Cell And Release Of Viruses

FIGURE 17.03 Virus Life Cycle

The life cycle of a virus starts when the viral DNA or RNA enters the host cell. Once inside, the virus uses the host cell to manufacture more copies of the virus genome and to make the protein coats for assembly of virus particles. Once multiple copies of the virus have been assembled, the host cell is burst open to allow the viral progeny to escape and find new host cells to infect.

Before entering the host cell, a virus must bind to a receptor on the cell surface.

spikes or prongs sticking out from the surface. Most bacterial and plant viruses abandon their protein coat when they infect a new host cell. Only the genetic material (DNA or RNA) enters the cell. Animal viruses vary in regard to when exactly they disassemble their protein coat.

Many animal viruses have an extra envelope outside the protein shell. This is made of membrane stolen from the previous host cell into which virus proteins have been

Nucleocapsid enters

FIGURE 17.04 Enveloped Viruses Merge with the Animal Cell Membrane

Nucleocapsid enters

FIGURE 17.04 Enveloped Viruses Merge with the Animal Cell Membrane

When the virus particle left the previous host cell, it surrounded itself with a layer of the host cell membrane. This outer layer contains viral recognition proteins previously inserted into the host cell membrane during virus infection. The recognition proteins bind to the cell membrane receptors of another animal cell. The protein complex triggers the animal cell to take in the particle by fusing the two membranes. The nucleocapsid structure enters the animal cell.

Viruses do not divide. Instead their genes code for components which are assembled into new virus particles.

inserted. These virus-encoded proteins detect and bind to receptors on the next target cell. When an enveloped virus enters a new animal cell, its envelope layer merges with the cell membrane and the inner protein shell containing the nucleic acid (the "nucleocapsid") enters (Fig. 17.04). Once inside, the protein shell disassembles, exposing the genome.

Once inside the host cell, the virus genome has two major functions. First, it must replicate to produce more virus genomes. Second, it must subvert the cell to manufacture lots of virus proteins for the assembly of new virus particles. Note that viruses do not divide like cells. They are assembled from components manufactured by the host cell using genetic information in the virus genome (Fig. 17.05).

The genes of viruses are often divided into "early genes" and "late genes". The early genes have promoters that resemble those of the host cell and encode for proteins responsible for replicating the virus genome. Consequently, they are transcribed by host cell RNA polymerase and are expressed immediately after infection. In very small viruses, host enzymes are largely responsible for replicating the virus genome so there may be very few "early genes" involved in replication. Conversely, in viruses that have large numbers of genes, such as bacteriophage T4 or the poxviruses of animals, regulation is obviously more complex and there may be several sub-categories of genes such as "immediate early", "delayed early" etc.

The late genes have promoters that are not recognized by host polymerase alone. These genes are expressed late in infection and encode the structural proteins of the virus particle together with proteins involved in the assembly and packaging processes and in lysing the host cell. Some viruses, such as bacteriophage T7, encode their own RNA polymerase in order to express late genes, others, such as bacteriophage T4, modify the host RNA polymerase. For example, T4 gene 55 encodes an alternative sigma factor that recognizes the promoters of T4 late genes.

early genes Genes expressed early during virus infection and that mainly encode enzymes involved in virus DNA (or RNA) replication late genes Genes expressed later in virus infection and that mainly encode enzymes involved in virus particle assembly nucleocapsid Inner protein shell of a virus particle that contains the nucleic acid


Replication mRNA

Translation mRNA




Virus particles

Virus particles

Virus proteins

FIGURE 17.05 Synthesis and Assembly of Virus Components

The viral genome (in pink) directs the host cell to replicate many copies of the virus genome. The viral genome is also transcribed and the mRNA is translated, giving viral proteins. The viral genome carries all the genes needed for making the protein coat. Finally, the coat proteins and the viral genomes are assembled to give new virus particles.

Bacterial viruses leave their protein shell behind and only their genome enters the host cell.

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