Viruses (Table 11.1) are not considered to be living organisms because they cannot replicate (copy) themselves without the aid of a host cell. Hosts for viral infections include many organisms other than humans, such as bacteria and plants. Viruses lack the enzymes for metabolism and contain no ribosomes, and therefore cannot make their own proteins. They also lack cytoplasm and membrane-surrounded organelles. Viruses are really nothing more than packets of nucleic acid (DNA or RNA) surrounded by a protein coat.

The genetic material, or genome, of a virus can be DNA or RNA; it can be double stranded or single stranded; it can be linear or circular. For example, the herpes virus has a double-stranded DNA genome, and the polio virus has a single-stranded RNA genome. The genes of a virus can code for the production of all the proteins required to produce more viruses.

Media Activity 11.2 The Revenge of the Chickenpox: The Virus Strikes Back

Figure 11.3 The anthrax threat. In 2002, investigators wore protective clothing to prevent anthrax spores from landing on their skin and a respirator to avoid inhaling spores.

Media Activity 11.3 New Antidote for the Anthrax Bacterium

The protein coat surrounding a virus is called its capsid. Many of the viruses that infect animals develop an additional layer called the viral envelope. The envelope is derived from the cell membrane of the host cell and may contain some additional proteins encoded by the viral genome.

Infection by an enveloped virus occurs when the virus gains access to the cell by fusing its envelope with the host's cell membrane. An unenveloped virus uses its capsid proteins to bind to receptor proteins in the plasma membrane of a host cell. Some capsid proteins function as enzymes that digest holes in the plasma membrane, thereby allowing the viral genome to enter into cells. Once inside the host cell, the capsid is removed.

Whether the virus is enveloped or not, after the genome enters the host cell, the infection continues when the virus makes copies of itself. First the genome is copied, then the virus uses the host-cell ribosomes and amino acids to make viral proteins for building new capsids and synthesizing some of the envelope proteins. Once assembled, the new virus exits the cell, leaving behind some viral proteins in the host's cell membrane, and moves to another cell to spread the infection.

The genome, composed of DNA or RNA, is replicated when the virus uses the host cell's enzymes and nucleotides to produce new nucleic acids. Viruses with DNA genomes use the host cell's DNA polymerase, the DNA-copying enzyme, to replicate their own DNA. As explained in Chapter 9, RNA viruses, such as HIV, copy their genomes with the help of a virally encoded enzyme called reverse transcriptase (Figure 11.4). Recall that transcription is the general process of copying DNA to make RNA—reverse transcriptase transcribes RNA into DNA. When a virus has an RNA molecule as its genome, the RNA must be converted to DNA, because DNA—whether produced by copying a

(a) Duplication of DNA genome

(b) Duplication of RNA genome



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