Ribosomes

Ribosomes are one of the most important cell organelles, made up of RNA and protein, and they convert genetic code into amino acid chains. Understanding ribosomes is key to grasping how information encoded in genes is translated into the proteins that perform the majority of the cell's functions. Ribosomes is a topic of the chapter Cell: The Unit of Life in Biology.

What are Ribosomes?

A ribosome is a complex molecular machine found inside living cells that synthesises proteins from amino acids, a process known as protein synthesis or translation.

Ribosomes can be described as molecular machines within a cell that are dedicated to putting together proteins by translating the message of messenger RNA into polypeptide chains. They are located within all living cells and play a major role in the production of proteins.

Ribosomes are made of large and small subunits and are made of ribosomal RNA and proteins. They can be free-floating in the cytoplasm or attached to the endoplasmic reticulum to form the rough endoplasmic reticulum. They are the actual sites for the translation of genetic information presented in an mRNA molecule into a protein.

Ribosomes are large and complex parts of the macromolecules and play a central role in the process of protein biosynthesis—hence crucial to the life and functionality of cells. The translation of genetic information is read from the sequence of mRNA, joining amino acids, according to the sequence. This translation of the genetic code into functional proteins is the basis of cell growth, repair, and regulation of other functions taking place in a cell.

George Emil Palade, a cell biologist of the 1950s, was the pioneer in presenting a detailed description of ribosomes, for which he was awarded a Nobel Prize in 1974.

Ribosomes Structure

• Ribosomes are made up of rRNA and proteins, which are again represented in the form of two subunits: the large subunit and the small subunit.

• The rRNA molecules are involved in the actual translation of the mRNA message and protein synthesis at the site of the ribosome, whereas proteins serve to stabilize the whole structure of the ribosome.

• Each ribosome has its own specific sequence to properly decode mRNA for translation into a polypeptide. In prokaryotes and eukaryotes, the ribosomal subunits dissociate after protein synthesis to cycle back through the subunit recycling.

Subunits

• A small subunit decodes the sequence within mRNA; a large subunit assembles amino acids into a polypeptide chain.

• Ribosomes of prokaryotes (70S) are smaller, comprised of two subunits: 50S large subunit and 30S small subunit. On the other hand, eukaryotes have a larger ribosome (80S), comprised of a 60S large subunit and a 40S small subunit.

• These differences represent the complexity and various size differences between these two cell types.

• Knowledge of these structural differences is essential for the field of medical research in the development of antibiotics which can specifically target prokaryotic ribosomal subunits without affecting the structure and function of eukaryotic ribosomes.

Also Read-

• MCQ Practice on Ribosomes
• Cell: The Unit of Life
• Plasma Membrane
• Cell Wall

Functions of Ribosomes

The functions of ribosomes is described below-

Protein Synthesis

• Ribosomes are the main place in protein synthesis where translation takes place.

• A ribosome works on this translation phase by lying on the sequence of mRNA to decode the sequence in groups of three nucleotides, or codons, and put together the corresponding amino acids in a polypeptide chain.

• Hence, the ribosomes are a reoccurring site for three types of key RNA interactions: mRNA, rRNA, and tRNA. mRNA is the information template's carrying vehicle for identifying the codon sequence.

• At the same time, tRNA translocates the correct amino acids. rRNA catalyses peptidyl transferase formation.

• The word translation implies the conversion process, a change from the language of nucleotides consisting of the codon structure used by messenger RNA to the language of amino acid structure.

• The process of translation can be defined by an elongation point, an initiation point, and a point of termination.

• At the initiation phase, the ribosome must engage the target mRNA, and so must the first tRNA.

• The point of elongation is the time during which the ribosome rims the mRNA, forming bonds between amino acids.

• The point of termination is when the ribosome reaches the stop codon and then releases the polypeptide.

Polysomes (Polyribosomes)

• Polysomes, or polyribosomes, are described as groups of ribosomes translating a single mRNA molecule simultaneously.

• All these components serve to allow for virtually simultaneous multiple copies of a protein to be made from a single mRNA transcript.

• Polysomes are very significant as they increase the rate of protein synthesis a cell can conduct when it needs to do so.

• The mechanism of polysomes is the inclusion of several ribosomes spaced along an mRNA strand, with each at a different stage of translating the mRNA.

• It is an effective assembly line, and in certain cells, such as dividing cells or certain somatic cells with high protein demands, the assembly line needs to work smoothly.

Types of Ribosomes

Based on their location and functions, ribosomes are of two types: free ribosomes and bound ribosomes.

Free ribosomes

• Free ribosomes are found throughout the cytosol and, most free ribosomes synthesize proteins that function within the cytosol and other fluid parts of the cell.

• Such proteins include those that are involved in the metabolic pathways of respiration, for example, as well as protein enzymes active in these pathways and structural proteins, necessary to maintain the cell's flexibility and tensile strength, for example.

• Free ribosomes account for the majority of proteins synthesized within a cell and are key in the daily synthesis and turnover of a cell's components.

Bound ribosomes

• Bound ribosomes occur together with a portion of the endoplasmic reticulum (ER) called the rough ER.

• These ribosomes make proteins that are destined for outside the cell, for movement across the membrane of the cell, or for importation into the lysosome.

• These bound ribosomes enable the resultant polypeptides to rapidly transfer across the membranes of the ribosomes, entering ER lumens in which they can fold into secondary, tertiary, and quaternary structures.

• This localization to the ER membrane is important for sorting and localization to their final sites.

Biogenesis of Ribosomes

The ribosome biogenesis is explained below-

Ribosome Assembly

Ribosome assembly in itself is a very complicated process. It starts further with the nucleolus, a subnuclear structure that is highly specialized. A nucleolus actively coordinates the synthesis and assembly of ribosomal RNA and ribosomal proteins. First, the rRNA genes transcribe into precursor rRNA, which is later processed into the mature rRNA.

Afterwards, rRNA combines with ribosomal proteins, which are cytoplasmically imported, to form both the small and the large ribosome subunits. Assembled subunits are then moved out of the nucleolus into the cytoplasm, where they will be paired up into their functional ribosomal form for the synthesis of proteins.

Regulation of Ribosome Production

The production of ribosomes is highly regulated and under strong control by the availability of nutrients, in response to cellular stress and growth signals. Ribosome synthesis and biogenesis are important in cell function and growth to meet the need for these organelles to synthesize proteins required in cellular functions.

Thus, cells must regulate the number of ribosomes in accordance with their metabolic activity without wasting the resources for this process. Disruption of ribosome biogenesis and dysregulation of the ribosomal pathways provide a basis for several human diseases, including cancer and ribosomopathies.

Ribosome Function in Different Cellular Contexts

The various aspects of the function of ribosomes are explained below-

Prokaryotic Ribosomes

• 70S prokaryotic ribosomes are then made up of a 50S large subunit and a 30S small subunit. These are small and relatively simple compared to eukaryotic ribosomes.

• An important property of prokaryotic ribosomes is their sensitivity to particular antibiotics, including tetracyclines and streptomycin, which can prevent protein biosynthesis by the particular zone in bacterial ribosomes.

• Such selective inhibition is important for the effectiveness of certain antibiotics to treat bacterial infections without harming eukaryotic cells.

Eukaryotic Ribosomes

• On the other hand, eukaryotic ribosomes are generally larger and are designated by the nomenclature 80S. The eukaryotic ribosome can be broken down into a 60S large subunit and a 40S small subunit.

• Compared to prokaryotic ribosomes, they are more complex and contain further rRNA and protein molecules. Generally, for eukaryotic cells, the ribosomes synthesize proteins that are needed in the cells, like all the various enzymes, structural proteins, and signalling proteins.

• Further, eukaryotic unique inhibitors, for example, cobomycin or cycloheximide, are narrow-spectrum inhibitors that specifically target the eukaryotic protein that synthesizes it, thereby indicating the difference in ribosomal structure and function of pro and eukaryotes.

Mitochondrial and Chloroplast Ribosomes

• The mitochondria and chloroplasts possess their own ribosomes. Their function goes together with the synthesis of proteins required for mitochondrial and chloroplast functions.

• The similarity of these ribosomes with the prokaryotic ones indicates their common origin in the process of evolution. Prokaryotic antibiotics may lead to potential alterations in these organelles.

What Distinguishes Ribosomes from the other Organelles?

Following describe the distinction between ribosomes and other organelles-

Ribosomes vs. Other Organelles

Comparison with Other Cellular Components: What distinguishes ribosomes from the other organelles is that they house the protein synthesis process and lack a membrane that would differentiate the structures' boundaries. In terms of structure, ribosomes are, in fact, complexes of ribosomal RNA and proteins in the way that a particle is comprised of smaller subunits.

Functionally, they serve to translate the genetic code of nucleic acids from the form of "messenger RNA" to the form of the amino acids that make up protein molecules, serving a diverse function from that of the other organelles.

These two organelles, endoplasmic reticulum and Golgi, are closely interlinked with ribosomes. The membrane-bound ribosomes of the rough endoplasmic reticulum are involved in the synthesis of primarily those proteins whose destination is either outside the cell or inside the membranes.

Then, these proteins are transferred to the Golgi apparatus, which further processes, packs, and dispatches these products to their final destinations. Such a close collaboration of functions is of critical importance to support cellular function and facilitate the execution of complex biochemical pathways.

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