mutations on human immunodeficiency virus type 1

In host cell, virus replicates its nucleic acid and synthe sizes its proteins, then assembles them to form progeny vir al particles that are released by budding or cell lysis... Fusion

Medical Microbiology

Li Mei Department of Microbiology

November, 2006

Virology

Chapter 19 General Properties of Viruses

Structure Replication

What is virus?

Viruses the smallest infectious and acellular microbe consisting only one kind of nucleic acid (DNA or

RNA), and which obligately replicate inside host cells.

 Virions

The complete mature viral particles.

(The intact infectious virus particles.)

Distinctive features

• Acellular microbes

• Pass through 0.2μm filters

• Obligatory intracellular para

sites

• Contain either DNA or RNA

• Self-replication

I Size, shape and structure

A Size:

The unit of measurement nm

parvoviruses poxviruses

B Shape:

I Size, shape and structure

Tobacco mosaic virus:

rod-shaped

Poxvirus: brick-shaped

Spherical

VSV (Vesicular stomatitis virus) :

bullet-shaped

Bacteriophage T4: tadpole-shaped

Ebola Virus: filamentous shape

 Basic structure:

Core: Viral nucleic acid (DNA or RNA)

Capsid : Protein shell

capsomers (morphological subunit)

polypeptide molecules (chemical subu nit)

Core + Capsid → nucleocapsid

I Size, shape and structure

Others: enzymes, etc.

e.g Retrovirus has reverse transcriptase

I Size, shape and structure

I Size, shape and structure

2 Symmetry of viral nucleocapsids

is decided by arrangement of capsomers

Influenza virus

– Animal diseases:

e.g., Scrapie Bovine spongiform encephalopathy (BSE)

a single circular RNA molecule without a protein coat which mainly cause plant diseases

infectious agents composed of a si ngle glycoprotein with MW 27-30 kDa

In host cell, virus replicates its nucleic acid and synthe sizes

its proteins, then assembles them to form progeny vir

al particles that are released by budding or cell lysis

II Replication

A Normal Replication

– Adsorption /Attachment – Penetration

 Attachment / Adsorption

II Replication

Attachment / Adsorption

B Fusion between cell membrane and viral envelope

The enveloped viruses

II Replication

C Nucleic acid translocation:

Some bacteriophages and naked viruses

II Replication

• penetration

inner

A B C D

Viral genome replication

Viral protein synthesis

II Replication

Types: dsDNA, ssDNA, dsRNA, +ssRNA, -ssRNA, Retrovirus

dsDNA viruses: e.g., Herpes simplex virus

progeny viral DNA late mRNA late proteins

-ssRNA virus

Translation Transcription

Structural protein

RNA Polymerase

e.g., influenza virus

RNA Polymerase

 Assembly

Naked virus: capsid + viral genome → nucleocapsid (virion)

Site: a DNA viruses (except poxvirus): cell nucleus;

b RNA viruses and poxvirus: cell cytoplasm;

Manner: a assemble as empty shell (procapsids), then viral genome fill in

Enveloped virus: nucleocapsid + envelope → virion

b Viral capsomers array around the viral genome to form helical symmetry

II Replication

 Release

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 bridge s.



SSPE (Subacute sclerosing panencephalitis)

II Replication

Host cell lysis Budding

• release

B Abnormal replication:

– Defective viruses

– Abortive infection

II Replication

Defective viruses:

are genetically deficient and incapable of producing infectious progeny virions.

Helper virus:

can supplement the genetic deficiency and make

defective viruses replicate progeny virions when they simultaneously infect host cell with defective viruses.

II Replication

e.g., AAV & adenovirus

• Defective viruses which can occupy the cell

machinery necessary for normal virus replication to prevent virus production, are called "defective

interfering particles" (DIP).

Defective interfering particles (DIP)

2 Abortive infection:

Virus infection which does not produce infectious progeny

because the host cell cannot provide the enzyme, energy or

materials required for the viral replication

Range of interference occurrence

between the different species of viruses;

between the same species of viruses;

between the inactivated viruses and live viruses.

III Viral interference:

Mechanisms of viral interference:

a.Virus A may inhibit virus B adsorption by blocking or destroying receptors on host cell

b.Virus A may compete with virus B for replication materials like polymerase, translation initiation factors, etc

c.Virus A may induce the infected cell to produce interferon that can

prevent viral replication

III Viral interference

Significance of interference:

Advantage

a Stop viral replication and lead to patient recovery

b Inactivated virus or live attenuated virus can be used as vaccine

Antibiotic, interferon, etc.

IV Reaction to physical & chemical agents

-196℃

Growth on cell free