Two Cycles and A Bit of Review Remember that viruses are not able to reproduce on their own. They rely on a ‘host cell’ for reproduction In the Lytic Cycle viruses infect the host cell, use it to replicate and then destroy it In the Lysogenic Cycle the virus infects the host cell and inserts its DNA into the DNA of the host cell.

Life Cycles of Viruses

Entry into the host cell… Most bacterial viruses (bacteriophages) inject their nucleic acid into host Eucaryotic viruses usually enter the cytoplasm with the genome still enclosed.

Non-enveloped viruses Binding takes place between viral capsid and receptor. Enveloped viruses Binding takes place between viral envelope proteins and receptor.

6 Stages of Lytic Infection For animal viruses, there are six steps in lytic infection: Attachment Penetration Uncoating Biosynthesis Maturation Release

The Lytic Cycle 1.Proteins on the surface of the virus bind with proteins on the surface of the host cell  this is specificity 2.The Virus injects its DNA into the host cell 3.The DNA from the virus shuts down and takes over the host cell 4.The virus uses the materials of the host cell to make thousands of copies of its own protein coat and DNA

The Lytic Cycle 5. The cell lyses (bursts) and the releases hundreds of virus particles This process is called a lytic infection

The Lysogenic Cycle 1.Proteins on the surface of the virus bind with proteins on the surface of the host cell  this is specificity 2.The Virus injects its DNA into the host cell 3.The viral DNA is inserted into the host cell’s DNA 4.The viral DNA is now known as a prophage

5.The prophage may remain as part of the host cell for many generations 6.The inserted DNA may be useful. It can block other viruses from entering the cell or create useful proteins 7.Eventually the viral DNA removes itself from the DNA of the host cell and starts reproducing viruses

RETROVIRUSES Retroviruses contain RNA They produce a DNA copy of their genes when they enter the cell This DNA is entered into the host cell’s DNA The DNA is then replicated and passed on to daughter cells each time the host cell divides