Structure and Replication of Virus — Revision Notes

⚡ 30-Second Revision

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 $\rightarrow$ Penetration $\rightarrow$ Uncoating $\rightarrow$ Biosynthesis $\rightarrow$ Assembly $\rightarrow$ 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 $\rightarrow$ DNA synthesis.
RNA Replicase: — Enzyme in some RNA viruses for RNA $\rightarrow$ RNA synthesis.

2-Minute Revision

Viruses are non-cellular, obligate intracellular parasites, meaning they must infect a living host cell to replicate. Their basic structure includes genetic material (either DNA or RNA, never both) encased in a protein coat called a capsid, composed of capsomeres.

Some viruses also possess an outer lipid envelope, derived from the host cell membrane, which often has viral glycoproteins (spikes) for attachment. The entire infectious particle is known as a virion.

Viral replication follows a general pattern: first, the virus attaches (adsorption) to a specific host cell, then enters (penetration), and sheds its capsid (uncoating) to release its genetic material.

This genetic material then takes over the host cell's machinery for biosynthesis (replication of viral genome and synthesis of viral proteins). Finally, new viral components assemble (maturation) into new virions, which are then released from the host cell, often by lysis or budding.

Bacteriophages exhibit two distinct cycles: the lytic cycle, which leads to immediate host cell destruction, and the lysogenic cycle, where the viral DNA integrates into the host genome (prophage) and replicates passively without immediate harm.

5-Minute Revision

Let's consolidate the core concepts of virus structure and replication. Remember, viruses are unique biological entities, acellular and completely dependent on a host cell for survival and reproduction – hence, 'obligate intracellular parasites.

' Structurally, every virus has a nucleic acid core (its genetic material, which is either DNA or RNA, never both in a single virion) and a protective protein capsid made of repeating capsomeres.

The capsid can exhibit helical, icosahedral, or complex symmetry. Many animal viruses also possess an outer envelope, a lipid bilayer stolen from the host cell membrane, studded with viral glycoproteins (spikes) crucial for host recognition and entry.

A complete, infectious particle is termed a virion.

The viral replication cycle is a masterclass in molecular hijacking, typically involving six sequential steps:

1
Adsorption: — Specific binding of viral surface proteins to host cell receptors.
2
Penetration: — Entry of the virus or its genetic material into the host cell (e.g., direct injection by phages, endocytosis, membrane fusion).
3
Uncoating: — Release of the viral genetic material from the capsid within the host cell.
4
Biosynthesis: — The host cell's machinery is reprogrammed to replicate the viral genome and synthesize viral proteins. This step varies significantly based on the viral genome type (e.g., retroviruses use reverse transcriptase to make DNA from RNA; some RNA viruses use RNA replicase).
5
Assembly (Maturation): — Newly synthesized viral genomes and proteins self-assemble into new virions.
6
Release: — Progeny virions exit the host cell, either by lysis (bursting the cell, common for naked viruses and lytic phages) or budding (acquiring an envelope from the host membrane, common for enveloped viruses).

For bacteriophages, two distinct cycles exist: the lytic cycle, where rapid replication leads to immediate host cell lysis, and the lysogenic cycle, where the phage DNA integrates into the host chromosome as a prophage and replicates passively with the host, without immediate cell death.

Key examples to remember include TMV (helical, ssRNA), Bacteriophage (complex, dsDNA, lytic/lysogenic), and HIV (enveloped, ssRNA retrovirus, reverse transcriptase). Understanding these details is crucial for NEET.

Prelims Revision Notes

Virus Structure and Replication: NEET Essentials

1. Basic Characteristics:

Acellular: — Not composed of cells; lack cytoplasm, organelles, cell membrane.
Obligate Intracellular Parasites: — Absolutely require a living host cell for replication and metabolism.
Size: — Extremely small (20-300 nm), visible only with electron microscope.

2. Viral Structure (Virion):

Genetic Material (Nucleic Acid Core):

* Either DNA or RNA, NEVER both in a single virion. * Can be single-stranded (ss) or double-stranded (ds). * Can be linear, circular, or segmented.

Capsid:

* Protein coat surrounding the genetic material. * Composed of protein subunits called capsomeres. * Protects the genome and aids in host cell attachment/entry. * Symmetries: * Helical: Rod-shaped (e.g., Tobacco Mosaic Virus - TMV). * Icosahedral (Polyhedral): Spherical/20-sided (e.g., Adenovirus, Poliovirus). * Complex: Neither purely helical nor icosahedral (e.g., Bacteriophages, Poxviruses).

Envelope (Optional):

* Outer lipid bilayer membrane, derived from host cell membrane during budding. * Contains viral glycoproteins (spikes/peplomers) for attachment. * Enveloped viruses are generally more fragile than naked viruses.

Nucleocapsid: — Genetic material + Capsid.

3. Viral Replication Cycle (General Steps):

1
Adsorption (Attachment): — Virion binds specifically to host cell surface receptors.
2
Penetration (Entry): — Virus enters host cell (e.g., direct injection, endocytosis, membrane fusion).
3
Uncoating: — Viral capsid is removed, releasing genetic material into host cell.
4
Biosynthesis: — Host cell machinery is reprogrammed.

* Genome Replication: Viral DNA/RNA copied. * Protein Synthesis: Viral structural and enzymatic proteins produced. * Key Enzymes: * Reverse Transcriptase: In retroviruses (e.g., HIV); RNA $\rightarrow$ DNA. * RNA-dependent RNA Polymerase: In some RNA viruses; RNA $\rightarrow$ RNA.

1
Assembly (Maturation): — New viral genomes and proteins self-assemble into new virions.
2
Release: — New virions exit host cell.

* Lysis: Host cell bursts (e.g., naked viruses, lytic phages). * Budding: Virions acquire envelope as they exit (e.g., enveloped viruses).

4. Bacteriophage Cycles:

Lytic Cycle: — Virulent phages; immediate replication, host cell lysis, release of progeny phages.
Lysogenic Cycle: — Temperate phages; phage DNA integrates into host chromosome (prophage), replicates with host DNA, no immediate lysis. Host cell is a lysogen. Can be induced to enter lytic cycle.

5. Important Examples:

TMV: — Helical, ssRNA, naked, plant virus.
Bacteriophage: — Complex, dsDNA, naked, bacterial virus (lytic/lysogenic).
HIV: — Enveloped, ssRNA, retrovirus, uses reverse transcriptase, infects T-helper cells.

6. Key Distinctions:

Viruses vs. Bacteria: Acellular vs. cellular; obligate parasite vs. independent; DNA/RNA vs. DNA; no ribosomes vs. ribosomes; antivirals vs. antibiotics.
Lytic vs. Lysogenic: Immediate lysis vs. integration/latency.

Common Mistakes to Avoid:

Assuming viruses have their own metabolic machinery.
Believing antibiotics work against viruses.
Confusing the genetic material types (DNA vs. RNA) or capsid symmetries of different viruses.

Vyyuha Quick Recall

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

Adsorption
Penetration
Uncoating
Biosynthesis
Assembly
Release