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Microbiology - Lecture Thirty One : Introduction to viruses

Q: What are the basic structural characteristics of viruses ?

Acellular microorganisms→ aren’t made of cells Obligate intracellular parasites→ cannot survive without a host cell’s molecular resources No metabolic abilities→ relies completely on the biosynthetic machinery of a host cell Most common biological entities on Earth Infect all types of cells

Q: What are the differences between 'naked' and 'enveloped' viruses ?

Genetic Material Can be DNA or RNA Can be single (ss) or double stranded (ds) The genome itself (ssRNA, dsRNA, ssDNA, dsDNA) can have the following shapes: linear, circular or segmented Capsid Protein coat that surrounds and protects genetic material Also important in viral interaction with host cells Envelope (In Some Viruses) Lipid membrane Often contains proteins as well Surrounds the capsid → ‘Naked’ viruses lack an envelope

Q: What are the different types of viral capsid symmetry ?

Capsomers Protein based subunits that make up the viral capsid Arranged within a precise/highly repetitive pattern around genetic material Capsid Symmetry Helical : Rod/filament shaped - e.g. plant viruses like tobacco mosaic virus (TMV) Icosahedral : Very common symmetry - e.g. adenoviruses Complex : Head= icosahedral; tail= helical - e.g. bacterial viruses (bacteriophages)

Q: Explain the lytic replication cycle of bacteriophage

Attachment : The phage attaches to the surface of the host Penetration : The viral DNA enters the host cell Biosynthesis : Phage DNA replicates and phage proteins are made Maturation : New phage particles are assembled Lysis : The cell lyses, releasing the newly made phages

Q: Explain the lysogenic replication cycle of bacteriophage

Induction : When the viral genome removes itself from the host cell’s genome. Process is initiated by particular growth and environmental factors Viral genome can then replicate and enter a lytic cycle The host bacterium carrying the viral genome is called a lysogen or lysogenic bacterium

Q: What is the replication cycle of enveloped mammalian cell viruses ?

Attachment : Virus binds to cell receptors Fusion : HIV envelope + cell membrane fuse Reverse Transcription : HIV reverse transcriptase enzyme converts ssRNA → cDNA (single stranded DNA). 2x cDNA : dsDNA dsDNA enters nucleus : ssRNA cannot enter nucleus Integration : HIV integrase enzyme integrates viral DNA with host’s DNA→ provirus formed Replication and Protein Synthesis : - Viral DNA transcribes : viral RNA - Viral RNA is then translated : viral proteins Self Assembly : viral RNA + proteins associate near cell membrane. Vesicles containing viral glycoproteins are transported to plasma membrane. Release : Newly formed virus buds off cell membrane; membrane envelopes the virus, glycoproteins attach to the exterior.

6 important questions on Microbiology - Lecture Thirty One : Introduction to viruses

What are the basic structural characteristics of viruses ?

Acellular microorganisms→ aren’t made of cells
Obligate intracellular parasites→ cannot survive without a host cell’s molecular resources
No metabolic abilities→ relies completely on the biosynthetic machinery of a host cell
Most common biological entities on Earth
Infect all types of cells

What are the differences between 'naked' and 'enveloped' viruses ?

Genetic Material

Can be DNA or RNA
Can be single (ss) or double stranded (ds)
The genome itself (ssRNA, dsRNA, ssDNA, dsDNA) can have the following shapes: linear, circular or segmented

Capsid

Protein coat that surrounds and protects genetic material
Also important in viral interaction with host cells

Envelope (In Some Viruses)

Lipid membrane
Often contains proteins as well
Surrounds the capsid

→ ‘Naked’ viruses lack an envelope

What are the different types of viral capsid symmetry ?

Capsomers

Protein based subunits that make up the viral capsid
Arranged within a precise/highly repetitive pattern around genetic material

Capsid Symmetry

Helical : Rod/filament shaped

- e.g. plant viruses like tobacco mosaic virus (TMV)

Icosahedral : Very common symmetry

- e.g. adenoviruses

Complex : Head= icosahedral; tail= helical

- e.g. bacterial viruses (bacteriophages)

Explain the lytic replication cycle of bacteriophage

Attachment : The phage attaches to the surface of the host
Penetration : The viral DNA enters the host cell
Biosynthesis : Phage DNA replicates and phage proteins are made
Maturation : New phage particles are assembled
Lysis : The cell lyses, releasing the newly made phages

Explain the lysogenic replication cycle of bacteriophage

Induction :

When the viral genome removes itself from the host cell’s genome.
Process is initiated by particular growth and environmental factors
Viral genome can then replicate and enter a lytic cycle

The host bacterium carrying the viral genome is called a lysogen or lysogenic bacterium

What is the replication cycle of enveloped mammalian cell viruses ?

Attachment : Virus binds to cell receptors
Fusion : HIV envelope + cell membrane fuse
Reverse Transcription : HIV reverse transcriptase enzyme converts ssRNA → cDNA (single stranded DNA).
2x cDNA : dsDNA
dsDNA enters nucleus : ssRNA cannot enter nucleus
Integration : HIV integrase enzyme integrates viral DNA with host’s DNA→ provirus formed

Replication and Protein Synthesis :

- Viral DNA transcribes : viral RNA
- Viral RNA is then translated : viral proteins

Self Assembly : viral RNA + proteins associate near cell membrane. Vesicles containing viral glycoproteins are transported to plasma membrane.
Release : Newly formed virus buds off cell membrane; membrane envelopes the virus, glycoproteins attach to the exterior.