20. The Classification and Evolution of Organisms


20.3. Acellular Infectious Particles


All of the groups discussed so far are cellular forms of life. They all have at least the following features in common. They have (1) cells that have an outer plasma membrane and contain organelles, (2) DNA as genetic material that is involved in reproduction, (3) enzyme-controlled metabolic pathways, (4) an ability to reproduce, and (5) an ability to adapt to their environment physiologically and evolutionarily.

However, some particles show some characteristics of life and cause disease but do not have a cell structure and cannot perform some life functions without the assistance of living cells. Because they lack a cell structure, they are referred to as acellular (a = lacking). Because they enter cells, cause disease, and can be passed from one organism to another, they are often called infectious particles. In causing disease, they make copies of themselves. There is no clear explanation of how these particles came to be. Therefore, they are not included in the classification system for cellular organisms. There are three kinds of acellular infectious particles: viruses, viroids, and prions.



A virus is an acellular infectious particle consisting of a nucleic acid core surrounded by a coat of protein (figure 20.15). Viruses are often called obligate intracellular parasites, which means they are infectious particles that can function only when inside a living cell. Viruses are not considered to be living because they are not capable of living and reproducing by themselves and because they show some characteristics of life only when inside living cells.



FIGURE 20.15. Typical Viruses

Viruses consist of a core of nucleic acid, either DNA or RNA, surrounded by a protein coat. Some have an additional layer, called an envelope, surrounding the protein coat.


Viruses vary in size and shape, which helps in classifying them. Some are rod-shaped, others are spherical, and still others are in the shape of a coil or helix. Most are too small to be visible through a light microscope and require an electron microscope to make them visible. Most viruses are identified by the disease symptoms they cause when they infect cells. It is very likely that all species on Earth serve as hosts to viruses (table 20.2).


TABLE 20.2. Viral Diseases


Type of Virus



Warts in humans


Mumps and measles in humans; distemper in dogs


Respiratory infections in most mammals



Wound-tumor viruses

Diseases in corn and rice


Diseases in potatoes


Infections in many types of bacteria


How Did Viruses Originate?

Soon after viruses were discovered in the late nineteenth century, biologists began to speculate on how they originated. One early hypothesis was that they were descendents of the first precells that did not evolve into cells. This idea was discarded as biologists learned more about the complex relationship between viruses and host cells. The recent discovery of a giant virus—mimivirus—in amoebas has added evidence that in their evolutionary past, viruses were more complex. The mimivirus is as large as some bacteria and has as many genes. It has both DNA and RNA. All other known viruses have only DNA or RNA as genetic material—not both. However, its structure is similar to that of other viruses and it shows a typical virus life cycle.

A second hypothesis was that viruses developed from intracellular parasites that became so specialized that they needed only their nucleic acid to continue their existence. Once inside a cell, this nucleic acid can take over and direct the host cell to provide for all of the virus’s needs.

A third hypothesis is that viruses are runaway genes that have escaped from cells and must return to a host cell to replicate. Regardless of how the viruses came into being, today they are important as parasites in all forms of life.


How Do Viruses Cause Disease?

Viruses are typically host-specific, which means that they usually attack only one kind of cell. The host is a specific kind of cell that provides what the virus needs to function. Viruses can infect only the cells that have the proper receptor sites to which the virus can attach. This site is usually a glycoprotein molecule on the surface of the cell membrane. For example, the virus responsible for measles attaches to the membranes of skin cells, hepatitis viruses attach to liver cells, and mumps viruses attach to cells in the salivary glands. Host cells for the human immunodeficiency virus (HIV) include some types of human brain cells and several types belonging to the immune system.

Once it has attached to the host cell, the virus either enters the cell intact or injects its nucleic acid into the cell. If it enters the cell, the virus loses its protein coat, releasing the nucleic acid. Once released into the cell, the virus’s nucleic acid may remain free in the cytoplasm, or it may link with the host’s genetic material. Some viruses contain as few as 3 genes; others contain as many as 500. A typical eukaryotic cell contains tens of thousands of genes. Most viruses need only a small number of genes, because they rely on the host to perform most of the activities necessary for viral multiplication.

Some viruses have DNA as their genetic material but many have RNA. Many RNA viruses can be replicated directly, but others must have their RNA reverse-transcribed to DNA before they can reproduce. Reverse transcriptase, the enzyme that accomplishes this has become very important in the new field of molecular genetics, because its use allows scientists to make large numbers of copies of a specific molecule of DNA.

Viruses do not divide like true cells; they are replicated. Virus particles are replicated by using a set of genetic instructions from the virus and new building materials and enzymes from the host cell. Viral genes take command of the host’s metabolic pathways and use the host’s available enzymes and ATP to make copies of the virus. When enough new viral nucleic acid and protein coat are produced, complete virus particles are assembled and released from the host (figure 20.16). In many cases, this process results in the death of the host cell. When the virus particles are released, they can infect adjacent cells and the infection spreads. The number of viruses released ranges from 10 to thousands. The virus that causes polio affects nerve cells and releases about 10,000 new virus particles from each human host cell. Some viruses remain in cells and are only occasionally triggered to reproduce, causing symptoms of disease. Herpes viruses, which cause cold sores, genital herpes, and shingles, reside in nerve cells and occasionally become active.



FIGURE 20.16. Viral Invasion of a Bacterial Cell

The viral nucleic acid takes control of the activities of the host cell. Because the virus has no functional organelles of its own, it can become metabolically active only while it is within a host cell. The viral genetic material causes the host cell to make copies of the virus, ultimately resulting in the destruction of the host cell.


Some viruses cause serious disease; others cause mild symptoms. It is also highly likely that there are viruses that require cells for reproduction but go unrecognized because they do not cause the death of cells or cause disease symptoms.


Viroids: Infectious RNA

A viroid is an infectious particle composed solely of a small, single strand of RNA in the form of a loop. To date, no viroids have been found to infect animal cells. The hosts in which they have been found are cultivated crop plants, such as potatoes, tomatoes, and cucumbers. Viroid infections result in stunted or distorted growth and may cause the plant to die. Pollen, seeds, and farm machinery can transmit viroids from one plant to another. Some scientists believe that viroids are parts of normal RNA from plant cells that have gone wrong.


Prions: Infectious Proteins

Prions are thought to be proteins that can be passed from one organism to another and cause disease. All the diseases of this type cause changes in the brain that result in a spongy appearance called spongiform encephalopathies. Because these diseases can be transmitted from one animal to another, they are often called transmissible spongiform encephalopathies. The symptoms typically involve abnormal behavior and eventually death. There are many scientists who are still cautious about accepting prions as a cause of disease. Although it appears that proteins are involved in these diseases, they suggest that environmental factors may be causing changes in the proteins rather than having the proteins passed from organism to organism.


Examples of Prion Diseases

In animals, the most common examples are scrapie in sheep and goats and mad cow disease in cattle. Scrapie got its name because one of the symptoms of the disease is an itching of the skin associated with nerve damage that causes the animals to rub against objects and scrape their hair off.

In humans, there are several similar diseases. Kuru is a disease known to have occurred in the Fore people of the highlands of Papua New Guinea. The disease was apparently spread because the people ate small amounts of the brain tissue of dead relatives. (This ritual was performed as an act of love and respect for their relatives.) When the Fore people were encouraged to discontinue this ritual, the incidence of the disease declined. Creutzfeldt-Jakob disease (CJD) is found throughout the world. Its spread is associated with medical treatment; contaminated surgical instruments and tissue transplants, such as corneal transplants, are the most likely causes of transfer from infected to uninfected persons.

The occurrence of mad cow disease (bovine spongiform encephalopathy—BSE) in Great Britain in the 1980s and 1990s was apparently caused by the spread of prions from sheep to cattle. This occurred because of the practice of processing unusable parts of sheep carcasses into a protein supplement that was fed to cattle. It now appears that the original form of BSE has changed to a variety that is able to infect humans. This new form is called vCJD; scientists believe that BSE and CJD are, in fact, caused by the same prion.

A form of transmissible spongiform encephalopathy called chronic wasting disease is present in elk and deer in parts of the United States and Canada. It is called chronic wasting disease because the animals lose muscle mass and weight as a result of the prion infection. Similar diseases occur in mink, cats, and dogs.


How Prions Cause Disease

How are prions formed and how do they multiply? Prion multiplication appears to result from the disease-causing prion protein coming in contact with a normal body protein and converting it into the disease-causing form, a process called conversion. Because this normal protein is produced as a result of translating a DNA message, scientists looked for the genes that make the protein and have found it in a wide variety of mammals. The normal allele produces a protein that does not cause disease but is able to be changed by the invading prion protein into the prion form. Prions do not reproduce or replicate as do viruses and viroids. A prion protein (pathogen) presses up against a normal (not harmful) body protein and may cause it to change shape to that of the dangerous protein. When this conversion happens to a number of proteins, they stack up and interlock, as do the individual pieces of a Lego toy. When enough link together, they have a damaging effect—they form plaques (patches) of protein on the surface of nerve cells, disrupting the flow of the nerve impulses and eventually causing nerve cell death. Brain tissues taken from animals that have died of such diseases appear to be full of holes, thus the name spongiform encephalopathy.

A person’s susceptibility to acquiring a prion disease, such as CJD, depends on many factors, such as his or her genetic makeup. If a person produces a functional protein with a particular amino acid sequence, the prion may not be able to convert it to its dangerous form. Other people may produce a protein with a slightly different amino acid sequence that can be converted to the prion form. Once formed, these abnormal proteins resist being destroyed by enzymes and most other agents used to control infectious diseases. Therefore, individuals with the disease-causing form of the protein can serve as the source of the infectious prions. There is still much to learn about the function of the prion protein and how the abnormal, infectious protein can cause copies of itself to be made. A better understanding of the alleles that produce proteins that can be transformed by prions will eventually lead to the prevention and effective treatment of these serious diseases in humans and other animals.



11. Why do viruses invade only specific types of cells?

12. Describe how viruses reproduce.

13. Describe how viruses and viroids differ in structure.

14. What is the chemical structure of a prion?

15. How does a prion cause disease?



To facilitate accurate communication, biologists assign a specific name to each species that is cataloged. The various species are cataloged into larger groups on the basis of similar traits.

Taxonomy is the science of classifying and naming organisms. Phylogeny is the science of trying to figure out the evolutionary history of a particular organism. The taxonomic ranking of organisms reflects their evolutionary relationships. Fossil evidence, comparative anatomy, developmental stages, and biochemical evidence are used in taxonomy and phylogeny.

The first organisms thought to have evolved were prokaryotic, single-celled organisms of the domain Bacteria. Current thinking is that the domain Archaea developed from the Bacteria and, ultimately, more complex, eukaryotic, many- celled organisms evolved. These organisms have been classified into the kingdoms Protista, Fungi, Plantae, and Animalia within the domain Eucarya.

There are three kinds of acellular infectious particles: viruses, viroids, and prions. Viruses consist of pieces of genetic material surrounded by a protein coat. Viroids consist of naked pieces of RNA. Prions are proteins. Viruses and viroids can be replicated within the cells they invade. Prions cause already existing proteins within an organism to deform, resulting in disease.


Basic Review

1. In the binomial system for naming organisms, an organism is given two names: the _____ and the specific epithet.

2. The most inclusive group into which an organism can be classified is the

a. phylum.

b. genus.

c. domain.

d. kingdom.

3. Phylogeny is the study that attempts to

a. name organisms.

b. organize organisms into groups based on how they evolved.

c. decide on the names of phylums.

d. classify organisms.

4. Closely related organisms should have very similar reproductive stages. (T/F)

5. Fossils

a. provide information about when organisms lived.

b. are found in sediments that form rock.

c. of soft-bodied animals are rare, compared with those with hard body parts.

d. All of the above are true.

6. The Bacteria and Archaea are prokaryotic. (T/F)

a. cows

b. mushrooms

c. algae

d. Bacteria

7. Which one of the following organisms has not been placed in the kingdom Protista?

a. protozoa

b. algae

c. yeast

d. slime mold

8. Which one of the following kinds of plants does not have seeds?

a. ferns

b. pine trees

c. roses

d. apple trees

9. All viruses have DNA. (T/F)

10. Prions are proteins. (T/F)

11. The kingdom Protista is a valid phylogenetic category. (T/F)

12. All of the following are members of the domain Eucarya EXCEPT

13. The Archaea

a. are eukaryotic.

b. have ribosomes.

c. have a nucleus.

d. All of the above are correct.

14. Plants have a life cycle that shows alternation of _____.

15. All animals and fungi are heterotrophs. (T/F)



1.genus 2.c 3.b  4.T  5.d  6.T  7.c   8.a 9.F 10.T 11.F 12.d 13.b 14. generations 15.T


Thinking Critically

Examine Life

A minimum estimate of the number of species of insects in the world is 750,000. Perhaps, then, it would not surprise you to see a fly with eyes on stalks as long as its wings, a dragonfly with a wingspread greater than 1 meter, an insect that can revive after being frozen at -35°C, and a wasp that can push its long, hairlike, egg-laying tool directly into a tree. Only the dragonfly is not presently living, but it once was. What other curious features of this fascinating group can you discover? Have you looked at a common beetle under magnification? It will hold still if you chill it.