Replication and Reproduction



Virus Life Cycles

Viruses can only reproduce within a host cell. Without the metabolic enzymes and ribosomes in the host cell, a virus is essentially inert. The two cycles of phage reproduction are the lytic and lysogenic cycles.

Lytic Cycle

Once a virus enters the host cell, it releases its DNA and capsid proteins. Enzymes within the host replicate the viral genome. While this is occurring, other enzymes in the host cell transcribe the viral genome into viral mRNA, which is used to make more capsid proteins. After the viral genomes and capsid proteins are produced, they assemble into new virus particles, which leave the cell in the hopes of infecting other host cells. The host cell dies in this process, so viruses must find new host cells to use for reproduction. CMV is a lytic virus and reproduces in this way.

Lysogenic Cycle

The lysogenic cycle facilitates phage genome replication without killing the host cell in the process. A phage will first inject its DNA genome into the host cell, which forms into a circle. The phage DNA then integrates into the bacterial host’s chromosome, which is now called a prophage. The host reproduces as usual, but it copies the prophage and sends it to daughter cells. The host continues to reproduce with the prophage in its chromosome and transmit the prophage to the cells it produces.