{ Viral Replication

 Virus: A biological particle composed of nucleic acid and protein  Intracellular Parasites: organism that must “live” inside a host

 All Have:  1) Capsid: coat of protein that surrounds DNA or RNA  2) Nucleic Acid: RNA or DNA  Some Have:  Tail Fibers: Used for attachment (not legs)  Shapes vary

 Cannot reproduce w/o a host cell  Virulent: cause disease  Bacteriophages (infect bacteria) are often studied  Replication is similar with many animal viruses

How do viruses get into their host cells?  Viruses cannot enter any cell they wish. There are certain components that MUST be present on the viral capsid/envelope AND on the potential host cell  The virus will have specific shapes for their glycoprotein spikes that MUST match the shapes of receptors on the host cell  NO MATCH=NO ENTRY=NO INFECTION Mode of Entry

Glycoproteins and Receptors MUST Match!!!

 There are 2 ways that viruses reproduce inside a host cell. The following steps are common to both cycles. Basic Steps in Viral Replication

 This step is one of the most critical steps in the viral replication process.  Remember, if the virus cannot attach itself to the host cell… NO MATCH=NO ENTRY=NO INFECTION  The receptors that are located on the outside of the host cell may be made of protein, carbohydrates, or lipids. Attachment

 Penetration of the virus varies depending on the type of virus  Enveloped:  Entry may be gained by fusing the viral envelope with the cells plasma membrane. This allows for the viral genome (capsid) to be taken directly to the host cell’s cytoplasm. Penetration

 Enveloped  Some viruses will enter the host cell via endosomes. One the virus docks to the cell receptors, the cell membrane will engulf the virus and take it into the cytoplasm. From there, the viral genome (capsid) is released into the cytoplasm and heads for the nucleus. Penetration

 Non-Enveloped  The non-enveloped viruses may either cross the plasma membrane on their own, or become engulfed by endocytosis. The capsid remains intact. Penetration

T-4 Bacteriophage penetrating a bacterium T-4 Bacteriophage penetrating a bacterium Source: Penetration

 The capsid surrounding the nucleic acid material is broken down, releasing the DNA or RNA to be replicated into the host cell Uncoating

 As the viral genome takes over the host cells machinery, new DNA or RNA is transcribed and translated much like the original cells DNA would have been. The host cell spends all of its energy and resources creating identical genetic material to create many viruses Synthesis of viral nucleic acid and protein

 As the virus takes over the production line of the host cells genome factory, it also plays a role in hijacking the production of new materials for the cell. The host cell now spends all of its time assembling new viruses to be sent out of the cell. Assembly and Maturation

 Viruses may be released due to cell lysis, or if enveloped, may bud from the cell. Budding viruses do not necessarily kill the cell  **Not all viral particles released will become infectious Release of Virus

 Because the most understood virus is the bacteriophage, most examples will reference this virus. This does not mean, however, that the bacteriophage is the only virus to replicate in this manner. Lysogenic and Lytic Cycles

 In the Lysogenic Cycle, the viruses genetic material becomes imbedded into the host cell’s genome. As the host cell goes about it’s normal function and divides, the viral genome is passed to each daughter cell. At this stage, the viral particles are inert, and no infection will occur  The Lysogenic bacterium will continue to replicate normally until it becomes stressed by an outside factor that causes the virus to enter the Lytic Cycle Lysogenic Cycle

 The Lysogenic Cycle The Lysogenic Cycle The Lysogenic Cycle Lysogenic Cycle Source: xKitsd0&playnext=1&list=PLB1AC4F7A9C99C847

 Once the virus has included its genetic material into the cytoplasm of the host cell, it can hijack the host cells resources to create MANY more viruses Lytic Cycle

 The viral genome creates proteins that destroy the host cells DNA allowing the virus to take over the control center of the host cell. The host cell now spends all of it’s energy and time creating new viral particles.  After the viruses are assembled, enzymes are released to weaken the cell wall or membrane. The host cell will eventually burst, or lyse, allowing the newly created viruses to spread to surrounding cells.  The infection process continues Lytic Cycle

 **Note: Viruses may also “bud” from the host cell. This mechanism allows the viral particles to spread to new cells, but does not destroy the host cell (example: influenza virus) Lytic Cycle

 Lytic Cycle Lytic Cycle Lytic Cycle  Source: Lytic Cycle

 Thanks to:  Pathmicro.med.sc.edu  Pediatrics.med.unc.edu  Dipartimentobiologia.it  Nature.com  Jcb.rupress.org  Microvirology.blogspot.com  Newworldencyclopedia.org  Cellsalive.com