Viruses

1. Viruses By: Brittnie, Kevin, and Cristian

2. What Is a Virus? Viruses… • Are particles of nucleic acid, protein, and in some cases, lipids. • Are not alive • Can only reproduce by infecting living cells. • Differ widely in terms of size and structure • All enter living cells and, once inside, use the machinery of the infected cell to produce more viruses.

3. Most viruses are so small that they can only be seen with a powerful electron microscope. • A typical virus is composed of a core of DNA or RNA surrounded by a protein coat called a capsid. • Simplest viruses contain only a few genes • Most complex have more than 1 hundred

4. A capsid includes proteins that enable a virus to enter a host cell. The capsid proteins of a typical virus bind to receptors on the surface of a cell and tricks the cell into allowing it inside. • When the virus gets in the cell, the viral genes are expressed and the cell transcribes and translates the viral genetic info into viral capsid proteins. • Sometimes this causes the host cell to make copies of the virus, destroying the host cell.

5. Most viruses are specific about the cells they invade. Bacteriophages are viruses that infect bacteria.

6. Viral Infection • Once the virus invades the cell, two different processes occur • Lytic Infections • Lysogenic Infections

7. Lytic Infection • In a lytic infection, a virus enters a cell, makes a copy of itself, and causes the cell to burst. This releases the virus particles.

8. An example… • Bacteriophage T4 has a DNA protein capsid that activates when it comes into contact with the host cell. It then injects its DNA into the cell. The host becomes confused and makes messenger RNA from the virus DNA. The viral RNA is made into proteins that shut down the cell. The virus then makes copies of its own DNA, assembling new virus particles. The cell lyses,or bursts, and releases hundreds of virus particles that go infect other cells.

9. Lysogenic Infection • Lysogenic infections are when a host cell makes copies of the virus indefinitely. A virus integrates its DNA into the DNA of the host cell and the viral genetic information replicates along with the host cells DNA.

10. Lysogenic viruses remain inactive for a long period of time until they are triggered to activate. The viral DNA embedded in the host’s DNA is called Prophage. The prophage may remain inactive for many generations but will eventually be triggered to activate.

12. Retroviruses • “Retro” = backward, getting there name from how information is copied backward from RNA to DNA • Retroviruses are viruses that contain RNA as their genetic information instead of DNA • When they infect a cell, they produce a DNA copy of their RNA and insert it into the host cell. The retroviruses then become dormant, producing new viruses and killing the host cell. Ex. Aids and Some types of cancers

13. Viruses and Living Cells Viruses… • Must infect a living cell to grow and reproduce • Also take advantage of the host’s respiration, nutrition, etc. • Are parasites • Are not alive but have many characteristics of living things (can reproduce, evolve, and regulate gene expression). They are borderline on living and non-living.

14. Virus Origin • Although viruses are smaller and simpler than the smallest cells, it is unlikely that they could have been the first living things. • Since viruses depend on living things. It is most likely that they developed after living cells. • The first viruses may have evolved from genetic material on living cells and continued to evolve, along with the cells they infect, over billions of years.