytoplasm.
So, in prokaryotes, the process of ribosome formation takes place in the cytoplasm with the transcription of many ribosome gene operons.
The ribosome gene operons contain those DNA segments (genes) in the single circular chromosome that will code for the ribosomal proteins.
The ribosome gene operons also include the genes for RNA polymerase and elongation factors (used in RNA translation). Regulation of all of these genes at once illustrates the coupling between transcription and translation in prokaryotes.
Altogether, there are 52 different genes that encode the ribosomal proteins, and they can be found in 20 ribosome gene operons.
There are ribosomal DNA (rDNA) sections as well in the single circular chromosome that will code for ribosomal RNA (rRNA). The encoded rRNAs differ in size, being distinguished as either large or small.
Now, after the production of ribosomal proteins and rRNA is completed they will assemble themselves into subunits and will remain freely floating in the cytoplasm. Sooner or later, they will form the complete ribosome.
The large rRNA will bind to many ribosomal proteins to form a large 50S subunit. On the other hand, the small rRNA will bind to many ribosomal proteins to form a small 30S subunit.
The rRNA strand is a type of non-coding RNA that acts as an enzyme called ribozyme to carry out the transcription of mRNA to Proteins.
So, the 70S prokaryotic ribosome that will be formed will consist of a large 50S subunit sitting on top of a small 30S subunit to form the complete ribosomal structure whenever the need arises.
If you see more precisely then, the 30S subunit contains 16S rRNA that is 1540 nucleotides long bonded with 21 ribosomal proteins.
On the other hand, the 50S subunit contains a 5S rRNA that is 120 nucleotides long, a 23S rRNA that is 2900 nucleotides long, all bonded with 31 ribosomal proteins.
How are ribosomes made in eukaryotic cells?
In eukaryotic cell, there’s a well-defined nucleus located in the center of the cytoplasm. This well-defined nucleus is bounded by a double-layered nuclear membrane that has the nucleoplasm within it.
Inside the nucleus, there’s a small dense region that has a majority of the cells’ DNA content, along with ribosomes and numerous proteins. This dense region is the nucleolus.
In the eukaryotes, unlike the prokaryotes, the nucleolus is a region within the cell nucleus where the production of rRNA and its binding with the ribosomal proteins to form the ribosomal subunits takes place.
It is also to be noted that, in eukaryotes, the synthesis of ribosomal protein doesn’t take place in the nucleolus, but only the production of rRNA takes place in the nucleolus.
Meaning that just like most of the protein synthesis, ribosomal protein synthesis also occurs in the cytoplasm from the ribosomal mRNA very rapidly. And, once the ribosomal proteins are formed then they get imported into the nucleolus through the nuclear-membrane pores.
Inside the nucleolus, the production of rRNA from rDNA has already taken place. So, as soon as the ribosomal proteins enter the nucleolus from the cytoplasm, the binding of rRNA with the various ribosomal proteins takes place.
Thus, the subunits of the ribosomes that are the 60S large subunit and 40S small subunit production take place there inside the nucleolus.
In general, if you see more precisely then, the 60S small subunit contains a 5S rRNA that is 120 nucleotides long, a 5.8S rRNA that is 160 nucleotides long, a 28S rRNA that is 4700 nucleotides long, all bonded with 49 ribosomal proteins.
On the other hand, the 40S small subunit contains an 18S rRNA that is 1900 nucleotides long and is bonded with 33 ribosomal proteins.
Now, these large and small subunits come out of the nucleus through the nuclear pores and remain there in the cytoplasm. These subunits will later assemble in the cytoplasm to make functioning ribosomes whenever the need for transcription arises.
In the case of the eukaryotes, most of the ribosomal proteins fall into various energy-consuming enzyme families, and so, almost 60% of the cell’s energy is spent on ribosome production and maintenance.
In eukaryotes, there are two types of ribosomes that are formed: Free Ribosomes and Bound Ribosomes.
Free Ribosomes are located freely in the cytoplasm and create proteins that can be used within the cell.
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.
What is ribosome biogenesis?
Ribosome biogenesis is the biological process of production of rRNA from rDNA, and also the production of various ribosomal proteins by transcription and translation of the various ribosome gene operons.
In simple words, ribosome biogenesis is the process of formation and maintenance of ribosomes inside the cell.
In eukaryotes, this process takes place both in the cytoplasm and the nucleolus. And, in the prokaryotes, this process takes place in the cytoplasm as there are no nucleus in prokaryotic cells.
There are more than 200 proteins involved in the synthesis and processing of the three prokaryotic rRNAs (5S RNA, 16S RNA, 23S rRNA), or four eukaryotic rRNAs (5S rRNA, 5.8S rRNA, 28S rRNA, 18S rRNA) during ribosome biogenesis.
The 200+ proteins are also involved in precisely binding of these rRNAs with the ribosomal proteins to form the structure of the large subunit and the small subunit of the ribosome.
Most of the ribosomal proteins fall into various energy-consuming enzyme families including ATP-dependent RNA helicases, AAA-ATPases, GTPases, and kinases.
And so due to the involvement of a lot of energy-consuming enzyme proteins during the biogenesis process, they use about 60% of a cell’s energy on ribosome production and maintenance.
Thus, it can also be concluded that ribosome biogenesis is a very energy-consuming process for the biological body.
In prokaryotes, it has been seen that ribosome biogenesis and other components of the protein biosynthetic machinery are directly dependent on nutrient availability and stress-related signals.
Whereas in multicellular eukaryotes, additional hormonal, growth factor-derived signals, along with the nutrient availability and stress-related signals also play important roles.
Also, it is to be noted that the molecular biological structure of the ribosomal proteins that are bonded with rRNAs to form the ribosomal submits may be a bit different from species to species.
Just, for instance, the bacterial ribosome consists of three rRNA molecules and approximately 55 proteins, components that are put together in an intricate and tightly regulated way.
It is also to be noted that the production of ribosomes that is the ribosomal biogenesis process happens more in those cells that are continuously involved in the production of proteins, like brain cells, pancreatic cells, etc. as compared to the other normal cells of the body.
What is so unique about ribosomes?
Ribosomes are those tiny organelles that are also considered as macromolecules that are composed of two macromolecules: ribosomal proteins and rRNA.
Many of the ribosomes are free-floating in the cytoplasm and are always in the state of continuous involvement for the translation of mRNA to proteins.
While many membrane-bound ribosomes remain attached to the outer surface of the Rough Endoplasmic Reticulum. These are also indulged in the synthesis and secretion of many protein molecules.
A single cell may contain up to 10 million ribosomes. Without these ribosomes, cells would not be able to produce protein and so would not be able to function properly.
Most cell organelles are membrane-bound whereas a few like ribosomes, cytoskeleton, and cell wall are not membrane-bound organelles.
There’s a important use of the two subunits that make up the ribosome. The smaller subunit reads the coding of mRNA, and the larger subunit functions to link the amino acids with peptide bond to form various protein chains.
The production and maintenance of ribosomes is the most energy-expensive process for the cell. Cells conduct ribosome production only in conditions of high ATP energy availability.
Ribosomes are special because they are one of those few organelles cum macromolecules that are found in both prokaryotes and eukaryotes, although the structure of ribosomes can be a bit different between the two types of cells.
Ribosomes don’t always remain as ribosomes. In fact, they do remain as freely floating subunits (one large and another small) when they don’t have to do their job of mRNA translation.
But, when it is time to make the proteins that is during the mRNA translation, the two subunits come together and combine with the mRNA. The subunits lock onto the mRNA forming a complete ribosome and start the protein synthesis.
And soon after the protein synthesis process has been completed, the two subunits separate into freely floating tiny macromolecules again.