Ribosomes are tiny, dense, sub-microscopic, membrane-less granular ribonucleoprotein organelles that are seen in all living cells.
What are the protein factories of the cell? Ribosomes are the sites of protein synthesis, they can provide all the required enzymes for the same purpose. As a result, these are referred to as “protein factories”.
Ribosomes were discovered by George E. Palade in the 1950s. He described them as small particles seen in the cytoplasm, which preferentially are associated with the endoplasmic reticulum membrane.
Types of Ribosomes
Two types of ribosomes have been identified based on the sedimentation coefficient, which is measured in Svedberg Units or S units. The sedimentation coefficient is the measure of the rate of sedimentation of a particle or a molecule. In other words, it describes how fast or slow a particle of given shape and size settles. The value of “S” is greater if the particle is larger and denser.
The different types of ribosomes hence are:
- 70 S ribosomes (usually seen in prokaryotic cells and chloroplasts)
- 80 S ribosomes (usually seen in eukaryotic cells)
That being said, other ribosomes with varying sedimentation value are also known to exist in different phyla, such as 77 S seen in fungal mitochondria and 55 S in the mammalian mitochondria.
Ribosomes – Number and Distribution
Ribosomes are seen in both eukaryotic and prokaryotic cells. However, they are not seen in mature RBC and sperm. Ribosomes in prokaryotic cells are seen dispersed in the cytoplasm in eukaryotic cells however, ribosomes occur free in the cytoplasmic matrix, attached to the exterior surface of the rough endoplasmic reticulum and the nuclear envelope.
These organelles can also be seen in the matrix of mitochondria and the stroma of the plastids in eukaryotic cells. Such ribosomes are referred to as organeller ribosomes to key them out from the cytoplasmic ribosomes.
Ribosomes can occur as monosomes (singly) or as polysomes (clusters). During the synthesis of proteins, temporarily 6-8 ribosomes associate with an mRNA forming clusters referred to as poly some or poly ribosome or ergosome. In a cell, the number of ribosomes is based on the active synthesis of proteins. There are about 10 million ribosomes said to be present in a eukaryotic cell.
Ribosomes as Protein Factories of the cell
Protein synthesis takes place at ribosomes. These are complexes of rRNA molecules and proteins which can be seen in electron micrographs of cells. A few ribosomes in eukaryotes are associated with the internal membranes where proteins are synthesized, which will in a matter of time occupy such membranes or get involved in performing secretion.
In the ribosomes, the rRNA molecules channelize the catalytic process of the synthesis of proteins – the coming together of the amino acids to create a molecule of protein. The rRNA, in fact, at times is referred to as catalytic RNA or ribozyme.
Synthesis of Proteins
The synthesis of new proteins is initiated in the nucleus, wherein the ribosomes acquire instruction to start the process. The genes or sections of DNA, encoding of particular proteins are passed onto the mRNA strands through a process of transcription.
Upon completion of transcription of DNA to the mRNA, translation is up next wherein such mRNA is read to create proteins. Each of the mRNA instructs the order to follow for amino acids to be added to the protein chain as and when synthesized. Proteins are constructed with the help of amino acids as bricks.
In order to build proteins, the small and large ribosomal subunits gather, forming a complete ribosome. These comprise binding sites for tRNA and mRNA molecules. The larger subunits are found at the top of the smaller subunit while the mRNA template is found in between these two subunits. The ribosome initiates its process of protein synthesis once these structures are assembled and operate as a unit.
Ribosomes are read in sets of three-nucleotide sequences moving along the mRNA referred to as codons. These encode particular amino acids. The tRNA gets such amino acids – the building blocks of proteins, to the ribosomes. Each molecule of tRNA has 2 different sites or ends – one which binds to particular amino acids and the other which associates with the corresponding mRNA codon. In the process of translation, such tRNAs carry amino acids to the ribosome and associate with its complementary codons found on the mRNA. Subsequently, these are translated to the accurate amino acids in the new protein chains.
With the help of rRNA molecules, the amino acids hence assembled are stitched together, which guides the mechanism of creation of a new protein chain. This process is repeated for each of the amino acids, the entire protein is constructed through the process of elongation. Such a growing chain of proteins stops only when it is faced with a stop codon on the mRNA. This indicates the end of the polypeptide chain at the time of translation. Upon the accurate formation of amino acids, the protein chain that is newly synthesized can be passed to the cytoplasm (prokaryotes) or to the Golgi apparatus (eukaryotes).
|Also see: tRNA Structure|
Functions of Ribosomes
- The organelles that are the protein factories for the cell are the Ribosomes, they are involved in the synthesis of proteins as they contain the required enzymes for it
- These organelles render enzymes and factors for the condensation of amino acids for the formation of a polypeptide
- Ribosomes comprise rRNA to provide attachment points to mRNA and tRNA
- For mRNA, ribosomes comprise a tunnel for it to be translated properly
- A polypeptide that is newly synthesized is rendered protection from the cytoplasmic enzymes by enveloping them in the groove of large ribosomal subunits until they acquire secondary structure
This was a brief on protein factories – ribosomes. Explore more such concepts for NEET preparation at BYJU’S.