To reproduce, viruses must infect cells by introducing their genetic material into them. This process begins when the virus adheres to the cell wall or membrane, binding to certain receptor molecules on the surface of cells. Some viruses attack cells, invading them with the capsid, and others inject only their genetic material. But the fact is that, once inside, the virus starts to control the metabolism of the infected cell, inactivating most of the genes and using the substances existing inside the cell, in order to multiply its own genetic material and manufacture capsids for the newly generated viruses.
In the case of viruses such as bacteriophages, whose genetic material is DNA, reproduction can occur in two ways. In one, the DNA begins to multiply immediately after being injected into the host cell and, at the same time, the synthesis of proteins that will form the capsids of the newly formed viruses begins. Once the bacteria is full of viruses, its cell wall breaks and the viruses are released, which can infect many other bacteria and restart the cycle. Often, however, instead of multiplying as soon as it invades the cell, DNA bacteriophage is incorporated into the bacterium's DNA, being called a provirus. In that case, the bacterial genes are not inactivated and the provirus replicates along with the bacterial DNA. In this way, it is inherited by the infected bacteria's daughter cells.
Also in viruses that have RNA as a genetic material, there may be differences in the viral cycle. In influenza-causing viruses, for example, as soon as they invade the cell, the multiplication of their RNA begins and the synthesis of proteins that will form part of the capsids. Upon leaving the cell infected, influenza viruses do not necessarily cause its death, but carry fragments of the cell membrane that will form a lipoprotein envelope of the capsid. In the so-called retroviruses, in addition to the RNA molecules, the capsid involves some molecules of an enzyme called reverse transcriptase, which, once inside the cell, will act in the manufacture of DNA from viral RNA. Therefore, the opposite of what occurs during the transcription process that occurs in cells.
