MDCAT Biology Lecture: Viruses – Classification and Structure
📘 Introduction
- Viruses
are acellular, ultramicroscopic infectious agents.
- They
occupy a gray area between living and non-living things:
- Non-living
outside the host: cannot reproduce, grow, or metabolize.
- Living
inside host: show replication, evolution, and infection.
- They
are obligate intracellular parasites, meaning they must infect a
host cell to replicate.
🧬 1. Classification of
Viruses
Viruses can be classified based on three major criteria:
🧫 A. Based on Host Type
Viruses are host-specific. They infect only certain
organisms or even specific cells within those organisms.
|
Type |
Infects |
Example |
|
Animals and humans |
HIV, Influenza, Rabies |
|
|
Plants |
Tobacco Mosaic Virus (TMV) |
|
|
Bacteria |
||
|
Fungi |
Mycoviruses |
|
|
Archaea (extremophiles) |
🔬 B. Based on Structure
The structure of a virus depends on capsid symmetry and
presence of envelope:
|
Structure Type |
Features |
Example |
|
Rod-shaped; capsomeres arranged in a helix |
TMV |
|
|
20 triangular faces forming a spherical shape |
Adenovirus |
|
|
Complicated shape with tail, fibers, etc. |
Bacteriophage |
|
|
Lipid envelope surrounding capsid |
HIV, Influenza |
|
|
Only capsid; no lipid membrane |
Norovirus, Polio |
🧬 C. Based on Nucleic
Acid (Genome)
Viruses are classified by the type and strand of
their nucleic acid:
|
Type |
Details |
Examples |
|
Contain DNA (single- or double-stranded) |
Herpesvirus |
|
|
Contain RNA (single- or double-stranded) |
Influenza, HIV |
|
|
RNA acts directly as mRNA |
Poliovirus |
|
|
RNA must be transcribed into mRNA |
Rabies, Influenza |
|
|
RNA genome, reverse transcribed to DNA |
HIV |
🧠 Key Fact: A virus
contains either DNA or RNA, but never both.
🔍 2. Detailed Structure
of Viruses
A. Capsid
- Made
of protein subunits called capsomeres.
- Protects
genetic material.
- Determines
virus shape and symmetry.
B. Genetic Material
- Either
DNA or RNA (never both).
- May be
single-stranded (ss) or double-stranded (ds).
C. Envelope (Optional)
- Found
in enveloped viruses.
- Made
of lipid bilayer from host cell membrane.
- Contains
glycoprotein spikes for host cell recognition.
- Makes
viruses less stable in external environment.
D. Tail and Base Plate (in complex viruses like
bacteriophages)
- Used
for attachment and injection of genetic material.
E. Enzymes
- Some
viruses carry enzymes such as:
- Reverse
transcriptase (in retroviruses like HIV)
- Integrase,
RNA polymerase, etc.
🎯 3. Summary Table – Key
Features of Viruses
|
Feature |
Description |
|
Acellular |
Not made up of cells |
|
Obligate parasites |
Cannot replicate without a host |
|
DNA or RNA only |
Never both simultaneously |
|
Capsid |
Protein coat protecting genetic material |
|
Envelope |
Optional lipid membrane with spikes |
|
Host specificity |
Infects specific organisms or cell types |
|
High mutation rate |
Especially in RNA viruses |
|
Can be crystallized |
Unique to viruses (e.g., TMV) |
|
Reproduction cycles |
Lytic (active) or Lysogenic (dormant integration) |
📝 4. Practice MCQs –
Classification & Structure of Viruses
✅ A. Recall-Based MCQs
- Which
type of virus infects bacteria?
A) TMV
B) T4 phage
C) HIV
D) Influenza
➤ Answer: B) T4 phage - What
is the basic unit of a viral capsid?
A) Virion
B) Capsomere
C) Envelope
D) Genome
➤ Answer: B) Capsomere - Which
scientist first crystallized a virus?
A) Ivanovsky
B) Twort
C) Stanley
D) Beijerinck
➤ Answer: C) Stanley - Which
virus contains RNA and an envelope?
A) Polio
B) Adenovirus
C) HIV
D) T4 phage
➤ Answer: C) HIV - What
shape does an icosahedral virus have?
A) Rod-shaped
B) Helical spiral
C) 20-sided polygon
D) Oval
➤ Answer: C) 20-sided polygon
💡 B. Conceptual-Based
MCQs
- Why
are RNA viruses more prone to mutations?
A) They lack a nucleus
B) RNA is double-stranded
C) RNA lacks proofreading enzymes
D) They infect more cells
➤ Answer: C) RNA lacks proofreading enzymes - Which
structural feature helps viruses attach to host cells?
A) Envelope
B) Genome
C) Capsid
D) Glycoprotein spikes
➤ Answer: D) Glycoprotein spikes - What
role does the envelope play in viral infection?
A) Protects against enzymes
B) Enables viral replication
C) Helps in host cell attachment
D) Kills the host cell
➤ Answer: C) Helps in host cell attachment - How
does a bacteriophage insert its genetic material?
A) Endocytosis
B) Fusion
C) Tail contraction and injection
D) Diffusion
➤ Answer: C) Tail contraction and injection - Which
feature is shared by all viruses?
A) Envelope
B) Tail fibers
C) Host specificity
D) DNA genome
➤ Answer: C) Host specificity
🧠 C. Analytical-Based
MCQs
- A
virus has RNA genome, an envelope, and reverse transcriptase. What is it?
A) Positive-sense RNA virus
B) Retrovirus
C) DNA virus
D) Negative-sense virus
➤ Answer: B) Retrovirus - A
non-enveloped virus is found to be very stable outside the host. What does
this suggest?
A) It contains both DNA and RNA
B) It is more resistant to heat and detergents
C) It cannot infect animals
D) It lacks a capsid
➤ Answer: B) It is more resistant to heat and detergents - A
virus infects only T-helper cells in humans. What can be inferred?
A) It is an archaeal virus
B) It lacks specificity
C) It recognizes CD4 receptors
D) It can replicate without a host
➤ Answer: C) It recognizes CD4 receptors - Which
virus would be inactivated by alcohol-based sanitizers?
A) Polio virus
B) TMV
C) HIV
D) Norovirus
➤ Answer: C) HIV - A
newly discovered virus has a head, tail sheath, and tail fibers. It most
likely infects:
A) Plants
B) Bacteria
C) Fungi
D) Animals
➤ Answer: B) Bacteria
📌 Conclusion
- Viruses
show great diversity in shape, genome, and host range.
- All
viruses share common structural elements like capsid and genetic
material.
- Understanding
their classification and structure is essential for diagnosing,
treating, and preventing viral diseases.
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