General Properties of Viruses
What is virus?
The smallest infectious and acellular microbe.
Consisting only one kind of nucleic acid (DNA or RNA), and which obligately replicate inside host cells.
The complete mature viral particles.
(The intact infectious virus particles.)
- The simplest: acellular microbes contain either DNA or RNA
- The smallest: Pas through 0.2μm filters
- Obligatory intracellular parasites.
I. Size, shape and structure
The unit of measurement nm
Comparative sizes of virions and bacteria
1. Staphylococcus aureus
5. Bacteriophage of E. coli
6. Influenza virus
8. Encephalitis B virus
Tobacco mosaic virus: rod-shaped
VSV (Vesicular stomatitis virus): bullet-shaped
Bacteriophage T4: tadpole-shaped
Ebola Virus: filamentous shape
Core: Viral nucleic acid (DNA or RNA)
Capsid: Protein shell
capsomers (morphological subunit)
polypeptide molecules (chemical subunit)
Core + Capsid → nucleocapsid
Size, shape and structure
Others: enzymes, etc.
e.g. Retrovirus has reverse transcriptase
Symmetry of viral nucleocapsids: Is decided by arrangement of capsomeres
(e.g., tobacco mosaic virus)
(e.g., poxviruses )
Viral nucleic acid: ssDNA, dsDNA, ssRNA, dsRNA
protection, mediate the attachment of virue to specific receptors on host cell surface
determine species and organ specificity
important antigens, superantigen
Human Hepatitis D
NOTE: a single circular RNA molecule without a protein coat which mainly cause plant diseases.
Proteinaceous infectious particle
Bovine spongiform encephalopathy (BSE)
NOTE: infectious agents composed of a single glycoprotein with MW 27-30 kDa.
In host cell, virus replicates its nucleic acid and synthesizes
its proteins, then assembles them to form progeny viral
particles that are released by budding or cell lysis.
- Adsorption /Attachment
i. Adsorption / Attachment
Specific binding of a viral attachment protein (VAP) with a receptor on the surface of host cell;
VAP (on virion ) --- viral surface protein
Spike – enveloped virus
Capsid protein – naked virus
Viral receptor (on host cell)
Glycoprotein, carbohydrate or glycolipid
e.g., CD4 (HIV), CD46 (measles virus), Sialic acid (influenza virus)
Some enveloped viruses
Most naked virus
B. Direct fusion of cell membrane with viral envelope:
Only enveloped viruses
C. Nucleic acid translocation:
Some bacteriophages and naked virus
The process of removing capsid and releasing viral nucleic acid into the cytoplasm;
Acidification of the content of the endosome
Proteases are needed;
- Viral genome replication
- Viral protein synthesis
dsDNA; ssDNA; dsRNA; +ssRNA; -ssRNA; retrovirus
+ssRNA with DNA intermediate in life cycle (HIV);
dsDNA with RNA intermediate (HBV);
+ssRNA virus (Poliovirus, HAV)
Viral genomic RNA serve as mRNA;
Enzymes for replication are made after infection, not carried in virion;
(Extracted) Viral genomic RNA is infectious
-ssRNA virus e.g., influenza virus
Virion carries RDRP;
First step: Transcription of viral genome;
Extracted -ssRNA not infectious;
Naked virus: capsid + viral genome → nucleocapsid (virion, complete structure)
Enveloped virus: capsid + viral genome → nucleocapsid (incomplete structure)
a. DNA viruses (except poxvirus): cell nucleus;
b. RNA viruses and poxvirus: cell cytoplasm;
a. assemble as empty shell (procapsid), then viral genome fill in.
b. Viral capsomeres array around the viral genome to form helical
The process of progeny viruses getting out of host cell.
Naked viruses:released by cell lysis.
Enveloped viruses:usually released by budding.
During budding enveloped viruses acquire their envelope.
Defective measles virus: release from cell to cell via cell bridges.
enveloped virus replication （1）
enveloped virus replication （2a）
enveloped virus replication （2b）
enveloped virus replication （3）
enveloped virus replication （4）
Two aspect factors:
non-permissive cells → Abortive infection
are genetically deficient and incapable of producing infectious progeny virions.
can supplement the genetic deficiency and make defective viruses replicate progeny virions when they simultaneously infect host cell with defective viruses.
e.g., HDV & HBV
- Defective viruses lack gene(s) necessary for a complete infectious cycle;
- helper viruses provide missing functions;
- 100:1 (defective to infectious particles)
- DIP (defective interfering particle) : When the defective viruses can not replicate, but can interfere other congeneric mature virion entering the cells, we call them defective interfering particles (DIP).
Virus infection which does not produce infectious progeny because the host cell cannot provide the enzyme, energy or materials required for the viral replication.
The host cells that cannot provide the conditions for viral replication.
The host cells that can provide the conditions for viral replication.
III. Viral interference:
When two viruses infect simultaneously one host cell, One type of virus
may inhibit replication of another type of virus.
Range of interference occurrence
- between the different species of viruses;
- between the same species of viruses;
- between the inactivated viruses and live viruses.
Main mechanisms of viral interference:
a. One type of virus inhibit or prevent subsequent adsorption and penetration
of another virus by blocking or destroying receptors on host cell.
b. The competition of two viruses for replication materials, e.g., receptor
polymerase, translation initiation factors, etc.
c. One type of virus may induce the infected cell to produce interferon that
can prevent viral replication.
The mechanism of IFN function
Significance of viral interference:
a. Stop viral replication and lead to patient recovery.
b. Inactivated virus or live attenuated virus can be used as vaccine to
interfere with the infection of the virulent virus.
May decrease the function of vaccine when bivalent/trivalent vaccine is used.
Just for your practice see the answers at the end.
Fill in the blank
1-The surrounding protein coat of a virus is called the _______ and it is composed of protein subunits called _________.
2-Viruses that are only covered with a protein coat outside viral genome are called ______ viruses, while those that have an additional lipid-containing membrane covering are called ________ viruses.
3. The general steps of the viral replication cycle include ( in the order of their occurrence) ___________________, ___________, ____________, __________, _________________.
3-Attachment, Penetration, Uncoating, Biosynthesis, Assembly and release.