Virus Structure
and Classification
Master the structural components, genome types, and classification systems of viruses: essential knowledge for the MCAT.
MCAT Biology
What Is a Virus?
A virus is an obligate intracellular parasite: a tiny infectious agent that can only replicate inside a living host cell. Viruses are not considered living organisms because they lack the machinery for independent metabolism and reproduction.
Core Structural Components
Genome
DNA or RNA (never both), single- or double-stranded, linear or circular
Capsid
Protein coat made of capsomeres that surrounds and protects the nucleic acid
Envelope
Lipid bilayer (from host membrane) found in some viruses with glycoprotein spikes
Anatomy of a Virion
Viral Genome Types
Unlike cells (which always have dsDNA), viruses can have any combination of nucleic acid characteristics. This is a major MCAT distinction.
dsDNA
Herpesvirus, Adenovirus, Poxvirus, Bacteriophage T4
ssDNA
Parvovirus (one of the few ssDNA families)
dsRNA
Reovirus (segmented genome, causes gastroenteritis)
ssRNA
(+) sense: Coronavirus, HIV (retrovirus); (-) sense: Influenza, Ebola
(+) Sense vs (-) Sense RNA
| (+) Sense RNA | (-) Sense RNA | |
|---|---|---|
| Function | Serves directly as mRNA | Must be converted to (+) strand first |
| Translation | Immediate by host ribosomes | Requires viral RNA-dependent RNA polymerase |
| Enzyme needed | Not carried in virion | Carries its own RdRp in the virion |
| Examples | Poliovirus, Hepatitis C | Influenza, Rabies |
Capsid Symmetry Types
Enveloped vs Naked Viruses
| Enveloped | Naked (Non-enveloped) | |
|---|---|---|
| Outer layer | Lipid bilayer + glycoproteins | Capsid only (no lipid layer) |
| Stability | Fragile: sensitive to detergents, drying, heat | Hardy: resists harsh environments |
| Transmission | Respiratory droplets, blood, direct contact | Fecal-oral, fomites, environment |
| Exit mechanism | Budding (acquires envelope) | Cell lysis |
| Examples | HIV, Influenza, Herpes | Adenovirus, Norovirus, HPV |
The Baltimore Classification System
David Baltimore classified viruses into seven groups based on how they produce mRNA from their genome. This is the most important classification system for the MCAT.
Seven Baltimore Groups at a Glance
I
dsDNA: Transcribed by host RNA polymerase (like host genes). Examples: Herpes, Adenovirus.
II
ssDNA: Converted to dsDNA first, then transcribed. Example: Parvovirus.
III
dsRNA: Uses viral RdRp to transcribe (-) strand into mRNA. Example: Reovirus.
IV
(+)ssRNA: Genome IS mRNA: directly translated. Example: Coronavirus, Hepatitis C.
V
(-)ssRNA: Must carry RdRp to convert to (+) mRNA. Example: Influenza, Ebola.
VI
ssRNA-RT (Retroviruses): RNA genome is reverse-transcribed into DNA, integrated into host. Example: HIV.
VII
dsDNA-RT: dsDNA genome replicates via an RNA intermediate and reverse transcriptase. Example: Hepatitis B.
The Retrovirus Life Cycle
Key Viral Enzymes
Reverse Transcriptase
\( \text{RNA} \rightarrow \text{DNA} \). Found in retroviruses (HIV) and Hepatitis B. Lacks proofreading: high mutation rate.
Integrase
Inserts viral DNA into host chromosome. Creates provirus that can remain latent.
Protease
Cleaves viral polyproteins into functional pieces. Target of protease inhibitor drugs.
Lytic vs Lysogenic Cycles
| Lytic Cycle | Lysogenic Cycle | |
|---|---|---|
| Outcome | Host cell lyses (dies) | Viral DNA integrates into host |
| Viral DNA state | Replicates independently | Becomes prophage in host genome |
| New virions | Produced immediately | None until induction |
| Host survival | Cell destroyed | Cell survives, divides normally |
| Trigger to switch | Default for virulent phages | Stress/UV induces lytic switch |
How to Classify a Virus
1
Identify the genome type: Is it DNA or RNA? Single-stranded or double-stranded?
2
Determine sense (for ssRNA): Is it (+) sense (directly translatable) or (-) sense (needs RdRp)?
3
Check for reverse transcriptase: If present, it's Group VI (RNA-RT) or Group VII (DNA-RT).
4
Envelope status: Enveloped (lipid bilayer) or naked (capsid only)?
5
Capsid symmetry: Icosahedral, helical, or complex?
6
Assign Baltimore group (I-VII) based on the path from genome to mRNA.
Example: Classify Influenza
Given: Influenza has a segmented RNA genome, is enveloped, and must carry its own RNA polymerase to replicate.
1
Genome: RNA (not DNA).
2
Strandedness: Single-stranded (segmented into 8 pieces).
3
Sense: It must carry RdRp, so genome cannot directly serve as mRNA. This means (-) sense.
4
Baltimore Group: Group V [(-) sense ssRNA].
Example: Classify HIV
Given: HIV has two copies of ssRNA genome, is enveloped, and integrates into the host chromosome using reverse transcriptase and integrase.
1
Genome: ssRNA (diploid: 2 copies).
2
Special enzyme: Contains reverse transcriptase (\( \text{RNA} \rightarrow \text{DNA} \)).
3
Baltimore Group: Group VI (ssRNA-RT = Retrovirus).
4
Envelope: Yes, acquires from host cell membrane via budding. gp120 and gp41 are surface glycoproteins.
Example: Bacteriophage T4
Given: Bacteriophage T4 infects E. coli, has a linear dsDNA genome, and a complex structure with a contractile tail.
Genome
dsDNA, linear: Baltimore Group I
Structure
Complex: icosahedral head + contractile tail + tail fibers
Lifecycle
Strictly lytic: injects DNA, hijacks host, lyses cell
Common Mistakes
✕ Viruses have both DNA and RNA
A single virus has EITHER DNA or RNA as its genome, never both. Only cells have both.
✕ All RNA viruses need RdRp
(+) sense ssRNA viruses (Group IV) act directly as mRNA. They do NOT need to carry RdRp for initial translation.
✕ Confusing HIV's group
HIV is Group VI (ssRNA-RT), NOT Group V. The presence of reverse transcriptase defines it as a retrovirus.
✕ Enveloped = more resistant
Enveloped viruses are MORE fragile. The lipid envelope is disrupted by detergents and drying. Naked viruses are hardier.
High-Yield Summary
| Virus | Genome | Baltimore | Envelope |
|---|---|---|---|
| Influenza | (-)ssRNA | V | Yes |
| HIV | ssRNA-RT | VI | Yes |
| Adenovirus | dsDNA | I | No |
| Poliovirus | (+)ssRNA | IV | No |
| Hepatitis B | dsDNA-RT | VII | Yes |
| Parvovirus | ssDNA | II | No |
Question 1 of 5
Which of the following is NOT a component found in all viruses?
Not all viruses have a lipid envelope. Naked (non-enveloped) viruses like adenovirus and poliovirus have only a protein capsid surrounding their genome. The envelope is derived from the host cell membrane and is present only in enveloped viruses.
Question 2 of 5
A researcher discovers a virus with a (+) sense single-stranded RNA genome. Which statement is most accurate?
(+) sense ssRNA functions directly as mRNA and can be immediately translated by host ribosomes. This is Baltimore Group IV. (-) sense ssRNA (Group V) requires RdRp. Retroviruses (Group VI) use reverse transcriptase.
Question 3 of 5
HIV is classified as a retrovirus. Which unique enzyme allows it to convert its RNA genome into DNA?
Reverse transcriptase synthesizes DNA from an RNA template: the defining enzyme of retroviruses (Baltimore Group VI). DNA pol III is a bacterial enzyme, RdRp is used by RNA viruses that replicate RNA from RNA, and helicase unwinds DNA but does not convert RNA to DNA.
Question 4 of 5
During the lysogenic cycle, a bacteriophage:
In the lysogenic cycle, the phage DNA integrates into the host chromosome and becomes a prophage. It replicates passively with the host DNA and does not produce virions or lyse the cell until it is induced to enter the lytic cycle.
Question 5 of 5
Enveloped viruses are generally more susceptible to inactivation by detergents than non-enveloped viruses because:
The lipid envelope is derived from the host cell membrane. Detergents dissolve lipids, destroying the envelope and the glycoproteins needed for host cell attachment. Without the envelope, the virus cannot infect new cells. Non-enveloped viruses lack this vulnerable lipid layer.