Viruses (like the ones pictured above) are very tiny nonliving particles. Most biologists do not consider viruses to be living because they do not fulfill all the criteria for life. Viruses do not carry out respiration. They also do not grow or reproduce on their own. A virus needs a living cell in order to reproduce. The living cell in which the virus reproduces is called a host cell. 

Let's look at what happens when a virus attacks our cells. Most viruses reproduce through a process called lytic infection. During lytic infection, a virus enters the host cell, makes a copy of itself, and causes the cell to burst, or lyse. In the video Virus Lytic Cycle, a bacteriophage, which is a virus that infects and replicates within a bacterium, attaches itself and infects the host cell. 

Directions: Watch Virus Lytic Cycle to see the entire process of the lytic cycle.

Directions: Study the Lytic Cycle Diagram. It shows the steps of the lytic cycle, starting with a bacteriophage attachment to a host cell. The steps shown in the diagram are also outlined in the Lytic Cyle Overview.

Lytic Cycle Overview:

1. Attachment: Virus attaches to the host cell.
2. Entry: Genetic material is injected into the host cell.
3. Replication: The virus takes over the cell's metabolism, causing the creation of new proteins and nucleic acids by the host cell's organelles.
4. Assembly: Proteins and nucleic acids are assembled into new viruses.
5. Release: Virus enzymes cause the cell to burst and viruses are released from the host cell. These new viruses can infect other cells.

Directions: Below are pictures of a virus in various steps of the lytic cycle, and labels for each step. After looking at the pictures and labels, drag them to the appropriate place in the diagram. The first step—attachment—has been done for you.

Unlike a lytic virus, a lysogenic virus does not cause the host cell to lyse away. A lysogenic virus can remain inactive for a period of time. In lysogenic infection, viral DNA gets integrated with the host cell's DNA, where it is copied along with the host cell's DNA when the host cell replicates. Viral DNA multiplies as the host cell multiplies. Each new daughter cell created is infected with the virus' DNA.

Viral DNA that becomes embedded in a bacterial host cell's DNA is called a prophage. Viral DNA that becomes embedded in an eukaryotic cell's DNA is called a provirus. The prophage/provirus may remain part of the DNA of the host cell for many generations. Influences from the environment, such as radiation, heat, and certain chemicals, trigger the prophage/provirus to become active. It then removes itself from the host cell's DNA and enters the lytic cycle.

Directions: Watch Virus Lysogenic Cycle to see the complete process of the lysogenic cycle. 

Directions: Study the Lysogenic Cycle Diagram. It shows the steps of the lysogenic cycle, starting with bacteriophage attachment to a host cell. The steps shown in the diagram are also outlined in the Lysogenic Cyle Overview.

Lysogenic Cycle Overview: 

1. Attachment: Virus attaches to the host cell.
2. Entry: Genetic material is injected into the host cell.
3. Integration: Viral DNA integrates into the host cell's genome.
4. Replication (lysogenic cycle): When the host cell replicates, viral DNA is copied along with host cell DNA. Each new daughter cell is infected with the virus.
5. Induction: When the infected cells are exposed to certain environmental conditions, viral DNA is activated and enters the lytic cycle.
6. Replication (lytic cycle): The virus takes over the cell's metabolism, causing the creation of new proteins and nucleic acids by the host cell's organelles.
7. Assembly: Proteins and nucleic acids are assembled into new viruses.
8. Release: Virus enzymes cause the cell to burst and viruses are released from the host cell. These new viruses can infect other cells.