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Viral replication and entry

Recall that viruses can only replicate in a host cell (they are obligate parasites). There are several steps involved in viral replication:

  1. Viral entry: the virus must first enter the host cell. Methods of viral entry depend on whether or not the virus is enveloped and the type of host.

    Viruses can only enter plant cells through damaged regions in the cell wall.

    The type of molecules present on the viral envelope determine how it can attach to the host cell's membrane.

  2. Viral replication: once inside a cell, the genome must be replicated. The type of viral genome influences the way it is replicated.

    In general, a DNA virus replicates in the nucleus (if there is one), and an RNA virus replicates in the cytoplasm .

  3. Viral shedding: once the virus has been replicated and the resources within the host cell have been depleted, the virus leaves the cell to find a new host.

The influenza virus replicates through the lytic cycle once inside the host.

  1. Entry: The glycoprotein haemagglutinin(HA) on the viral envelope allows the influenza virus to fuse to the cell membrane of the host and enter the cell.

  2. Replication: Once inside the cell, uncoating removes the RNA viral genome from the capsid and viral envelope, using the cell's own acidic conditions.

    RdRp (RNA-dependent RNA polymerase) replicates the RNA genome. Viral proteins are synthesised by enzymes and machinery present in the host cell.

  3. Exit: Once the genome has been replicated and protein synthesis has been completed, the capsid proteins assemble around the replicated RNA genomes, creating the virus particle.

    The glycoprotein neuraminidase (NA) helps the virus to exit the cell by breaking down part of the cell membrane.

    The virus then leaves by exocytosis, taking some of the host cell's membrane with it to form the viral envelope.

Viral entry methods depend on the type of host cell.

Bacteriophages do not have viral envelopes and normally inject only their genome into the host cells.

Animal viruses are enveloped; the viral envelope allows them to fuse to the cell membrane.

As a result, both the genome and the capsid enter the cell. The viral envelope often remains fused to the cell membrane.

Plant viruses cannot enter the cell by the same method because plant cells have a cell wall.

Plant viruses enter the cell through breaches (holes) in the cell wall. Initial infection tends to be through cell walls that have been damaged.

Once inside the cell, the virus can pass to other cells through plasmodesmata (intercellular cytoplasmic links).