STEPTRACKERMICROBIOLOGY + GENETICS | VIROLOGY PRINCIPLES | HIGH-YIELD REVIEW

Viral Structure & Genetics

How a virus is built predicts where it replicates, how it survives outside the body and how it changes — genome type, envelope and polarity explain most exam rules.

FOCUS ON
Rules with exceptions: DNA viruses are double-stranded (except parvo), icosahedral (except pox), replicate in the nucleus (except pox); RNA viruses are single-stranded (except reo/rota), replicate in cytoplasm (except influenza, retroviruses). Naked viruses survive acid, bile and drying → fecal-oral spread. Positive-sense RNA is infectious by itself; negative-sense must carry RNA-dependent RNA polymerase. Genetic events: reassortment (segmented → antigenic shift, pandemics), recombination, complementation, phenotypic mixing.
CONCEPT
WHAT IT MEANS
EXAMPLES
WHY IT MATTERS
Capsid symmetry
Icosahedral (20 faces) or helical (nucleocapsid coils)
Most DNA viruses icosahedral; pox complex; many − RNA viruses helical
Classification
Envelope
Lipid bilayer taken from host membrane (plasma, nuclear, ER/Golgi)
Herpesviruses get envelope from nuclear membrane
Enveloped viruses are fragile — need close contact, respiratory, sexual, blood
Naked (non-enveloped)
Capsid only
DNA: papilloma, adeno, parvo, polyoma · RNA: picorna, calici, reo, hepevirus
Resistant to drying, acid, detergents → fecal-oral, fomites; alcohol hand gel less effective
Positive-sense RNA
Genome acts as mRNA → translated directly
Picorna, flavi, toga, calici, corona, retro (but needs RT)
Purified genome is infectious
Negative-sense RNA
Must be copied to + strand first
Orthomyxo, paramyxo, rhabdo, filo, arena, bunya
Virion carries RNA-dependent RNA polymerase · naked genome not infectious
Segmented genome
Genome in several pieces
Orthomyxo (8), reo (10–12), arena (2), bunya (3)
Enables reassortment
Reassortment
Two strains co-infect a cell and swap whole segments
Influenza A (human + avian/swine strains)
Antigenic shift → pandemics
Recombination
Exchange of genes by crossing over between two chromosomes
Two related DNA or RNA viruses
New gene combinations
Complementation
One virus supplies a functional protein the other lacks
HDV uses HBsAg from HBV
Defective virus replicates only with helper
Phenotypic mixing
Genome of virus A packaged in coat of virus B
Co-infection progeny
Progeny infect like B once; next generation reverts to A's coat (genome unchanged)
Antigenic drift
Point mutations in surface proteins
Influenza HA/NA yearly
Seasonal epidemics · annual vaccine update
Live attenuated vaccines
Weakened replicating virus → cellular + humoral immunity
MMR, varicella, yellow fever, oral polio (Sabin), rotavirus, intranasal influenza
Avoid in pregnancy and immunodeficiency (MMR OK in HIV with CD4 ≥ 200)
Inactivated / subunit vaccines
Killed virus or protein → humoral immunity
Rabies, injected influenza, Salk polio, hepatitis A · HBV, HPV (subunit/VLP)
Safe in immunocompromised; boosters often needed

Mechanism — from genome to protein

+ ssRNA
→
Translated directlyPolyprotein
− ssRNA
→
Viral RdRpMakes + strand
→
Translated
Retrovirus + ssRNA
→
Reverse transcriptasedsDNA
→
IntegratesHost transcribes

Exceptions to memorize

RULE
EXCEPTION
REASON
DNA viruses are ds
Parvovirus (ss)
Smallest DNA virus
DNA viruses in nucleus
Poxvirus (cytoplasm)
Carries own DNA-dependent RNA polymerase
DNA viruses icosahedral
Poxvirus (complex)
Brick-shaped
RNA viruses are ss
Reovirus/rotavirus (ds)
Double-capsid gut virus
RNA viruses in cytoplasm
Influenza, retroviruses (nucleus)
Need host splicing / integration
DNA viruses linear
Papilloma, polyoma (circular); HBV (partly ds circular)
—

How it's tested

COMMON CONFUSIONAntigenic shift (reassortment, big change, pandemics, only influenza A) vs drift (point mutations, small change, seasonal, A and B).
THE TRAPIn phenotypic mixing the progeny genome is unchanged — the borrowed coat lasts one generation only.
YOUR APPROACHAsk: DNA or RNA → strands → sense → envelope → segmented? Then predict replication site, spread and genetic behavior.

If you see… think…

Virus survives on hands after alcohol gel, spreads fecal-oral → naked virus (norovirus)
Purified RNA alone produces infection → positive-sense RNA virus
Virion must carry its own RNA polymerase → negative-sense RNA virus
Only single-stranded DNA virus → parvovirus B19
DNA virus replicating in the cytoplasm → poxvirus
Double-stranded RNA virus → rotavirus/reovirus
New pandemic flu strain from swapped segments → reassortment (antigenic shift)
HDV packaged in HBsAg → complementation
Envelope acquired from nuclear membrane → herpesviruses
Live vaccine to avoid in pregnancy → MMR, varicella