Although some viruses are not harmful, other viruses are known to
Infect and harm all types of living organisms. A virus is a nonliving
Strand of genetic material within a protein coat. Most biologists don’t
Consider viruses to be living because they do not exhibit all of the characteristics
Of life. Viruses have no organelles to take in nutrients or use
Energy, they cannot make proteins, they cannot move, and they cannot
Replicate on their own. In humans, some diseases, such as those listed
In Table 18.2, are caused by viruses. Just as there are some bacteria
That cause sexually transmitted disease, some viruses can cause sexually
Transmitted diseases – such as genital herpes and AIDS. These
Viruses can be spread through sexual contact. Diseases caused by these
Viruses have no cure or vaccine to prevent them.
Virus size Viruses are some of the smallest disease-causing structures
That are known. They are so small that powerful electron microscopes
Are needed to study them. Most viruses range in size from 5 to
300 nanometers (a nanometer is one billionth of a meter). It would take
About 10,000 cold viruses to span the period at the end of this sentence.
Virus origin Although the origin of viruses is not known, scientists
Have several theories about how viruses evolved. One theory, now considered
To be most likely, is that viruses came from parts of cells. Scientists
Have found that the genetic material of viruses is similar to cellular genes.
These genes somehow developed the ability to exist outside of the cell.
Virus structure Figure 18.11 shows the structures of adenovirus,
Influenza virus, bacteriophage, and tobacco mosaic virus. Adenovirus
Infection causes the common cold, and influenza virus is responsible
For causing the flu. A virus that infects bacteria is called a bacteriophage
(bak TIHR ee
uh fayj). Tobacco mosaic virus causes disease in
Tobacco leaves. The outer layer of all viruses is made of proteins and is
Called a capsid. Inside the capsid is the genetic material, which could
Be DNA or RNA, never both. Viruses generally are classified by the
Type of nucleic acid they contain.
The virus that causes smallpox is a DNA virus.
Outbreaks of smallpox have occurred in the human population for
Thousands of years. A successful program of worldwide vaccination
Eliminated the disease and routine vaccination was stopped. For a
Closer look at the history of the discovery of the virus that causes
Smallpox and smallpox vaccination, examine Figure 18.12.
In order to replicate, a virus must enter a host cell. The virus attaches
To the host cell using specific receptors on the plasma membrane of the
Host. Different types of organisms have receptors for different types of
Viruses, which explains why many viruses cannot be transmitted
Between different species.
Once the virus successfully attaches to a host cell, the genetic material
Of the virus enters the cytoplasm of the host. In some cases, the entire
Virus enters the cell and the capsid is broken down quickly, exposing the
Genetic material. The virus now uses the host cell to replicate by either
The lytic cycle or the lysogenic cycle.
Lytic cycle In the lytic cycle, illustrated in Figure 18.13, the host cell
Makes many copies of the viral RNA or DNA. The viral genes instruct
The host cell to make more viral protein capsids and enzymes needed for
Viral replication. The protein coat forms around the nucleic acid of new
Viruses. These new viruses leave the cell by exocytosis or by causing the