Transcript Virus Notes
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 1. How much time is associated with the replication cycle of a lytic virus? A lysogenic virus? 2. Which of these may be caused by lytic viruses? Explain? 3. Which may be caused by lysogenic viruses? Explain? 4. 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.