Chapter 19 Microbial Models: The Genetics of Viruses and Bacteria
Chapter 19: Viruses
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Transcript of Chapter 19: Viruses
Chapter 19: Viruses
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).
Essential Knowledge
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. Infectious3. Reproduction4. Alcohol resistance
Not immediately killed by alcohol (like your cells)
What is a virus?
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
Infection and Reproduction
1. Genome - the genetic information2. Capsids and Envelopes - the outer covering
Viral Structure
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
Capsid
Membranes cloaking the capsids of some viruses
Made from host membrane, but may have virus-originated glycoproteins added
Envelopes
1. Cell Entry2. Genome Replication3. Capsid Protein formation4. Self-assembly of offspring5. Exit from Host
General Steps for Viral Replication
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
Cell Entry
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
Genome Replication
Host's protein synthesis machinery used to make virus proteins
Capsid Protein Formation
genome + capsid virus Particles usually assemble spontaneously
Self-Assembly of Offspring
Host cell may burst or lyse, releasing the virus
May "bud-off" host membrane, forming envelopes around the capsids
Exit From Host
Best understood of all viruses Called phages May show two types of virus life cycles:
◦ Lytic Cycle◦ Lysogenic Cycle
Bacterial Viruses
Virus reproductive cycle that kills the host cell
Note - the previously described virus life cycle was a Lytic Cycle
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
Ex: T4 Virus on E. coli
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
Lysogenic Cycle
Belong to several viral classes Specific Interests:
1. Viruses with envelopes2. RNA viruses
Ex. of Infections caused by animal viruses: Cold sores/canker sores, influenza, common cold
Animal Viruses
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
Herpes Viruses
Classes III – VI Ex: yellow fever, rubella, common cold,
SARS, ebola Class VI - Retrovirus - use Reverse
Transcriptase to make DNA from an RNA template.
RNA Viruses
HIV - causes AIDS Retrovirus from chimps Destroys the body’s immune system,
allowing other diseases to kill
Human Immunodeficiency Virus
Measles Polio Smallpox Influenza
Other Viral Diseases
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
Treatments for Viruses
Infectious particles of naked RNA Affect plants Similar to viruses, but lack a capsid Ex:
◦ Coconut Blight◦ Chrysanthemum Wilt
Viroids
Infectious protein particles Affect animals Cause “mad-cow” and Creutzfeldt-Jakob
disease Transmitted in food Slow-acting Virtually indestructible Protein folding problem?
Prions
DNA circle (Chromosome) Plasmids - Small circle of DNA that is
independent of the chromosome Carries a small number of traits
Bacteria Genomes
1. Transformation2. Conjugation3. Plasmids
This is how bacteria mix genetic material into new combos
Bacterial Genetic Recombinations
Alteration of a cell's DNA by the uptake of foreign DNA
Ex: Griffith's experiment
Transformation
Direct transfer of genetic material between two bacterial cells
Bacterial "sex” Used to map genetic sequences in bacteria
Conjugation
Small circular piece of DNA Carry many important traits Ex: Fertility Factor
Antibiotic Resistance (R Plasmids)
Virus movie http://www.youtube.com/watch?v=Rpj0emE
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Plasmids