Lecture 14 - Viruses

Virus

non-cellular infectious agents that hijack a host cell to reproduce

Virion

a single infectious viral particle

Viruses: Sizes

Viruses are extremely small

Virusess: Cellularity

viruses are not made up of cells

Viruses: Structures

Capsid

Envelope (some viruses)

Spikes

Matrix (some viruses)

genetic material

Capsid

a protein shell held together by electrostatic interations that surrounds and protexts the virus' genetic material

Capsid: Functions

Help virus attach to receptors on host cells

Encapsulate genome in host cell, transport, and release to another host cell

Spike

structures made of protein/carbohydrate that stick out of viruses

Matrix

link the core/capsid with envelope; mediates assembly, exit and entry

Viruses: Metabolic Activity

No independent metabolism; must infect host to replicate

Viruses: Genome

DNA or RNA (not both); can be single-stranded or double-stranded; typically much smaller (a few thousand to hundreds of thousands of bases)

Virus: Classification

DNA or RNA

Naked or Enveloped

Double or SIngle Stranded

Lytic Cycle

1. Virion particle attach and penetrate host cell

2. Capsid gets separated from genetic material

3. Host gets hijacked by virus

4. Virus gets replicated in host cell

5. Host cell dies or buds, releasing the replicated virus

Lysogenic Cycle

1. Virion particle attach to host cell

2. Integrates viral DNA into host cell

3. Host cell integrates viral DNA into its own DNA

4. Viral DNA replicated by host and passed down to next generations

5. Excision of provirus

Virus matures and is released when host cell dies

Lytic versus Lysogenic Cycle

Lytic = Lysed = Dies

cell must die for virus to spread

Genic = gene = DNA

virus is integrated into he host's DNA to spread within generations

Viral Replication

1. Attachment

2. Penetration

3. Biosynthesis

4. Assembly

5. Release

6. Maturation

Attachment

Viral surface ligand collides with and binds to host receptor

ligand-receptor

highly specific; like a lock and key

Penetration

Virion or viral genome passes through cell membrane or cell wall depending on the type of cell

Processes in Penetration

Membrane Fusion

Endocytosis

Uncoating

Membrane Fusion

only enveloped viruses; like two bubbles merging

Endocytosis

process by which a cell takes material into the cell by infolding of the cell membrane

Uncoating

removal of capsid; releases genome into appropriate compartment

Biosynthesis

Sequential viral gene expression to:

- disrupt/take over host functions

- Enable viral genomes replication

- Make virion components

Genome Replication

DNA: similar to central dogma

RNA: (+) RNA ⇒ dsDNA ⇒ integration ⇒ provirus (using RdRp or RNA replicase)

Assembly

Components concentrated at a particular location in the host cell

Nucleocapsid forms, virion assembled

Release

Non-Enveloped: Lysis of cell

Enveloped: Exocytosis or "Budding"

Exocytosis

Process by which a cell releases large amounts of material

Maturation

Viral protein processing e.g., polyprotein cleavage by host or viral protease

Requried for immature virus to become infectious

Explain why naked viruses cannot enter a host cell through membrane fusion

naked viruses do not contain a membrane but just a capsid, preventing them to merge into the host's cell membrane

Explain why naked viruses cannot exit through exocytosis

No envelope means they cannot bud or use exocytosis. Instead, they usually exit by host cell lysis — the cell bursts, releasing new virions.

Stages of HIV

Attachment → Fusion → Reverse transcription → Integration → Transcription/translation → Assembly → Budding → Maturation

Viruses: Highly Specific

1. Host Specifity (i.e., HIV only affects humans)

2. Cell-Type Specificity (i.e., HIV infects CD4+ T cells, macrophages, dendritic cells (because they express CD4 + CCR5/CXCR4))

3. Receptor Binding (ligand and receptor must fit)

4. Intracellular Requirements (i.e., the virus must have the same enzymes to adapt to the host cell)