The cell is the structural and functional unit of life. The prokaryotic cells constitute bacteria, blue-green algae, mycoplasma, and PPLO( Pleuro Pneumonia Like Organisms). The size of prokaryotic cells is usually small ( 0.1- 5.0 μm ). It has one envelope organization. In prokaryotes, DNA is naked, that is, without an association with histones and it is usually circular. The size of the eukaryotic cells is larger than the prokaryotic cell ( 5-100 μm ). It has two envelope organization. In these cells, nuclear DNA is associated with histone proteins. Nuclear DNA is linear while extranuclear DNA is commonly circular. The cytoplasm is jelly-like semi-fluid bulk of protoplasm excluding the nucleus. It includes all other components – cytoplasmic matrix, cell organelles, and cell inclusions. Cell organelles are sub-cellular structures. They can carry out their functions even outside the cytoplasm provided they are supplied with substances, which are normally provided by the cell. Cell organelles include Mitochondira, Plastids, Endoplasmic Reticulum (ER), Golgi Complex, Lysosomes, Microbodies, Ribosomes, Cytoskeletal Structures, Flagella and Cilia, Centrioles, Vacuoles and Nucleus. Hereunder is a detailed explanation of Ribosomes and inclusion bodies.
Ribosomes are naked ribonucleoprotein protoplasmic particles (RNP) with a length of 200-340 Å and diameter of 170-240 Å which function as the sites for protein or polypeptide synthesis. Many ribosomes occur in a cell e.g., a single cell of E. coli has 20000-30000 ribosomes. In eukaryotic cells, their number is several times more. Ribosomes are known as protein factories. They are subspherical in outline and have no covering membrane. Each ribosome consists of two unequal subunits, larger dome-shaped, and smaller ovoid. The larger subunit has a bump, elevation, and bent. The smaller subunit which is about half the size of the larger subunit possesses a platform, cleft, head, and base. The smaller subunit fits over the large one at one end like a cap. Mg2+ is required for binding the two subunits. The two subunits dissociate below 0.0001 M Mg2+ while above this strength, the ribosomes can come together to form dimers. A role has also been attributed to Ca2+, Mn2+, and Co++.
Ribosomes may occur in spiral or helical groups called polyribosomes or polysomes. The different ribosomes of a polyribosome are connected with an mRNA strand. Polyribosomes are formed during periods of active protein synthesis when a number of copies of the same polypeptide are needed. Their maintenance requires energy.
Ribosomes happen in all living cells with the exception of mammalian erythrocytes. Ribosomes are of two kinds, cytoplasmic and organelle. The organelle ribosomes are found in plastids (plastid ribosomes) and mitochondria (mitoribosomes). The cytoplasmic ribosomes (mitoribosomes) may remain free in the cytoplasmic matrix or attached to the surface of the endoplasmic reticulum via the larger or 60S subunits. Prokaryotic cells have only free cytoplasmic ribosomes. All three types of ribosomes produce different types of proteins, e.g., structural proteins from free cytoplasmic ribosomes and globular proteins from ribosomes bound to ER. The bound ribosomes generally transfer their proteins to the cisternae of the endoplasmic reticulum for transport to other parts both inside and outside the cell. These proteins are also sent to intracellular organelles like the nucleus, mitochondria, and chloroplasts. Specific proteins called chaperones assist the newly formed proteins in their folding and transport.
The size of the ribosomes is determined by the sedimentation coefficient in the centrifuge which is measured as a Svedberg unit called S (S = 1 x 10–13 sec). The cytoplasmic ribosomes of eukaryotes are 80 S (have a size of 300-340Å x 200-240 Å and weight 4-4.5 million daltons) while those of prokaryotes are 70 S (have a size of 200-290Å x 170-210Å and weight 2.7-3.0 million daltons) The organelle ribosomes are also 70 S but in mammalian mitochondria, they have sedimentation coefficient of 55 S. The two subunits of 80 S ribosomes are 605 and 405 while 70S ribosomes have 50 S and 30 S subunits and a tunnel occurs between the two subunits for passage of mRNA. The larger subunit has a trough for pushing out the newly synthesized polypeptide. There are two reactive sites, P and A, a binding site for tRNA and many enzymes connected with protein synthesis.
80S ribosomes are synthesized inside the nucleolus but they do not become functional inside the nucleolus. Their subunits come out of the nucleus and become working in the cytoplasm. 70S ribosomes of prokaryotes are formed in the cytoplasm while those of semi-autonomous cell organelles are formed in their matrix.
The chemical composition of ribosomes is proteins and rRNA. More rRNA is present in 70S ribosomes as compared to protein (60-65: 35-40) while the reverse is true for 80S ribosomes (40-44: 56-60). 40S subunit of 80S ribosome contains 33 protein molecules and a single 18S rRNA. 30S subunit of 70S ribosome possesses 21 protein molecules and 16S rRNA. 60S subunit of 80S ribosome has 50 protein molecules and three types of rRNAs – 28S, 5.8S, and 5S, 50S subunit of 70S ribosome contains 32 protein molecules and two types of rRNAs-23S and 5S. Proteins are both structural and enzymatic. The ribosomes of liver cells may also contain lipids to the extent of 5-10%.
Types of Ribosomes
Ribosomes are of two types, free and fixed (also known as membrane-bound).
- Free Ribosomes: Some of the ribosomes located in the cytoplasm are not fixed to any other organelle. They are freely located in the cytoplasm in an unbound state. They are called free ribosomes. Free ribosomes synthesize proteins in the cytoplasm.
- Membrane-bound Ribosomes: Most of the ribosomes in a cell are placed on the surface of the endoplasmic reticulum. They are known as the attached or bound ribosomes. These ribosomes produce proteins that are exported from the cell to the outside.
- Ribosomes are the sites for polypeptide or protein synthesis. Free ribosomes synthesize structural and enzymatic proteins for use inside the cell while the attached ribosomes synthesize proteins for transport.
- They provide enzymes (e.g., Peptidyl transferase) and factors for the condensation of amino acids to form polypeptides.
- They contain rRNA for providing attaching points to mRNA and tRNAs.
- The ribosome has a tunnel for mRNA so that it can be translated properly.
- A newly synthesized polypeptide is provided protection from cytoplasmic enzymes by enclosing it in the groove of the larger subunit of ribosome till it arrives at the secondary structure.
Reserve material in prokaryotic cells is stored in the cytoplasm in the form of inclusion bodies. These are not membrane-bound systems and lie free in the cytoplasm, e.g., phosphate granules, cyanophycean granules, and glycogen granules. Gas vacuoles are found in blue-green and purple and green photosynthetic bacteria. Cell inclusions also called ergastic bodies are non-living substances present in the cells which may be present in soluble or insoluble states and can be organic or inorganic in nature. They belong to three categories – reserve food, excretory or secretory products, and mineral matter. Inclusion bodies are unusual structures with different sizes and shapes and are normally observed in nerve, epithelial, or endothelial cells.
Inclusion bodies can be seen in both prokaryotic and eukaryotic cells. They are a wide variety of inclusion bodies in distinct kinds of cells. In prokaryotes, they are mainly formed to store reserve materials. In animal cells, they store fats and sugars that are ready for cellular respiration, and in plant cells, they store granules of materials like glycogen, starch, etc.
General Features of Inclusion Bodies
- They act as reserve deposits.
- Cell inclusions are generally acidophilic.
- A large number of nutrients can be stored in them by the cells and utilized when there is a shortage in the environment.
- These can be seen as pink structures under the microscope when stained with gypsum or methylene blue dye.
Classification of Inclusion Bodies
Inclusion bodies can be classified into distinct types based on their location, either at the nucleus or cytoplasm or at both the cell organelles. The different types of inclusion bodies are as follows:
- Intranuclear inclusions
- Infection inclusion bodies.
- Intracytoplasmic inclusions.
- Physiological inclusion of bodies.
Inclusion bodies can exist in a bacterium or eukaryotic cell in the form of cystic lesions, fungal infections, virus-infected cells, bacterial infections, autoimmune diseases, neoplasms, and blood dyscrasias.
Viral Inclusion Bodies
Some of examples of viral inclusion bodies are:
- Negri bodies in rabies
- Paschen bodies in variola
- Bollinger bodies in fowlpox
- Handerson-Patterson bodies in Molluscum Contagiosum
- Eosinophilic inclusion bodies in boid inclusion body disease
- Torres bodies in yellow fever
- Cowdry type A in Herpes simplex virus
- Cowdry type B in Polio and adenovirus
- Cowdry type B in adenovirus
- Owl’s eye appearance in cytomegalovirus
Key Points on Inclusion Bodies
- Inclusion bodies are the bacteria using hydrogen sulfide as an electron source containing sulfur granules.
- These are cytoplasmic or nuclear combinations of stainable substances.
- A few bacteria keep in reserve excess carbon in the form of glycogen or polyhydroxyalkanoates.
- When the genes from one organism are expressed in some other organism, the protein is synthesized from inclusion bodies.
FAQs on Ribosomes and Inclusion Bodies
Question 1: Why ribosomes are called protein factories?
Ribosomes are the only cell organelle, involved in the synthesis of protein. Hence, they are called the protein factory of the cell.
Question 2: Which type of ribosomes are found in prokaryotes and eukaryotes?
Prokaryotes have 70S ribosomes and eukaryotes have 80S ribosomes.
Question 3: What is the chemical composition of a ribosome?
Ribosomes are made up of ribosomal proteins, ribosomal RNA (rRNA), enzymes, and metal ions.
Question 4: What are the inclusion bodies?
Inclusion bodies are found in bacteria as particles of aggregated protein. They have a higher density than many other cell components but are porous.
Question 5: What are the functions of ribosomes?
Ribosomes have two main functions — decoding the message and the formation of peptide bonds.
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