VirusesWhat is a virus?Viruses are composed of nucleic acids enclosed in a protein coat and are smaller than the smallest bacterium.• Most biologist consider viruses to be nonliving

because they don’t exhibit all the criteria for life. They don’t carry out respiration, grow, or develop.

• All viruses can do is replicate-make copies of themselves- and they can’t even do that without the help of living things.

Virus

• A cell in which a virus replicates is called the host cell.

• Because they are nonliving, viruses were not named in the same way as organisms. Viruses, such as rabies viruses and polioviruses were named after the diseases they cause.

• A virus that infects a bacterium is called a bacteriophage.

Viral Structure• A virus has an inner core of nucleic acid, either

RNA or DNA, and an outer protein coat called a capsid.

• Some large virus may have and additional layer, called and envelope, surrounding their capsid. Envelope is composed of same materials found in the plasma membrane.

• Viral nucleic acid either DNA or RNA and contains instructions for making copies of the virus.

Viral Structure

• The arrangement of proteins in the capsid of a virus determines the virus’s shape.

• The protein arrangement also plays a role in determining what cell can be infected and how the virus infects the cell.

Attachment of host cell

• Before a virus can replicate, it must enter a host cell.

• Virus do not infect all types of cells. • A virus recognizes and attaches to a host cell

when one of its proteins interlocks with a molecular shape that is the receptor site on the host cell’s plasma membrane.

• In general, viruses are species specific, and some also are cell-type.

• For example, polio viruses normally infect only intestinal and nerve cells.

Viral replication cycles

• Virus have two ways of getting into host cells.1. The virus may inject its nucleic acid into the host

cell like a syringe injects a vaccine into your arm.2. An enveloped virus, the plasma membrane of

the host cell surrounds the virus and produces a virus-filled vacuole inside the host cell’s cytoplasm. Then, the virus burst out of the vacuole and releases its nucleic acid into the cell.

Viral replication cycle

• Once the DNA is inside the cell it becomes part of the cells DNA.

• There are two kinds of cycles. Lytic cycle which kills the cell and the lysogenic cycle which does not kill the cell the cell.

• Lytic cycle-the virus takes over the cell and the cell just makes viruses and the cell burst and release the new viruses.

Viral replication cycle

• Lysogenic cycle- The virus DNA becomes part of the host cells DNA. It does not take over the cell’s genetic material immediately. The host cell still carries out it’s metabolic activity. Provirus is viral DNA that is integrated into the host cell’s chromosome.

Disease symptoms of proviruses

• The lysogenic process explains the reoccurrence of cold sores, which are caused b the herpes simplex I virus remains in your provirus.

• Herpes, hepatitis B, shingles.