Lecture 90 – Lecture 91 – Lecture 92 Viruses and Prions Ozgur Unal NIS - BIOLOGY Lecture 90 – Lecture 91 – Lecture 92 Viruses and Prions Ozgur Unal

Viruses “It is cold and flu season.” “1918 Spanish Flu Epidemic Kills Millions.” “New Cases of SARS reported.” “Human Cases of Bird and Swine Flu Reported.” …. all of these diseases are caused by viruses, and for none antibiotics work!!

Viruses A virus is a nonliving strand of genetic material within a protein coat. Viruses are considered to be nonliving, because: No organelles to use energy Can’t make protein Can’t move Can’t replicate on their own Although some viruses are not harmful, some are known to infect and harm all types of living things. Check out Table 18.2!!

Viruses Virus size: Viruses are one of the smallest disease causing structures that are known. They range in size from 5 to 300 nanometers!! Virus origin: The origin of viruses is not exactly known. According to one theory, viruses came from parts of cells. Genetic materials of viruses are similar to cellular genes  these genes somehow developed the ability to exist outside of the cell

Viruses Virus structure: The shape of virus changes from type to type  Figure 18.11 The outer layer of all viruses is made of proteins  capsid All viruses have a genetic material inside the capsid. The genetic material can be DNA or RNA, but never both!! Viruses generally are classified by the type of nucleic acid they contain.

Viruses Smallpox is caused by variola virus  highly contagious Symptoms include headache, fever, body ache, rashes etc. Lesions eventually spread over the body. Doctors take a throat swab to diagnose the disease. A smallpox vaccine had been used until 1977 when the last case of smallpox occurred in Somalia. Today, this vaccine is no longer used to prevent smallpox.

Viral Infection In order to replicate, a virus must enter a host cell. Virus attaches to the host cell using specific receptors on the plasma membrane of the host cell. Why is a virus common in plants very unlikely to affect humans? Once the virus successfully attaches to a host cell, the genetic material of the virus enters the cytoplasm of the host. Sometimes the entire virus enters the cell and the capsid is broken down quickly  genetic material exposed The virus now uses the host cell to replicate by either the lytic cycle or the lysogenic cycle.

Viral Infection Lytic cycle: In the lytic cycle, the host cell makes many copies of the viral DNA or RNA. The viral genes instruct the host cell to make viral protein capsids and enzymes needed for the replication. These new viruses leave the cell by exocytosis or by causing the cell to burst or lyse  viruses become free to infect other cells! Example: Common cold and influenza viruses

Viral Infection Lysogenic cycle: In some cases, the viral DNA might enter the nucleus of the host cell. In the lysogenic cycle, the viral DNA inserts or integrates into a chromosome in a host cell  permanently! The viral genes may remain dormant for a long time. Then at some future time, the viral genes might be activated  lytic cycle!! Example: Herpes simplex I

Retroviruses Some viruses have RNA instead of DNA  retroviruses Retroviruses have a complex replication cycle. Why? Example: HIV and some cancer causing viruses Retrovirus structure: RNA, reverse transcriptase, protein capsid, lipid envelope

Retroviruses HIV replication: When HIV attaches to a cell, the virus moves into the cytoplasm and the viral RNA is released. Reverse transcriptase synthesizes DNA using the viral RNA as template. Then the DNA moves into the nucleus and integrates into a chromosome. It might be activated later. Once it is activated, RNA is transcribed from the viral DNA  manufacturing and assembling new HIV particles.

Prions A protein that can cause infection or disease is called a proteinaceous infectious particle, or a prion. Prion is a protein that was first discovered in 1982. Prions normally exist in the cells, although their function is not well understood. Normal prions are shaped like a coil. Mutations in the genes that code for these proteins occur  misfolded proteins Mutated prions are shaped like a piece of paper folded many times. Mutated prions are associated with diseases such as mad cow disease in cows, CJD in humans etc.

Prions Prion infection: Check out Figure 18.15 for CJD disease! These misfolded proteins can cause normal proteins to mutate. These prions also infect nerve cells in the brain, causing them to burst. CJD can also affect younger people, in addition to the elderly  nvCJD