Viral Replication Viruses Part II Viral Replication It

Viral Replication Viruses Part II

Viral Replication • It was not until the 1950 s 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 T 4 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, m. RNA, 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, 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