Viral ReplicationViruses Part II

Viral Replication

• It was not until the 1950s that scientists were able to isolate and identify how viruses multiplied in a host

• Much of the early research was performed using a bacteriophage (or a virus that infects only a bacteria)• Specifically, scientists studied T4

bacteriophage that infected the E. coli bacteria

Lytic Cycle

• Virulence – the ability of a pathogen to affect cells

• Some viruses are nonvirulent to specific types of cells• Ex: human could consume the leaves of a tobacco

plant with TMV and not become infected

• Scientists call the activity of a virulent virus the lytic cycle

Lytic Cycle During the lytic cycle:

• the virus invades the cell,

• uses the resources of the host cell to produce multiple copies of the viral nucleic acid,

• destroys the host cell

• Releases new virions (complete virus particle) into the environment

Lytic cycle Steps

• Step 1 Attachment• The glycoprotein (“spikes”) on the outside

of a virus capsid adhere to the cell wall/cell membrane of its intended host

Lytic cycle – Step 2

• Step 2 – Entry • Once attached to the exterior of the host cell,

enzymes begin to break it down and the core (DNA/RNA) is injected into the host cell

Lytic cycle

• Step 3 – Replication and Transcription• Using the cell’s enzymes and organelles, the virus

begins to produce multiple copies of the viral nucleic acid, mRNA, enzymes, and proteins

Lytic cycle

• Step 4 – Assembly• The virus proteins form capsids around the

replicated viral DNA/RNA, forming new virions

Lytic Cycle• Step 5 – Release

• Eventually, the resources of the host cell are used up, the organelles being used wear out, and the enzymes coded by the virus cause the cell to lysis or burst open

• This releases the new virions into the environment and potentially infecting new host cells

Lytic cycle

• Certain bacteriophages, human polio, influenza are relatively virulent → meaning they can infect one cell and complete the lytic cycle within 30 minutes

Lysogenic cycle

• Not all viruses destroy the host cell immediately after the entry phase…

• During the lysogenic cycle, a virus that enters a cell may remain inactive (or latent) for long periods of time

• While in the latent stage, the virus has inserted its genome into the host’s DNA and it is replicated each time the cell undergoes cell division

• Essentially, the lysogenic viruses reproduce each time the cell divides so that all new cells have the virus in them

Lysogenic Viruses A lysogenic virus may lay dormant

until stimuli (like UV radiation, temperature change, or other agents) cause it to become virulent and undergo the lytic cycle

Other Kinds of Viruses Persistent infections – the host cell

does not go through the lytic cycle but slowly releases virus particles

Transforming viruses – significantly alter the cell’s metabolism by adding new genetic info but does not destroy the cell