Microbiology - Lecture Thirty One : Introduction to viruses
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 ?
- 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
- Protein coat that surrounds and protects genetic material
- Also important in viral interaction with host cells
- Lipid membrane
- Often contains proteins as well
- Surrounds the capsid
What are the different types of viral capsid symmetry ?
- Protein based subunits that make up the viral capsid
- Arranged within a precise/highly repetitive pattern around genetic material
- Helical : Rod/filament shaped
- Icosahedral : Very common symmetry
- Complex : Head= icosahedral; tail= helical
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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
- 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
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 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.
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