Viruses

What is a virus?

• A virus is classified as a non-cellular particle made up of protein-covered genetic material that can invade living cells

• Viruses are not considered to be alive because they do not respire or metabolize energy

• Viruses cannot reproduce unless they are in a living cell

Sizes of viruses

How a virus reproduces

• Viruses grow and develop in the cells of specific hosts and depend on these cells for respiration, nutrition, and all other functions of life to enable them to reproduce.

• When viruses enter the host’s cells, they “take over” by altering the host cells’ genetic make up.

Make-up of a virus

• Viruses have two components:

genetic material and

a protein covering called a capsid.

• A virus’s genetic material can either be DNA or RNA, depending on the type of virus.

The Lytic Cycle

The Lytic Cycle

The Lytic Cycle

The Lytic Cycle

The Lytic Cycle

Bacteriaphage

Lysogenic Cycle

Lysogenic cycle

Lysogenic cycle

Lysogenic cycle

Lysogenic cycle

Lysogenic cycle

RNA Viruses

• RNA viruses contain RNA, which can act as mRNA once in the cell.

• The mRNA can manufacture viral proteins and so bypass the host DNA.

• All RNA viruses are lytic since there is no way for the viral RNA to become part of the host’s DNA structure.

Retroviruses

• Retroviruses also contain RNA as their genetic material.

• When a virus infects a cell, it produces a copy of viral DNA from the viral RNA code.

• The new DNA becomes part of the hereditary apparatus of the infected human cell.

• The host cell does not burst, but it changes permanently in shape, metabolism, and growth.

• Because the host cell does not burst, the cycle of infection is lysogenic.

Viruses and Disease

• Most viruses cause some kind of disease in their hosts.

• Some common animal diseases caused by viruses are chickenpox/shingles, hepatitis (A, B, and C), measles, warts, distemper, mumps, yellow fever, infectious mononucleosis, equine encephalitis, and rabies.

Plant Viruses

• Most plant viruses tend to be RNA viruses.• Plant viruses can stunt plant growth and cause

low crop yields.• Viruses can be spread from plant to plant by

insects carrying the virus, or by farmers and gardeners using tools that are infected with the virus.

• Some examples of plant viral diseases are strawberry ringspot, bean leaf roll, tobacco mosaic, potato mottle, apple chlorotic,

Homework

1. Explain why it is difficult to classify viruses.

2. Draw diagrams to compare the lytic and lysogenic cycles of a bacteriophage.

3. Compare the life cycle of a eubacterium with the reproductive cycle of a virus.

4. Make a chart to compare how RNA viruses are similar to and different from DNA viruses.