VIRUSES ( Acellular life), Lecture notes of Biology

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Acellular Life

Viruses

What is a Virus? (Introduction &

Definition)

• (^) Definition: Viruses are acellular entities—they are not made of cells.
• (^) Nature: They are obligate intracellular parasites, meaning they can only replicate inside the living cells of a host (plants, animals, bacteria).
• (^) Size: They are ultramicroscopic and are not capable of independent replication.
• (^) Too small to be seen under the light microscope.
• (^) Only visible using the electron microscope.
• (^) Impact: Viruses cause numerous diseases, including influenza, polio, swine flu, dengue fever, etc.

History and Discovery of Viruses

• (^) W. M. Stanley (1935):
• (^) Successfully crystallized the Tobacco Mosaic Virus (TMV).
• (^) This demonstrated that viruses can be stored like chemicals, suggesting a non-living quality.
• (^) Virology: The study of viruses, only fully possible after the invention of the electron microscope.

Structure of a Virus Particle (Virion)

• (^) A complete, single virus particle is called a Virion.
• (^) Core Components:
• (^) Genetic Material (DNA or RNA): Viruses contain only one type of nucleic acid—either DNA or RNA, but never both.
• (^) Proteins (Enzymes): Various proteins necessary for replication and invasion.
• (^) Outer Structure:
• (^) Capsid Protein (Coat): A protective protein shell surrounding the core.
• (^) Envelope (Optional): An outer membrane-like structure derived from the host cell (not found in all viruses).

The Dual Nature: Living and Non-

Living Characteristics

• (^) Living Characteristics (Inside the Host)
• (^) (1) Genetic Material: They possess nucleic acid (DNA or RNA).
• (^) (2) Mutation: They are capable of undergoing genetic mutation, allowing them to evolve.
• (^) (3) Reproduction: They reproduce inside the living cell using the host's metabolic machinery.
• (^) (4) Pathogenicity: They enter host cells and cause disease (e.g., intracellular obligate parasitism).

Bacteriophage Structure

• (^) A bacteriophage, or phage, is a virus that infects bacteria. Its structure typically consists of:
• (^) Key Components
• (^) 1. Head: Contains genetic material (DNA or RNA).
• (^) 2. Tail: Attaches to the bacterial host and injects genetic material.
• (^) 3. Tail fibers: Recognize and bind to specific bacterial receptors.

Bacteriophage Life Cycle

• (^) The life cycle of a bacteriophage typically involves several stages:
• (^) 1. Attachment
• (^) The phage attaches to the bacterial surface using its tail fibers.
• (^) 2. Penetration
• (^) The phage injects its genetic material (DNA or RNA) into the bacterial cell.
• (^) 3. Replication
• (^) The phage genetic material takes over the host's cellular machinery, replicating itself and producing new phage components.
• (^) 4. Assembly
• (^) New phage particles are assembled using the replicated components.

• (^) 5. Lysis
• (^) The host cell is lysed (broken open), releasing the new phage particles.
• (^) 6. Release
• (^) The new phage particles are released into the environment, ready to infect other bacteria.
• (^) Types of Life Cycles
• (^) 1. Lytic cycle: The phage immediately replicates and lyses the host cell.
• (^) 2. Lysogenic cycle: The phage integrates its genetic material into the host's genome, remaining dormant until activated.

Lytic Cycle Replication

• (^) In the lytic cycle, bacteriophage replication involves:
• (^) Stages
• (^) 1. Injection: Phage DNA is injected into the host bacterium.
• (^) 2. Transcription: Phage genes are transcribed, producing mRNA.
• (^) 3. Translation: Phage proteins are synthesized using host ribosomes.
• (^) 4. DNA replication: Phage DNA is replicated.
• (^) 5. Assembly: New phage particles are assembled.
• (^) 6. Lysis: Host cell is lysed, releasing new phage particles.
• (^) Key Features
• (^) 1. Viral takeover: Phage hijacks host cellular machinery.
• (^) 2. Rapid replication: Phage replicates quickly, producing many progeny.
• (^) 3. Cell lysis: Host cell is destroyed, releasing new phage particles.