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

GShQ

Plasmids