By themselves, viruses do not have all the characteristics of living organisms and are not considered "alive" by most definitions. Viruses are not cells, but consist of single or double stranded RNA or DNA surrounded by a protein shell called a capsid. The two major shapes of viruses are helical and polyhedral.  Some viruses also have a protein/lipid outer membrane or envelope surrounding the capsid.  Viruses do not grow, maintain homeostasis or metabolize on their own.   

The structure and replication mode of viruses varies widely; however, all viruses can multiply only within a host cell (including bacteria). Phages (viruses that infect bacteria) are the best understood of all viruses and research has led to the discovery that some double stranded DNA viruses are able to reproduce using two alternative processes, the lytic and the lysogenic cycles.  In the lytic cycle, the virus attaches to the host cell, injecting its DNA.  The viral nucleic acid directs the host to produce new viral DNA and phage proteins.  After assembly, new viral offspring particles are released when the host cell disintegrates, or "lyses."  In the lysogenic cycle, viral genomes remain dormant for long periods of time inside the host cell.

The reproductive cycle of viruses that infect bacteria, called bacteriophages, or simply phages for short, are known in great detail. Phages can undergo two alternative life cycles: lytic and lysogenic. Bacteriophages that reproduce through a lytic cycle end up killing their host cell. Lytic bacteriophages attach to the cell wall of a bacterial cell and inject viral DNA into the cell. The phage takes over the cell’s metabolic machinery, degrades the cell’s DNA, and makes copies of its own genome and proteins. Once these components assemble into new phage particles, the viruses lyse, or burst open the cell, resulting in death to the bacterial host. 

The alternate cycle is called a lysogenic cycle (illustrated in the next slide); in this case, the phage reproduces without killing its host. Once the virus genome enters the cell, the circularized DNA will become inserted into a host cell chromosome; phage DNA that has been incorporated into a host chromosome is referred to a prophage and is essentially “hidden” in the host cell’s DNA. The prophage is replicated along with the bacterial host chromosome during cell replication. A single prophage can quickly multiply as each of the daughter cells arising from bacterial cell division will contain a copy of the prophage. On occasion, a switch from a lysogenic to a lytic cycle is triggered. This usually occurs as a result of an environmental factor, such as radiation or certain chemicals. The prophage is released from the host chromosome and returns to an active replication cycle that will lyse the host cell. 

Bacteriophages that reproduce using only the lytic cycle are known as virulent phages, while phages that can undergo both modes of replication are called temperate phages. Bacteriophage T4 an example of a virulent phage; lambda is an example of a temperate phage. The study of temperate phages, and the process whereby the lysogenic or lytic pathway is activated, has led to fundamental discoveries in mechanisms that control the regulation of gene expression. Viruses that infect animal and plant cells differ in many particular aspects from bacteriophage life cycles, but the basic pattern of reproduction is similar. 

The life cycles of viruses that infect bacteria, called bacteriophages, or simply phages for short, are known in great detail. Phages can undergo two alternate life cycles: lytic or lysogenic. Bacteriophages that reproduce through a lytic cycle end up killing their host cell (see previous slide). Lytic bacteriophages attach to the cell wall of a bacterial cell and inject viral DNA into the cell. The phage takes over the cell’s metabolic machinery, degrades the cell’s DNA, and makes copies of its own genome and proteins. Once these components assemble into new phage particles, the viruses lyse, or burst open the cell, resulting in death to the bacterial host. 

The alternate cycle is called a lysogenic cycle. In this case, the phage reproduces without killing its host. Once the virus genome enters the cell, its circularized DNA will become inserted into a host cell chromosome. Phage DNA that has been incorporated into a host chromosome is referred to as a prophage and is essentially “hidden” in the host cell’s DNA. The prophage is replicated along with the bacterial host chromosome during cell replication. A single prophage can quickly multiply as each of the daughter cells arising from bacterial cell division will contain a copy of the prophage. On occasion, a switch from a lysogenic to a lytic cycle is triggered. This usually occurs as a result of an environmental factor, such as radiation or the introduction of certain chemicals. The prophage is released from the host chromosome and returns to an active replication cycle that will lyse the host cell. 

Bacteriophages that reproduce using only the lytic cycle are known as virulent phages, while phages that can undergo both modes of replication are called temperate phages. Bacteriophage T4 is an example of a virulent phage; lambda is an example of a temperate phage. The study of temperate phages, and the process whereby the lysogenic or lytic pathway is activated, has led to fundamental discoveries in mechanisms that control the regulation of gene expression. Viruses that infect animal and plant cells differ in many particular aspects from bacteriophage life cycles, but the basic pattern of reproduction is similar.