Viruses

Is a Virus Alive?

• Viruses are not considered living because they are missing key characteristics of living organisms.

Viral Structure

• All viruses have nucleic acid and a capsid.

Nucleic Acids

-The genetic material of a virus can be either RNA or DNA.

RNA versus DNA

• DNA viruses– The genetic material can become inserted into

the host cell’s DNA or may remain separate– The virus makes copies of its DNA by using the

host cell’s enzymes and nucleotides.

• RNA viruses– When the genetic material of an RNA virus

enters a host cell, reproduction can take place by 2 methods.

Capsid, Nuclear Envelope, and Bacteriophages

• Capsid- protein coat that encloses the genetic material.

• Envelope- a membrane surrounding the capsid.

• Bacteriophages- viruses that infect bacteria and have complicated structures.

Reproduction

• Viruses can reproduce by a lytic life cycle and a lysogenic life cycle.

Lytic Cycle

• The cycle of viral infection, reproduction, and cell destruction.

• Viruses that reproduce only by the lytic cycle are often called virulent.

Lysogenic Cycle

• When viral DNA becomes part of its host cell’s DNA, the virus is called a prophage.

• When the host cell replicates its own DNA the cell also replicates the provirus. New cells produced, then, also contain the provirus. In this process, the host cell is not destroyed. = lysogenic cycle.

• A virus whose reproduction includes the lysogenic cycle is called a temperate virus.

Viroids and Prions

• Viroids and prions are molecules that are able to reproduce and cause disease.– Viroids- a single strand of RNA that has no

capsid.– Prions- are misshapen versions of proteins that

are found in the brain.

Roles of Bacteria and Viruses

• Bacteria play an important role in the environment and in industry. Both bacteria and viruses are important in research.

Bacteria and the Environment

• Bacteria play a vital role in all Earth’s ecosystems.

Bacteria and Industry

• Bacteria are important in a variety of industries.– Example:

• Many of the foods we eat, such as pickles, soy sauce, and sourdough bread, are made using bacteria.

Bacteria, Viruses and Research

• Bacteria and viruses have been very important in genetic research.

Koch’s Postulates and Disease Transmission

• The 4-step procedure for identifying a disease-causing agent. (= a pathogen)– 1 = the pathogen must be found in an animal with the

disease and not in a healthy animal.– 2 = the pathogen must be isolated from the sick

animal and grown in the laboratory.– 3 = When the isolated pathogen is injected into a

healthy animal, the animal must develop the disease.– 4 = the pathogen should be taken from the second

animal, grown in the lab, and shown to be the same as the original pathogen.

Bacterial Diseases

• Bacteria can cause disease by producing toxins and destroying body tissues.

• Toxins- poisonous chemicals.

Antibiotic Resistance

• Antibiotics – chemicals that inhibit the growth of or kill micro-organisms.

• Development of Resistance– Antibiotic resistance is the ability of bacteria

to tolerate antibiotics.– Antibiotic resistance spreads when sensitive

populations of bacteria are killed by antibiotics. As a result, resistant bacteria thrive.

Consequences of Resistance

• Diseases that were once easy to treat with antibiotics, such as staphylococcal infections, are now more difficult to treat because of resistance to multiple antibiotics.

Viral Diseases

• Because viruses enter host cells to reproduce, it is difficult to develop a drug that kills the virus without harming the living host.

Emerging Diseases

• Emerging diseases are infectious diseases that are newly recognized, that have spread to new areas or a new host, or that have reemerged when a disease that was once considered under control begins to spread.