Lecture 4 - Viruses and Prions

7/27/2019 Lecture 4 - Viruses and Prions

1/15

An Introduction to

Viruses

Dr. Linroy Christian

7/27/2019 Lecture 4 - Viruses and Prions

2/15

Viruses Non-cellular genetic element that utilizes cells for its own

replication

The virus particle consists of nucleic acid (geneticmaterial) surrounded by a protein coat (capsid) Nucleic acid + capsid = nucleocapsid

The mature virus is also called a virion

Some viruses may have a lipid outer membrane(envelope) acquired from the host cell

The nucleic acid can be either DNA or RNA

Outside the cell the virus particle is metabolicallyinactive and does not carry out any metabolic functions

7/27/2019 Lecture 4 - Viruses and Prions

3/15

Viruses

Once inside the host, virus reproduction occurs The virus genetic material is reproduced and the

components that make up the virus coat are made

When a virus genome is introduced into the hostcell and is reproduced the process is calledinfection

The cell that the virus can infect is called a host

The viruses rely heavily on the host machineryfor replication

7/27/2019 Lecture 4 - Viruses and Prions

4/15

Virus Reproduction

(1) Attachment Virus recognizes host cell receptor Attaches to the host cell

(2) Penetration

Injects genetic material into the host cell

(3) Early Steps in Replication Host cell machinery is altered in preparation of virus nucleic

acid synthesis

Virus-specific enzymes may be made

Some RNA viruses (retroviruses) utilize ReverseTranscriptase to make DNA from RNA

7/27/2019 Lecture 4 - Viruses and Prions

5/15

Virus Reproduction

(4) Replication The virus nucleic acid is reproduced

(5) Synthesis of Proteins

Virus protein coat is made

(6) Assembly Virus protein coat is assembled and virus is packaged

(7) Release Mature virus is released from the cell

7/27/2019 Lecture 4 - Viruses and Prions

6/15

Virus Reproduction

7/27/2019 Lecture 4 - Viruses and Prions

7/15

Ve

7/27/2019 Lecture 4 - Viruses and Prions

8/15

Lytic vs. Lysogenic

7/27/2019 Lecture 4 - Viruses and Prions

9/15

Classes of Virus

Animal

Plant

Bacterial (Bacteriophages)

7/27/2019 Lecture 4 - Viruses and Prions

10/15

Virus Structure

Icosahedral Symmetrical structure

Roughly spherical is shape with 20 faces

Herpes virus, poliovirus

Helical

Protein and nucleic acid interact to form a coiled, ribbon-like

structure

Influenza virus, rabies virus, measles virus

Complex

Composed of separate parts with separate shapes andsymmetries

Poxvirus, bacteriophages

Viruses may or may not have an outer membrane

(envelope) derived from the host cell

7/27/2019 Lecture 4 - Viruses and Prions

11/15

Virus Structure

7/27/2019 Lecture 4 - Viruses and Prions

12/15

Viral Structure

7/27/2019 Lecture 4 - Viruses and Prions

13/15

Virus Classification

Based on the structure and composition of

the virus particle as well as the mode of

replication

DNA viruses (Herpesviridae)

RNA viruses (Retroviridae, Orthomyxoviridae)

Double-stranded RNA viruses (Reoviridae)

7/27/2019 Lecture 4 - Viruses and Prions

14/15

Prions An infectious particle comprised only of proteins

All known prion diseases affect the structure ofthe brain or neural tissue and are untreatableand fatal

Cause abnormal folding of normal prion proteins

in the brain Diseases do not produce an inflamatory

response

Typical prion diseases

Scrapie sheep Bovine Spongiform Encephalopathy (BSE) MadCow disease

Creutzfeldt-Jakob disease (CJD) humans

Kuru - humans

7/27/2019 Lecture 4 - Viruses and Prions

15/15

Viroids

Comprised only of RNA

Single-stranded, circular

RNA is not packaged

Associated with plant diseases

Not known to be associated with human

disease