Viruses Flashcards
(25 cards)
What does it mean when viruses are “obligate intracellular parasites”?
Obligate -> Uses host resources
Intracellular -> Inside cell
Parasite -> Parasitic in nature
Virus Structure
Nucleic Acids surrounded by Protein Coat (Capsid), a few enzymes, and sometimes a biological membrane stolen from the host called an “Envelope”
Shapes of viruses
1) Rod
2) Icosahedron (d20 die)
3) Spherical
4) Complex
Nucleic Acid Genome Properties
totally varied in nature…
-either DNA or RNA
-either single or double stranded
-either single or multiple copies
-either linear or circular
-either segmented or non-segmented (not all genes on a single genome; ex of segmented: chromosome 1,2,3,etc.)
Most common type of virus
Linear Double-Stranded DNA virus (most likely type to infect BACTERIA, which are NUMEROUS)
Viral Replicative Cycle
1) Attachment to SPECIFIC host cell surface receptors
2) Nucleic Acid enters cell (somehow)
3) Early Gene Expression (production of enzymes needed to replicate nucleic acids)
4) Replication of viral nucleic acid
5) Late Gene Expression (production of capsid proteins needed for assembly/leaving)
6) Virus assembly (near plasma membrane)
7) Release (Non-enveloped -> burst // Enveloped -> Exocytosis (Budding))
Important DNA viruses (Double-Stranded)
Phage T4 (Virulent)
Phage Lambda (Temperate)
Virulent
Virus cycle ends with death of host cell (lytic pathway)
Temperate
Can follow either lytic or lysogenic (dormancy) pathways
T4 Replicative Cycle
1) Entry
2) Release viral DNA
3) mRNA made
4) Viral Proteins made
5) Viral proteins degrade host genome and modify RNA Polymerase Sigma Factors
6) RNA Polymerase replicates viral DNA, making “late” viral dna
7) Synthesis of “late” viral proteins
8) Exit (Lysis)
Lambda Replicative Cycle
Same as T4, but makes a “molecular decision” (once inside cell) to do either lysogenic (Average cell growth conditions) or lytic (Strong/Weak cell growth conditions) pathway
Lysogeny
Viral genome integrated into host chromosome (prophage), cell becomes “immune” to infection by same type of phage
How does an RNA virus make a copy of its DNA?
Provides the code for a protein that is an RNA-DEPENDENT, RNA-SYNTHESIZING enzyme
Replicase (viral replicase, RdRp, viral RNAP)
RNA-dependent, RNA-synthesizing enzyme
Reverse Transcriptase
Makes ds-DNA from ss-RNA
(+) sense strand RNA viruses
Directly translatable by a host ribosome (as if it was mRNA)
(-) sense strand RNA viruses
RNA is complementary and antiparallel to mRNA
Retroviruses
ss-RNA virus that integrates w/ host genome
(+) sense strand RNA replication
1) Replicase is made directly
2) (-) sense strand made from (+) template
3) Replicase uses (-) strand makes more (+) strands (signal amplication)
(-) sense strand RNA replication
1) Replicase has to be brought in (can’t be directly translated by cell)
2) Replicase uses (-) strand to make (+) strands
3) (+) strands used as templates by ribosomes to churn out (-) strands and more replicase
HIV Components
-Viral Protein
-Viral RNA (single-stranded)
-Reverse Transcriptase
-Receptor-binding glycoprotein
-Integrase
-Protease (not really used, caught along during budding)
CD4 Receptor
Specific receptor for HIV
HIV Replicative cycle
1) HIV binds to CD4 Receptor
2) Viral RNA released
3) Reverse Transcriptase copies viral RNA to DNA
4) Integrase (has a NLS) binds to viral DNA and integrates it into the host DNA (not lysogenic)
5) Host makes viral RNA from viral DNA in host genome
6) viral mRNA is translated
7) Protease can cleave ITSELF, cleaves the polyprotein to make many different proteins (remember, our DNA is monocistronic)
Latent Virus (Latency)
Viral DNA lies dormant in cell, NOT LYSOGENY (no choice)
