An enveloped virus contains all of the structures that a non-enveloped virus has including a capsid made of capsomere and the nucleic acid (either DNA or RNA). However this virus also has an envelope which was obtained when the virus left the previous host. This envelope has the same structure as the plasma membrane of future hosts; therefore it can be used to enter new hosts through the process known as fusion. There can also be viral structures known as spikes found in this envelope structure. These will be used to attach to receptors found on host cells.
The first step in the infection cycle of a virus is attachment to the host cell. This is accomplished when the virus binds to specific structures known as receptors. If there were free receptors (those not attached to host cells) and virus bound to them, it would inhibit the infection. Though this is a way to try and prevent infection, most of these free receptor molecules would probably be cleared from the cell.
Viruses use many parts of the host cell’s machinery during the course of an infection:
Viruses bind to receptors that have a different function for the host cell e.g. rhinovirus attaches to ICAM-1, which the cell usually uses in inflammatory responses.
Some non-enveloped viruses use the host cell’s receptor-mediated endocytosis machinery to enter the host cell.
Once inside the cell and uncoated, viruses use cellular transport mechanisms (cytoskeletal structures such as microtubules) to move through the host cell cytoplasm. The association with the cell machinery can be direct or through membrane-coated vesicles. Some viruses use cell molecules called chaperones to move though the cell.
The entry of retroviral DNA (made from retroviral RNA by reverse transcriptase) into the host cell nucleus is correlated with the breakdown of the nuclear membrane at mitosis.
Replication of DNA virus genomes is carried out by the host cell DNA polymerase – the virus inhibits synthesis of host cell DNA and forms specialized sites in the host cell that are essentially viral replication factories.
Transcription of viral DNA is performed by host cell RNA polymerases and it is regulated by host cell proteins.
Both DNA and RNA viruses are totally dependent on host cell machinery for translation of mRNA into protein – proteins are made by cellular ribosomes and then folded and sorted by the endoplasmic reticulum and Golgi apparatus. Viruses can modify the translation apparatus so viral RNA is preferentially translated.
Newly synthesized viral components are transported round the cell by cellular transport mechanisms (microtubules).
Enveloped viruses can leave the host cell by using cellular exocytosis mechanisms.