Replication cycle

Viral populations do not grow through cell division, because they are acellular; instead, they use the machinery and metabolism of a host cell to produce multiple copies of themselves, and they assemble in the cell.

A typical virus replication cycle

Some bacteriophages inject their genomes into bacterial cells

The life cycle of viruses differs greatly between species but there are six basic stages in the life cycle of viruses:

• Attachment is a specific binding between viral capsid proteins and specific receptors on the host cellular surface. This specificity determines the host range of a virus. For example, HIV infects only human T cells, because its surface protein, gp120, can interact with CD4 and receptors on the T cell's surface. This mechanism has evolved to favour those viruses that only infect cells in which they are capable of replication. Attachment to the receptor can induce the viral-envelope protein to undergo changes that results in the fusion of viral and cellular membranes.
• Penetration follows attachment; viruses enter the host cell through receptor mediated endocytosis or membrane fusion. This is often called viral entry. The infection of plant cells is different from that of animal cells. Plants have a rigid cell wall made of cellulose and viruses can only get inside the cells after trauma to the cell wall. Viruses such as tobacco mosaic virus can also move directly in plants, from cell to cell, through pores called plasmodesmata.^[80] Bacteria, like plants, have strong cell walls that a virus must breach to infect the cell. Some viruses have evolved mechanisms that inject their genome into the bacterial cell while the viral capsid remains outside.
• Uncoating is a process in which the viral capsid is degraded by viral enzymes or host enzymes thus releasing the viral genomic nucleic acid.
• Replication involves synthesis of viral messenger RNA (mRNA) for viruses except positive sense RNA viruses (see above), viral protein synthesis and assembly of viral proteins and viral genome replication.
• Following the assembly of the virus particles, post-translational modification of the viral proteins often occurs. In viruses such as HIV, this modification, (sometimes called maturation), occurs after the virus has been released from the host cell.
• Viruses are released from the host cell by lysis—a process that kills the cell by bursting its membrane. Enveloped viruses (e.g., HIV) typically are released from the host cell by budding. During this process the virus acquires its envelope, which is a modified piece of the host's plasma membrane.

The genetic material within viruses, and the method by which the material is replicated, vary between different types of viruses.