Major Parts of a Virus Bacteriophage Capsid whole

Major Parts of a Virus - Bacteriophage Capsid (whole outer coat) Collar

II. Viruses A. What is a virus? Virus – particle made up of nucleic acid, protein, & in some cases lipids that can replicate only by infecting living cells. “Virus” is Latin for poison.

Viral characteristics 1. Viruses are NOT cells. 2. Have No cell structures. 3. Can’t move, feed, or grow outside of another organism’s cells. 4. Reproduce inside living cells. 5. Are very small – bacteria seem big. *A typical virus is made up of a core of DNA or RNA surrounded by a protein coat (capsid).

Major Parts 1. Capsid – protein coat that makes up most of a virus & gives it shape. 2. Nucleic Acid – genetic information in the form of DNA or RNA. 3. Envelope – an additional protective coating usually made of lipids, proteins, & carbs. *Found only in some viruses that infect animal cells.

Host Cell Dependency -The capsid of each virus is specific to the type of host that it invades. -Are classified by the hosts that they invade. Host – a living that another organism lives in. -Hosts shelter & nourish invaders, ex : bacteria, plants, or animals. -Viruses that infect bacteria are called bacteriophages.

Viral Infection Are viruses living or nonliving? -Nonliving- can’t reproduce without a host cell. -Don’t fit into any of the 6 kingdoms. B. Viral Infection *Viruses can begin replication immediately after they enter host cells or they may remain dormant (inactive) for long periods of time. 2 Types of Viral Replication 1. Lytic cycle – (virulent cycle) the viral replication process that rapidly kills a host cell. 2. Lysogenic cycle – (temperate cycle) the viral replication process in which a virus doesn’t immediately kill a host cell.

Steps of the Lytic (Virulent) Cycle 1. Virus attaches itself to the host cell using its tail fibers. 2. The genetic material (DNA or RNA) is injected into the host. 3. Viral DNA/RNA directs cell to make new viruses. 4. Viral pieces are made & assembled, finishing the copies of the virus. 5. Cell lyses (bursts) open and releases new viruses.

Steps of the Lysogenic (temperate) Cycle 1. Attachment to host cell. 2. Injection of viral nucleic acid. 3. Recombination – viral DNA segment (prophage) waits in the host’s chromosomes and gets replicated with the host’s chromosomes. 4. Activation by environmental stimulus. 5. Lysis of cells. Cycle may take years.

C. Retroviruses Retrovirus – a virus containing RNA as its genetic information, ex : HIV – causes AIDS. -Retroviruses work in reverse, “retro” referring to backward. 1. They inject a DNA copy of their RNA into their hosts. 2. Viral & host DNA combine. 3. Viral DNA is dormant until activated.

D. Viruses & Living Cells -Viruses must infect a living cell in order to grow & reproduce. -They must also take advantage of the host’s respiration, nutrition, & other life functions, making them parasites. Parasite – organism that depends entirely upon another living organism for existence, harming that organism in the process.