1. Viruses and Infections
Mr. Chapman
Biology 20

2. Viruses infect Prokaryotes and Eukaryotes
Different types of viruses infect different types of cells.
Remember that the types of cells viruses are able to infect depend on two things:
The types of protein structures on the surface of the virus (the key).
The types of surface receptors on the surface of the host cell (the lock).

3. Viruses infect Prokaryotes and Eukaryotes
Bacteriophages
Differ from bacteriophages, entering cell by endocytosis:
See next slide
Viruses that Infect Bacteria
Viruses that Infect Eukaryotes

4. Viruses infect Eukaryotes
The process of endocytosis

5. Endocytosis
Vesicles (membrane-bound sacs) are formed around the viral particles after they penetrate the cell membrane.
Viruses that are enveloped by a lipid membrane can also fuse directly to the cell membrane and release the capsid into the cell’s cytoplasm.
Once inside the cell, eukaryotic viruses target the nucleus of the cell.

6. Bacteriophage Infection
Recall that bacteriophages only infect bacterial cells.
One example is the T-bacteriophage which infects E. Coli. It has a 20-sided capsid connected to a long protein tail with spiky foot-like fibres.

7. Bacteriophage Infection
Notice that the capsid contains the genetic material.
The tail and spikes help to attach the virus to the host cell.
After attaching, the tail of the virus releases an enzyme that breaks down part of the bacterial cell wall.

8. Bacteriophage infection
After this step, the tail sheath retracts, and the tail core punches through the cell wall, injecting the DNA of the bacteriophage into the bacterial cell.
The bacteriophage (or phage for short), works like a syringe, injecting its DNA into the bacterial cell’s cytoplasm.
Remember: bacteria don’t have a nucleus, so the cytoplasm is where their DNA is found.

9. Viruses Cause Two Types of Infections

10. Lytic Infection
A lytic infection is an infection where the host cells bursts, or lyses.
When the cell lyses, new viruses are released from it and each one of them infects another cell.
The following are the steps of a lytic infection (taken from page 551 of the textbook).

11. Lytic Infection Steps
When the viral DNA enters the host cell, it takes over control of the host cell’s DNA, turning on the genes necessary to copy the viral genes.
Under direction of the viral genes, the host’s DNA undergoes transcription and translation, producing capsids and enzymes. The enzymes then help in the copying of the virus’s DNA.
Using energy from the host cell, the capsids and viral DNA assemble into new virions. Viral enzymes dissolve the host cell membrane, releasing the new virus particles into the host cell’s bloodstream or tissues – and destroying the host cell in the process.

13. Lysogenic Infection
In a lysogenic infection, a phage (virus) combines its DNA into the host cell’s DNA.
After entering the host cell, the viral DNA combines with the host’s DNA, forming a new set of genes called a prophage. A prophage is the phage DNA inserted into the host cell’s DNA. In organisms other than bacteria, this stage is called a provirus.

14. Lysogenic Infection
The prophage is copied and passed to daughter cells, with the host’s own DNA, when the host cell undergoes mitosis (cell division). Although this process does not destroy the cell, it changes some of the cell’s traits.
After the cell has been copied, there are two possible paths. A trigger, such as stress, can activate the prophage, which then uses the cell to produce new viruses. Or the prophage can remain as a permanent gene.

16. Comprehension Questions
What process do viruses that infect eukaryotes use to enter the cell? What is the name of the envelope that carries them through the cell?
What type of virus infects bacteria (prokaryotes)? How is it like a syringe?
What are the differences between lytic and lysogenic infections?