Viruses: Characteristics and Importance

Viruses are ultramicroscopic, non-cellular organisms that exist at the threshold between living and non-living worlds. They are obligate intracellular parasites, meaning they can only replicate inside a living host cell.

Key Characteristics

• Acellular Structure: They lack a cellular organization and cytoplasm.
• Genetic Material: They contain either DNA or RNA, but never both, surrounded by a protein coat called a capsid.
• Metabolic Inertness: Outside a host cell, they are inert and can be crystallized like non-living matter.

Economic Importance

Viruses play a dual role in our world, primarily known for causing diseases but also having beneficial applications:

• Harmful: They cause severe diseases in plants (e.g., Mosaic disease), animals (e.g., Rabies), and humans (e.g., Influenza, HIV).
• Beneficial: Used as vectors in genetic engineering to transfer genes. Bacteriophages are used to control harmful bacterial populations (phage therapy).

T-phage and TMV Structure

T-phage (Bacteriophage)

T-phages are viruses that infect bacteria, specifically E. coli.

• Structure: They typically have a "tadpole" shape with a head and a tail.
• Head: A protein capsid containing the double-stranded DNA.
• Tail: A complex structure used for attaching to the host and injecting the viral DNA.

Tobacco Mosaic Virus (TMV)

TMV was the first virus to be discovered and primarily infects tobacco plants.

• Structure: It is a rod-shaped virus.
• Composition: It consists of a single-stranded RNA molecule spirally coiled and enclosed in a cylindrical protein coat made of capsomeres.

Lytic and Lysogenic Cycles

These are the two primary mechanisms by which viruses replicate within a host cell.

Lytic Cycle

In the lytic cycle, the virus takes over the host's machinery to produce new viruses, eventually causing the host cell to burst (lyse).

1. Adsorption: Attachment of virus to the host cell surface.
2. Penetration: Injection of viral genetic material into the host.
3. Biosynthesis: Viral DNA directs the host cell to produce viral components.
4. Maturation: Assembly of new viral particles.
5. Lysis: Release of new viruses by breaking the host cell.

Lysogenic Cycle

In the lysogenic cycle, the viral DNA integrates into the host's chromosome and remains dormant as a "prophage".

• The viral genome is replicated along with the host's DNA during cell division.
• The prophage can be triggered by environmental stress to enter the lytic cycle.

Bacteria: Characteristics and Importance

Bacteria are prokaryotic, microscopic, unicellular organisms found in almost every environment on Earth.

General Characteristics

• Prokaryotic: They lack a well-defined nucleus and membrane-bound organelles.
• Nutrition: Diverse nutritional groups including autotrophs and heterotrophs.

Economic Importance

• Beneficial: Nitrogen fixation in soil (e.g., Rhizobium), antibiotic production (e.g., Streptomyces), and fermentation in food industry (e.g., Lactobacillus).
• Harmful: Pathogenic bacteria cause diseases like Tuberculosis, Tetanus, and Citrus Canker.

Bacterial Cell Structure

The bacterial cell is characterized by a relatively simple but highly efficient organization.

• Cell Wall: A rigid outer layer typically composed of peptidoglycan, providing shape and protection.
• Cytoplasmic Membrane: Regulates the passage of materials in and out of the cell.
• Nucleoid: A region containing the circular, double-stranded DNA molecule.
• Flagella: Hair-like structures used for locomotion (in some species).
• Pili/Fimbriae: Used for attachment to surfaces or other cells during conjugation.

Reproduction in Bacteria

Bacteria reproduce primarily through asexual methods but can also exchange genetic material.

Vegetative and Asexual Reproduction

• Binary Fission: The most common method where a single cell divides into two identical daughter cells.
• Endospore Formation: Production of highly resistant spores under unfavorable conditions for survival.

Genetic Recombination

While not true sexual reproduction, these processes lead to genetic diversity:

• Conjugation: Direct transfer of DNA between two bacteria via a conjugation tube or pilus.
• Transformation: Uptake of "naked" DNA from the surrounding environment.
• Transduction: Transfer of bacterial DNA from one cell to another by a bacteriophage.