Viruses: Key Ideas

• Why is a virus not considered a living organism?

• What two structures are characteristic of viruses?

• What are two ways that a virus can reproduce?

Is a Virus Alive?

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

• Viruses do have genetic material, but they cannot reproduce on their own.

• Viruses reproduce by infecting cells. Viruses use the cell’s ribosomes, ATP, enzymes, and other molecules to make more viruses.

Is a Virus Alive?, continued

• Viruses do not grow. Instead, they are assembled into their full size within a cell.

• Viruses do not carry out any metabolic activities, do not have any cytoplasm or organelles, and do not maintain homeostasis.

Viral Structure• The structure of a virus is relatively simple with two structures

that are characteristic of all viruses.

• All viruses have nucleic acid (either DNA or RNA) and a capsid. A capsid is a protein covering that surrounds the nucleic acid core in a virus.

• In addition to a capsid, viruses may have an envelope. An envelope is a membranelike layer (made from the membrane of the host cell) that covers the capsid of some viruses.

• Viruses that infect bacteria also have tail fibers.

Types of Viruses

Plant Viruses They are similar to animal viruses in most basic

characteristics but they can also be markedly different. Most plant viruses have RNA as the genetic material.

Tobacco Tobacco Mosaic Mosaic VirusVirus

Enteroviruses

cause many common illnesses such as polio, meningitis, encephalitis, common cold, and tonsillitis,

RotavirusViral gastroenteritis is the second most common

clinical disease in developed countries

Bacteriophages viruses that attack bacteria

Virus Reproduction

• A viral infection begins when the genetic material of a virus enters a host cell.

• Once inside the cell, a virus can reproduce by two different processes.

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

The Lytic and Lysogenic Life Cycle

Lytic cycle• Virus attaches to receptors on host cell and

injects its genetic material into the cell.

• The virus destroys the host DNA and uses the cell’s materials to create new viruses (100 – 200)

• The host cell splits open and the viruses are released.

• These viruses will infect new cells.

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

Lysogenic cycle• The virus injects its genetic material into the host cell. The

viral DNA is incorporated into the DNA of the cell (called prophage).

• Whenever the cell divides, the viral DNA is copied along with the cell DNA. This may continue for months or years.

• At some point, a trigger (such as stress or weakened immune system) may cause the virus to leave the lysogenic cycle and enter the lytic cycle, destroying the host cell.

• A virus that reproduces in this way is called a temperate virus.

Task

• 1) Sketch and label a typical virus (p 476 or 477 – your choice). Label DNA/RNA, capsid, envelope (if present), glycoprotein (if present, tail fibers (if present)

• 2) Sketch lytic and lysogenic reproductive cycles (p 478). Label lytic cycle (on left) and lysogenic cycle (on right). Describe each stage.