Biology Unit 3- Viruses

What is the basic structure of viruses?

Nucleic acid: Single or double stranded DNA or RNA (split into two groups depending on which it has)

Capsid: protein shell made of capsomers enclosing the viral genome (may be rod-shaped, polyhedral or more complex)

What are rod shaped viruses called?

Helical viruses

What other structures can viruses have?

Viral envelopes: derived from membranes of the host cell, containing their phospholipids and membrane proteins. Help infect host cell

Protein and glycoproteins of viral origin and viral enzyme molecules

How do viruses replicate?

Viruses are parasites, meaning they only replicate within a host cell

How do viruses identify host cells?

Using a “lock and key” fit between viral surface proteins and specific receptor molecules outside of cells, with some viruses having broad ranges of host cells

Ex. West Nile virus can infect horses, birds and humans while measles can only infect humans

Are some viruses limited to infecting certain types of tissues?

Yes. Human cold viruses only infect cells lining upper respiratory tract while AIDS and HIV viruses bind to receptors present only on certain types of immune cells

What is step 1 of the viral reproductive cycle?

Virus binds to the host cell and is uncoated, releasing viral DNA and capsid proteins (either inject DNA into a bacterium using a tail, taken up by endocytosis, or fusion of viral envelope with host’s plasma membrane)

Step 2 reproductive cycle

Proteins viral genome commands the host, reprogramming the cell to copy the viral genome and manufacture viral proteins

Step 3 reproductive cycle

Host enzymes transcribe the viral genome into viral mRNA, which host ribosomes use to make more capsid proteins

Step 4 reproductive cycle

Viral genomes and capsid proteins self-assemble into new virus particles, which exit the cell

What is the lytic cycle?

Phage replicative cycle that culminates in death of the host cell, referring to the last stage of infection during which the bacterium lyses (breaks open) and releases phages produced within the cell

What are the major steps of the lytic cycle?

1: Attachment- Phage binds to surface proteins of cell using tail fibers

2: Entry of phage DNA and degradation of host DNA- Sheath of tail contract, injecting phage DNA into cell (cells DNA hydrolyzed)

3: Synthesis of viral genomes and proteins- Phage DNA direct production of proteins using components within the cell

4: Self-assembly- 3 separate sets of proteins self assemble to form phage parts

5: Release- phage directs production of enzyme that destroys cell wall, allowing cell to burst

What is the lysogenic cycle?

Allows replication of the phage genome without destroying the host.

The virus integrates its genetic material into the host cell’s DNA, allowing it to replicate passively as the host cell divides

What are the major steps of the lysogenic cycle?

1: Attachment and entry- virus attaches to host cell and injects its material inside

2: Integration- viral DNA inserts itself into host cell’s chromosome, forming a combined structure called a prophage

3: Cell division- host cell lives and reproduces normally, copying the hidden viral DNA along with its own each time

4: Induction- environmental stress or damage can activate the viral DNA, which then breaks free and switches to the lytic cycle

What is the progressive hypothesis?

Viruses arose from genetic elements that gained the ability to move between cells

What is the regressive hypothesis?

Viruses are remanents of cellular organisms

What is the virus first hypothesis?

Viruses existed as self-replicating units before cells and later coevolved with hosts

What is the chimeric hypothesis?

Virus genomes arose first and recruited cellular proteins to form capsids

What is the symbiogenic hypothesis?

Viruses arose from genomic reduction of ancient viro-cells that coexisted with early cells

Why are viruses non-living?

No cells: viruses aren’t made of cell, the basic unit of life

No independent energy: lack mitochondria to generate their own energy and ribosomes to synthesize proteins

Inert outside a host: outside of a living host cell, a virus is dormant and cannot do anything

Why can viruses be considered as living things?

Genetic material: contain DNA or RNA

Evolution: adapt and evolve over time to survive changes in their environment

Reproduction: once inside a host cell, they hijack the cell’s machinery to make copies of themselves