Definition of Ribosome
Ribosomes are cellular structures that are not surrounded by a membrane, as occurs in cellular organelles ( mitochondria , plastids and vesicles, among others).
These are found in the cytoplasm of prokaryotic and eukaryotic cells . In the latter, they are also located in the membranes of the rough endoplasmic reticulum (RER) and within the mitochondria and chloroplasts. These organelles use their own ribosomes to synthesize the proteins encoded by the genetic material (DNA) inside them.
What are Ribosomes?
Ribosomes are cellular organelles that are present inside all cells and are made up of ribonucleic acids (RNA) and proteins. They can appear in different states of dissociation. When they are complete, they can be isolated or form groups (polysomes).
Its size depends on the types of cells, with the average size of the ribosomes of a prokaryotic cell being 200 Å in diameter.
In this sense, an active eukaryotic cell contains approximately 10 million ribosomes, while a prokaryotic cell contains approximately 15,000 to 20,000 ribosomes (25% of the total cell mass).
The word ribosome comes from 'ribo' which means ribonucleic acid and from the Greek 'soma' which means body.
In eukaryotic cells, the ribosomes of the cytoplasm are called 80 S. In mitochondria and plastids of eukaryotes as well as in prokaryotes they are 70 S. Both rRNAs and the subunits of ribosomes are usually named by their sedimentation coefficient in Svedberg subunits.
Who were the discoverers of Ribosomes?
The Belgian biologist Albert Claude was one of the first scientists to discover ribosomes during a study in 1943. Using the differential centrifugation technique, the biologist observed very small granules among the final residues that he called “microsomes.”
In 1958, it would be the American biologist Richard Brooke Roberts who modified this name from “microsomes” to “ribosomes”, taking into account that they were composed mainly of ribonucleic acids.
However, the mechanism of action of ribosomes was not discovered until the late 1990s by Ada E. Yonath, the American biochemist Thomas Arthur Steitz and the British biologist of Indian origin Venki Ramakrishnan.
His work helped determine the structure of ribosomes and discover the mechanism of action of ribosomes in protein synthesis. They were awarded the 2009 Nobel Prize in Chemistry for their discoveries about ribosomes.
Ribosome Structure
In prokaryotic cells ribosomes are 40% protein and 60% rRNA. On the other hand, in eukaryotic cells ribosomes are made up of 50% proteins and 50% rRNA. This RNA is synthesized from the transcription of certain DNA sequences and is packaged together with ribosomal proteins in the nucleolus. In prokaryotic cells, this occurs in the cytoplasm.
Each ribosome is made up of two subunits: the smaller subunit and the larger subunit. When the protein synthesis process begins, the two subunits join together and when the process stops they dissociate. During protein synthesis these two subunits clump together and form a polyribosome or polysome.
These subunits are coupled when the smaller one recognizes an mRNA (messenger). When this happens, the mRNA remains inserted in the groove that is formed between the contact surfaces of both subunits. Finally, the subunits are uncoupled when a stop sequence (stop codon) is recognized in the mRNA.
In addition to the mRNA binding site, each ribosome has two more sites that bind to tRNA. The A (amino acid) site is the place where the tRNA loaded with an amino acid is inserted . The anticodon of this tRNA pairs with a codon of the mRNA.
The P (peptide) site is the place that houses the tRNA attached to the growing chain of amino acids, which will give rise to the final protein. Since both sites are very close to each other, the tRNA from the P site pairs with the codon immediately upstream of the A site.
Types of Ribosomes
To carry out a classification of the types of ribosomes, it is necessary to take into account several aspects, taking into account their arrangement and the type of cell to which they belong. Normally there are two types of ribosomes: eukaryotes and prokaryotes . However, we can differentiate some more. This would be the classification.
Eukaryotic ribosome
It is the largest type of ribosome of all, whose subunits have a sedimentation coefficient of 60S (highest) and 40S (lowest). They are found in both the cellular cytosol and the rough endoplasmic reticulum. More than half of this type of ribosome is composed of proteins and less than half of rRNA. They are larger in size than prokaryotic ribosomes.
Prokaryotic ribosome
It is a type of ribosome smaller than eukaryotes with a sedimentation coefficient of 70S. Prokaryotic cells do not have endoplasmic reticulum, so prokaryotic ribosomes are found in the cell cytosol. They are mostly made up of rRNA and less than half is made up of proteins.
Mitochondrial ribosome
They are small ribosomes found inside the mitochondria. Its size is variable, although in the case of animals its sedimentation coefficient is generally 50S.
Plastidial ribosome
They are ribosomes similar to the prokaryotic ribosomes (70S) found inside the plastids (chloroplasts, chromoplasts and leukoplasts) typical of plants and algae.
Characteristics of Ribosomes
Ribosomes are very important in all cells because they are key in protein synthesis. The following are its features and functions:
- They are made up of rRNA and ribosomal proteins.
- The number of proteins that make up a ribosome varies depending on the species.
- The function and design of ribosomes is similar between prokaryotic and eukaryotic cells.
- They are cellular structures .
- They are found in the cytoplasm of prokaryotic and eukaryotic cells.
- They are essential components of all cells.
Functions of Ribosomes
Among the main function of ribosomes, we can highlight that they constitute the machinery that catalyzes protein synthesis, with rRNAs being the ones that play the central role in this function.
They are the link between mRNAs, which carry the information to synthesize proteins, and tRNAs, which "read" the codons of the mRNA and translate it into a sequence of amino acids (protein monomers).
The main function of ribosomes in any cell is to produce proteins. And these proteins, in turn, are used in almost all cellular functions. Ribosomes, together with transfer RNA and messenger RNA, manage to decode the DNA message and interpret it into a sequence of amino acids that will form all the proteins of an organism. This process is called translation.
To do this, the ribosome follows the following steps:
- Obtains and reads the information from the messenger RNA.
- It translates this information into a sequence of amino acids.
- The blocks that make up proteins are created in a predetermined and ordered way.
In this sense, the correct functioning of ribosomes is essential for organisms to fulfill their functions.
Some of the functions of ribosomes are:
- Participate in lipid metabolism.
- Create binding sites for two tRNA molecules.
- Be part of the amino acid assembly process.
- Produce cytochrome for electron transport during cellular respiration.
