What is the primary difference between a virus and a bacterium?

The fundamental difference lies in their cellular organization and metabolic capabilities. Bacteria are prokaryotic cells, meaning they are single-celled organisms with their own cellular machinery (ribosomes, cytoplasm, cell wall, cell membrane) and can metabolize and reproduce independently. Viruses, on the other hand, are acellular entities, lacking these cellular components. They are obligate intracellular parasites, meaning they cannot carry out metabolic processes or replicate without hijacking a host cell's machinery. Bacteria are typically much larger than viruses and are susceptible to antibiotics, while viruses are not.

Why are viruses considered obligate intracellular parasites?

Viruses are called obligate intracellular parasites because they absolutely require a living host cell to carry out their life cycle. They lack the necessary cellular machinery, such as ribosomes for protein synthesis, enzymes for energy production (metabolism), and organelles, to replicate independently. Instead, they must infect a host cell and commandeer its cellular resources – including ribosomes, ATP, amino acids, and enzymes – to synthesize their own genetic material and proteins, and then assemble new viral particles. Without a host cell, a virus is metabolically inert.

What is the role of the capsid in a virus?

The capsid is a crucial protein coat that encloses and protects the viral genetic material (DNA or RNA). It is made up of repeating protein subunits called capsomeres. Beyond protection, the capsid plays several vital roles: it helps in the attachment of the virus to specific receptors on the host cell surface, facilitates the entry of the viral genome into the host cell, and in some cases, aids in the release of new virions. The shape and symmetry of the capsid (e.g., helical, icosahedral) are important characteristics used in viral classification.

How do enveloped viruses differ from non-enveloped (naked) viruses in terms of structure and release?

Enveloped viruses possess an outer lipid bilayer membrane, called an envelope, which is derived from the host cell membrane during the budding process. This envelope often contains viral glycoproteins (spikes) that aid in attachment and entry into new host cells. Non-enveloped or naked viruses lack this outer membrane; their outermost layer is the protein capsid. In terms of release, enveloped viruses typically exit the host cell by budding, acquiring their envelope as they pass through a host membrane, often without immediately lysing (killing) the cell. Naked viruses, conversely, usually cause the host cell to lyse and burst open to release progeny virions.

What is reverse transcriptase and which type of virus uses it?

Reverse transcriptase is a unique viral enzyme that catalyzes the synthesis of DNA from an RNA template, a process known as reverse transcription. This enzyme is characteristic of retroviruses, a specific class of RNA viruses (e.g., HIV). After infecting a host cell, retroviruses use reverse transcriptase to convert their single-stranded RNA genome into a double-stranded DNA copy. This viral DNA can then integrate into the host cell's genome, becoming a provirus, which allows the viral genetic material to be replicated along with the host's DNA and transcribed into new viral RNA.

Can antibiotics be used to treat viral infections? Why or why not?

No, antibiotics are ineffective against viral infections. Antibiotics are designed to target specific structures and metabolic pathways found in bacteria, such as bacterial cell walls, bacterial ribosomes, or unique bacterial enzymes. Viruses, being acellular and obligate intracellular parasites, lack these bacterial targets. They rely on host cell machinery for replication, and therefore, antibiotics have no mechanism to inhibit viral processes. Using antibiotics for viral infections is not only futile but can also contribute to antibiotic resistance in bacteria, making future bacterial infections harder to treat. Antiviral drugs, which specifically target viral replication steps, are used for viral infections.

Structure and Replication of Virus

Biology

NEET UG

Version 1Updated 21 Mar 2026

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Viruses are obligate intracellular parasites, characterized by their acellular nature and reliance on host cellular machinery for replication. They consist of genetic material, either DNA or RNA, encased within a protein coat called a capsid, and sometimes an outer lipid envelope derived from the host cell membrane. Lacking their own metabolic machinery, viruses hijack the host cell's ribosomes, e…

Quick Summary

Viruses are acellular, obligate intracellular parasites consisting of genetic material (DNA or RNA) enclosed within a protein coat called a capsid. Some viruses also have an outer lipid envelope. They lack their own metabolic machinery and must hijack a host cell's resources to replicate.

The viral life cycle involves adsorption, penetration, uncoating, biosynthesis (replication and protein synthesis), assembly, and release of new virions. Bacteriophages can undergo either a lytic cycle, leading to host cell lysis, or a lysogenic cycle, where viral DNA integrates into the host genome as a prophage.

Key viral enzymes include reverse transcriptase (in retroviruses) and RNA-dependent RNA polymerase (in some RNA viruses). Viruses are classified based on their genetic material, capsid symmetry, and presence/absence of an envelope.

Understanding these basics is crucial for comprehending viral diseases and developing antiviral strategies.

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Viruses: — Acellular, obligate intracellular parasites.

Genetic Material: — DNA or RNA (never both); ss or ds.

Capsid: — Protein coat, made of capsomeres, protects genome. Symmetries: Helical, Icosahedral, Complex.

Envelope: — Outer lipid layer (optional, host-derived), with spikes (glycoproteins).

Virion: — Complete infectious viral particle.

Replication Steps: — Adsorption

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Penetration

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Uncoating

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Biosynthesis

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Assembly

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Release.

Lytic Cycle: — Rapid replication, host cell lysis (e.g., virulent phages).

Lysogenic Cycle: — Phage DNA integrates as prophage, replicates with host, no immediate lysis (e.g., temperate phages).

Reverse Transcriptase: — Enzyme in retroviruses (e.g., HIV) for RNA

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DNA synthesis.

RNA Replicase: — Enzyme in some RNA viruses for RNA

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RNA synthesis.

To remember the viral replication steps: All People Undergo Birth, And Release.

Adsorption

Penetration

Uncoating

Biosynthesis

Assembly

Release