Biology and Bacteria and Viruses for Nursing School Entrance Exam Study Guide
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Viruses: the simplest of all genetic systems, infectious particles the largest of which can barely be seen with a light microscope Viruses hover between life and nonlife, being either very complex molecules or very simple life forms. They lack the structure and most of the equipment of cells, including ribosomes, and they lack enzymes for metabolism; they are merely aggregates of nucleic acids and proteins—cores of nucleic acid packaged in protein coats called capsids. Some also bear an outer envelope of proteins and lipids. Viruses are parasites of animals, plants, and some bacteria, and can only metabolize and reproduce within a living host cell. The discovery of viruses began with the German scientist Adolf Mayer in 1883; however, most of the research done with viruses has been done in the last twenty years.
Structure: nucleic acid coated with a shell of protein called a capsid, and sometimes a membranous envelope (shell of protein and lipids) coating the capsid. The envelope may help the virus enter the host cell. Whereas other genes are made of double-stranded DNA, genomes of the virus may consist of double-stranded DNA, single-stranded DNA, double-stranded RNA, or single-stranded RNA.
Kinds of virus: DNA virus and RNA virus
Bacteria: unicellular organisms—prokaryotes—with no true nucleus
Bacteria are classified into two groups, gram-positive and gram-negative, based on differences in cell wall composition detected by Gram's staining. Gram-negative bacteria are more dangerous to other life forms than gram-positive bacteria. Bacteria are extremely adaptable with regard to their physiological adjustment to changes in the environment. They are the principal decomposers of most ecosystems. Bacteria were discovered by the Dutch maker of microscopes, Antoni van Leeuwenhoek (1632–1723).
- Structure, Shapes, Metabolism, and Life Cycle of Bacteria
- Shapes and Metabolic Requirements
The bacterial genome is mainly a single double-stranded DNA molecule. Prokaryotes lack membrane-enclosed organelles. (See Section A for more detail.)
Bacteria are initially grouped according to:
- Shape. Bacteria can be placed in three groups: cocci, with a spherical shape; bacilli, with a rodlike shape; and spirilla, with a spiral shape.
- Metabolic requirements. Bacteria are further classified as to, for example, whether they require oxygen. (For more on groupings of bacteria, see Classification of Bacteria.) Bacteria have greater metabolic diversity than all eukaryotes combined. With regard to procurement of energy and carbon, they fall into four categories:
- Photoautotrophs harness light energy for synthesis of organic compounds from carbon dioxide—for example, cyanobacteria (formerly called blue-green algae).
- Photoheterotrophs use light to generate ATP but can get carbon only in organic form.
- Chemoautotrophs obtain energy by oxidizing inorganic substances, although they need only CO2 as source of carbon—for example, Sulfobolus, which oxidizes sulfur.
- Chemoheterotrophs use organic molecules for both energy and carbon—the majority of bacteria are in this category.
Bacteria also vary in the effect oxygen has on metabolism (obligate aerobes, facultative anaerobes, obligate anaerobes), and in nitrogen metabolism.
- Life Cycle
In their life cycle, bacteria do not undergo mitosis or meiosis, although they may undergo genetic recombination by three mechanisms: transformation, conjugation, and transduction. Instead, they reproduce by binary fission, each daughter cell receiving a copy of the single parental chromosome. Bacteria are exceptionally resistant to environmental destruction; some cannot even be killed by boiling water, and endospores may remain dormant for centuries. Unchecked by unfavorable environmental conditions, their growth is geometric. Generation times are usually one to three hours, but some species may double every 20 minutes.
Bacteria used to be classified as plants; however, prokaryotes and plants have a completely different molecular composition. Instead of cellulose, bacterial walls are composed of peptidoglycan, which consists of polymers of modified sugars cross linked by short polypeptides that vary according to species. Classification of bacteria is still in flux. They are usually classified in the Kingdom Monera and are generally divided into two subkingdoms.
Archaebacteria may be descendants of the earliest forms of life. They include methanogens, extreme halophiles, and thermoacidophiles.
Eubacteria (or "true" bacteria) are sometimes said to belong to the order Schizomycetes, although, as noted previously, classification of bacteria is in flux. Eubacteria include, among others, actinomycetes (e.g., Mycobacterium), chemoautotrophic bacteria (e.g., Nitrobacter), cyanobacteria (e.g., Chroococcus), endospore-forming bacteria (e.g., Bacillus), enteric bacteria (e.g., Escherichia), mycoplasmas (e.g., Mycoplasma), myxobacteria (e.g., Myxococcus), nitrogen-fixing aerobic bacteria (e.g., Azotobacter), pseudomonads (e.g., Pseudomonas), rickettsias and chlamydias (e.g., Rickettsia and Chlamydia), and spirochetes (e.g., Borrelia).
- Viral Diseases
- Bacterial Diseases
Not all viruses are disease-causing; many viruses do no apparent harm. Diseases caused by viruses include the common cold, influenza, AIDS, herpes, viral pneumonia, meningitis, hepatitis, polio, and rabies in animals, and tobacco mosaic disease in plants. Types of viruses include adenovirus, arbovirus, herpesvirus, human immunodeficiency virus (HIV, the retrovirus that causes AIDS), myxovirus, papillomavirus, picornavirus, poxvirus, retrovirus, and (in plants) the tobacco mosaic virus. Bacterial viruses are called bacteriophages or simply phages and include, among many others, seven that infect Escherichia coli. Most E. coli are harmless, while some can cause serious food poisoning. E. coli is widely used in laboratory experiments and biotechnology.
Approximately half of all human diseases are caused by bacteria; they may be intruders from outside or opportunistic—that is, they live inside the body of a healthy host, becoming destructive only when the host's defenses are weakened. Pathogenic bacteria can disrupt the physiology of the host by growing inside and invading the tissues. Others exude poisons that are one of two types: exotoxins or endotoxins. (See Mechanisms of Infection/Bacteria.)
Examples of diseases caused by bacteria include pneumonia, caused by the bacterium Streptococcus pneumoniae; tuberculosis, caused by the bacterium Mycobacterium tuberculosis, which destroys parts of the lung tissue and is spread through inhalation and exhalation; syphilis, caused by the bacterium Treponema pallidum; and many others.
Lock-and-key fit is the method by which viruses identify their host. Some viruses can infect several species, for instance, the swine flu virus and the rabies virus; some can infect only a single species, for example, the human cold virus and HIV. Some viruses depend on coinfection by other viruses. The host range is the range of host cells a particular type of virus can infect.
- Lytic cycle: the reproductive cycle of virulent viruses that ends in the death of the host
- Lysogenic cycle: the reproductive cycle of temperate viruses, which coexist with the host rather than killing it
- Vaccines: variants or derivatives of pathogenic microbes that help the cell defend against infection (e.g., polio, rubella, measles, and mumps). There is little that can be done to cure a viral infection once it begins, as antibiotics are powerless; however, many new antiviral agents have been developed in recent years.
One mechanism of infection is growing and invading tissues. Bacteria that use this mechanism include rickettsias that cause Rocky Mountain spotted fever and typhus, and actinomycetes that cause tuberculosis and leprosy. Others produce toxins of two types:
- Exotoxins: proteins secreted by the bacterial cell; examples are Clostridium botulinum, which causes the often fatal disease botulism, and Vibrio cholerae, which causes cholera
- Endotoxins: not secreted by the bacterium, but are merely components of its outer membrane; examples are the various species of Salmonella, which cause food poisoning, and Salmonella typhi, which causes typhoid fever
Many bacteria are harmless or even beneficial; certainly they have had wide-ranging benefits to humankind. From bacteria, we have learned much about metabolism and molecular biology. Methanogens are used for sewage treatment by aerating sewage. Some soil species of pseudomonads are used to decompose pesticides and certain harmful synthetic substances. Bacteria are used to make vitamins, antibiotics, and certain foods—e.g., to convert milk to yogurt and some types of cheese.
Whether destructive or beneficial, bacteria do not act alone but form relationships with other bacterial species and organisms from other kingdoms through symbiosis, which means "living together"—if one symbiont is larger than another, it is known as the host. There are three categories of symbiotic relationships:
- Mutualism: both symbionts benefit
- Commensalism: one symbiont receives benefits while neither harming nor helping the other
- Parasitism: one symbiont benefits but harms the host
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