Viruses + Bacteria

What is a Virus Nucleic acids enclosed in a protein coat Smaller than the smallest bacteria Do not fit all criteria for life: Do not carry out respiration, grow, or develop.

Viruses and Host Cells All viruses can do is replicate In order to do so, they need a host cell. Viruses can use a variety of host cells Many are named for the tissue they infect: Ex: Adenovirus

Bacteriophage Virus that uses bacteria as a host cell. Has the “spider-like” structure associated with viruses

Viral Structure Has an inner core of nucleic acid (DNA or RNA) Nucleic acid contains instructions to replicate the virus Can have as few as 4 genes but may have hundreds. Covered in protein coat called a capsid.

Attachment to a Host Before a virus can replicate it has to attach to a host cell. Must recognize and attach to a receptor site on the plasma membrane of the host. Each virus has a specifically shaped attachment protein. Most viruses have only a few possible hosts.

Viral Replication Once attached, the virus enters the cell by injecting its nucleic acid into the cell. This causes the cell to make viral nucleic acids and proteins. Eventually new viruses are assembled.

Lytic Cycle Virus takes over host cell’s genetic material. The host uses its own raw materials to create new viruses. Eventually, new viruses burst from the host, killing it.

Lysogenic Cycle Not all viruses kill their hosts. Some integrate into the host cell’s chromosome. Called a provirus. They may not affect the activity of the host, but every time the host replicates, it replicates with the provirus.

Disease Symptom Incubation Measles Rash, fever 9-11 Days Shingles Pain, Itching on Skin Years Warts Bumpy areas on skin Months Influenza Aches, runny nose, fever 1-4 Days HIV Fatigue, weight loss, fever 2-5 Years How much time is associated with the replication cycle of a lytic virus? A lysogenic virus? Which of these may be caused by lytic viruses? Explain? Which may be caused by lysogenic viruses? Explain? What is a possible consequence of the fact that a person infected with HIV may have no symptoms for years?

Proviruses At any time, the provirus may activate and enter a lytic cycle. Many suspect that physical or emotional stress will lead to activation. Examples are cold sores and chicken pox.

Retroviruses Contain only RNA Most complex replication cycle Carry the enzyme reverse transcriptase with them. Reverse transcriptase helps integrate the RNA into the cell’s DNA. Ex: HIV

Cancer and Viruses Some viruses have been linked to cancer. Hepatitis B and liver cancer, HPV and cervical cancer Disrupt normal growth of cells and may lead to uncontrolled cell growth.

Prions and Viroids Prions are composed of proteins with no nucleic acids. Cause proteins to fold incorrectly Known to cause mad cow disease. Viroids are composed of a single strand of RNA with no protein coat Cause diseases in plants.

Plant Viruses First identified was tobacco mosaic virus. Over 400 viruses that affect plants. Require wounds to enter. Do not undergo lytic or lysogenic phases. Not all harmful. May cause striking color patterns in flowers.