Transcript Virus
3.a.1 – DNA, and in some cases RNA, is the primary source of heritable information (19.2). 3.c.3 – Viral replication results in genetic variation, and viral infection can introduce genetic variation into the hosts (19.1 & 19.2). An infectious particle consisting of nucleic acid enclosed in a protein coat Characteristics: 1. Small size Large - barely visible with the light microscope Small - down to 20nm Much smaller than cells (including bacteria) 2. Infectious 3. Reproduction 4. Alcohol resistance Not immediately killed by alcohol (like your cells) Infection: ◦ Viruses cause many diseases ◦ They can be spread from one organism to another Reproduction: ◦ Can not reproduce without a host ◦ Viruses are obligate parasites – must have host to survive 1. Genome - the genetic information 2. Capsids and Envelopes - the outer covering Protein shell that encloses the viral genome Made from a large number of protein subunits Number of kinds of subunits usually small Often geometric in shape Membranes cloaking the capsids of some viruses Made from host membrane, but may have virusoriginated glycoproteins added 1. 2. 3. 4. 5. Cell Entry Genome Replication Capsid Protein formation Self-assembly of offspring Exit from Host Locate host by "Lock-and-Key” fit between virus proteins and cell receptor molecules Infection - Begins when viral genome enters the cell Some viruses can enter many hosts ◦ Ex: West Nile – mosquitoes, birds and humans This step begins the actual viral infection Host cell is reprogrammed to copy virus genome Use host DNA polymerase Use virus enzymes to copy RNA to DNA Use host's resources to make copies of viral proteins Host's protein synthesis machinery used to make virus proteins genome + capsid virus Particles usually assemble spontaneously Host cell may burst or lyse, releasing the virus May "bud-off" host membrane, forming envelopes around the capsids Best understood of all viruses Called phages May show two types of virus life cycles: ◦ Lytic Cycle ◦ Lysogenic Cycle Virus reproductive cycle that kills the host cell Note - the previously described virus life cycle was a Lytic Cycle Uses Lytic Cycle Example of an Virulent Virus (a virus that only uses the lytic cycle) Has about 100 genes Completes cycle in 20-30 minutes Virus reproduction that doesn't immediately kill the host cell ◦ Usually doesn’t kill the host AT ALL Viral DNA is inserted into the host DNA, but not expressed Belong to several viral classes Specific Interests: 1. Viruses with envelopes 2. RNA viruses Ex. of Infections caused by animal viruses: Cold sores/canker sores, influenza, common cold Use nuclear membrane Causes: cold sores, shingles, chicken pox, mononucleosis Viral DNA integrated into Host DNA as a provirus Shows both lytic and lysogenic life cycles Classes III – VI Ex: yellow fever, rubella, common cold, SARS, ebola Class VI - Retrovirus - use Reverse Transcriptase to make DNA from an RNA template. HIV - causes AIDS Retrovirus from chimps Destroys the body’s immune system, allowing other diseases to kill Measles Polio Smallpox Influenza Vaccines (preventative) Some Drugs ◦ Ex: Ara-A Acyclovin (Herpes treatment) ◦ Ex: AZT – HIVirus Comment - some treatments are working on the reverse transcriptase Problems: ◦ Since viruses work within a host cell, they are very difficult to treat ◦ Best Treatment – prevention ◦ Can also cause cancer Infectious particles of naked RNA Affect plants Similar to viruses, but lack a capsid Ex: ◦ Coconut Blight ◦ Chrysanthemum Wilt Infectious protein particles Affect animals Cause “mad-cow” and Creutzfeldt-Jakob disease Transmitted in food Slow-acting Virtually indestructible Protein folding problem? DNA circle (Chromosome) Plasmids - Small circle of DNA that is independent of the chromosome Carries a small number of traits 1. 2. 3. Transformation Conjugation Plasmids This is how bacteria mix genetic material into new combos Alteration of a cell's DNA by the uptake of foreign DNA Ex: Griffith's experiment Direct transfer of genetic material between two bacterial cells Bacterial "sex” Used to map genetic sequences in bacteria Small circular piece of DNA Carry many important traits Ex: Fertility Factor Antibiotic Resistance (R Plasmids) Virus movie http://www.youtube.com/watch?v=Rpj0emEGShQ