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General Biology Lecture; Regis University; B. Rife; March 1992
Viruses and Monera
I Viruses
II. Kingdom Monera
III. Classification of Bacteria
I Viruses
Viruses are on the borderline between living and nonliving things.
Viruses - are tiny particles (5 nm to 200 nm) composed of at least two parts: an outer coat of protein
called the capsid and an inner core of nucleic acid of either DNA or RNA.
Viruses are capable of reproduction, but only within living cells; therefore, they are call obligate parasites. Outside living cells, viruses are nonliving and can be stored in much the same way as chemicals are stored.
Viruses typically have a specific host range.
Life Cycles
A. Bacteriophages
1. Lytic Cycle
Once inside, the viral DNA brings about disintegration of host DNA and takes over the operation of
the cell. Viral DNA replication, utilizing the nucleotides within the host cell, produces many copies
of viral DNA. Viral DNA and capsids are assembled to produce about 100 viral particles. In the
meantime, viral DNA has directed the synthesis of lysozyme, and enzyme that disrupts the cell
wall, and the particles are released.
2. Lysogenic Cycle
Some bacteriophages do not immediately undergo a lytic cycle. Instead, the viral DNA becomes
integrated into the bacterial DNA. In this stage it is called a prophage. Certain environmental
factors such as ultraviolet radiation can induce the lytic cycle.
B. Animal Viruses
The entire virus enters by endocytosis and uncoating, which releases viral nucleic acid from the
envelope and capsid, occurs inside the cell. Following multiplication and assembly, the virus exits
from the cell by exocytosis, also called budding. The process of budding, which does not
necessarily kill the host cell, provides the virus with its membranous envelope.
RNA Animal Viruses
Some animal viruses have RNA genes, that is, only RNA is enclosed within the capsid.
Other RNA viruses called retroviruses have an enzyme called reverse transcriptase that carries
out RNA --> DNA transcription. Retroviruses are of extreme interest because these are the viruses
that are apt to bring cancer-causing oncogenes into a host cell. The AIDS virus is also a retrovirus
C. Viroids and Prions
Viroids differ from viruses in that they consist only of a short chain of naked RNA.
Prions consist solely of a glycoprotein having only one polypeptide of about 250 amino acids.
D. Viral Infections
Some viral diseases in humans are influenza (respiratory), mumps, measles, polio, rabies, and
infectious hepatitis. Retroviruses cause AIDS and have been implicated in the development of
cancer.
II. Kingdom Monera
Kingdom Monera contains the prokaryotes, the first types of cells to evolve (endosymbiosis).
These organisms provide the first fossils, dated 3.5 billion years ago.
A. Structure
Prokaryotic cells lack a nucleus and do not have the cytoplasmic organelles found in eukaryotic
cells, other than ribosomes.
Bacteria occur in three basic shapes: rod (bacillus), round or spherical (coccus),
and spiral (helical shape called spirillum)
B. Reproduction
Prokaryotes reproduce asexually by means of binary fission.
Genetic recombination can occur among bacteria in three ways:
Conjugation - A process involving contact between two bacterial cells during which genetic
material is passed from on cell to the other.
Transformation - occurs when a bacterium picks up from the medium DNA released by dead
bacteria.
Transduction - Transfer of genetic material between bacterial cells by bacteriophages.
Since prokaryotes have no simple method for achieving genetic recombination as do plants and
animals, mutation becomes the most important avenue for evolutionary change.
When face with unfavorable environmental conditions, some bacteria can form endospores.
Endospores are amazingly resistant to extremes in temperature, drying, and harsh chemicals,
including acids and bases.
When conditions are again suitable for growth, the spore absorbs water, and becomes a typical
bacterial cell, reproducing once again by binary fission.
C. Metabolism
Obligate anaerobes - are unable to grow in the presence of oxygen (botulism, gangrene, tetanus)
Facultative anaerobes - are able to grow in either the presence or the absence of oxygen
Aerobic - require a constant supply of oxygen to carry on cellular respiration.
1. Autotrophic Bacteria
Autotrophic bacteria are capable of synthesizing organic nutrients directly from simple inorganic
substances, such as carbon dioxide and nitrogen.
Photosynthetic bacteria use light as a source of energy to produce their own food.
Chemosynthetic bacteria oxidize inorganic compounds such as ammonia, nitrites, and sulfides,
and trap the small amount of energy released from these oxidations to use in the reactions that
synthesize carbohydrates. Nitrifying bacteria are also chemosynthesizers that oxidize ammonia
(NH3) to nitrites (NO2) and nitrites to nitrates (NO3).
2. Heterotrophic Bacteria
The majority of bacteria are aerobic heterotrophs and feed on dead organic matter by secreting
digestive enzymes and absorbing the products of digestion. These saprophytic decomposers
can break down such a large variety of molecules.
Heterotrophs may be free-living or symbiotic forming mutualistic, commensalistic,
or parasitic relationships. Certain nitrogen-fixing bacteria (Rhizobium) reduce atmospheric
nitrogen (N2) to ammonia (NH3) and nitrates (NO3) and are mutualistic; they live in the root
nodules of legumes (soybeans & alfalfa) and provide organic nitrogen to their hosts.
III. Classification of Bacteria
A. Archaebacteria
Many archaebacteria are able to live in the most extreme of environment, perhaps representing the
kinds of habitats that were available when the earth first formed.
B. Eubacteria
One way to classify eubacteria is to consider their reaction to the Gram stain. The difference is
dependent on the construction of the cell wall.
Gram-positive bacteria appear purple. (Staphylococcus aureus)
Gram-negative bacteria appear red. (E. coli, rickettsias which cause Rocky Mountain spotted
fever)
Cyanobacteria
Cyanobacteria, formerly called blue-green algae, are the most prevalent of the photosynthetic
bacteria. The cyanobacteria carry on photosynthesis in a manner similar to that of plants; they
possess chlorophyll a and evolve oxygen.
The cyanobacteria are believed to be responsible for first introducing oxygen into the primitive
atmosphere. Cyanobacteria my be unicellular, filamentous, or colonial. In association with
fungi, they form lichens that can grow on rocks.