NOTES FOR BIOLOGY 1002
SECTIONS 004, 005, 006
Spring 2006
DR. STEVEN POMARICO
CHAPTER 21
PROKARYOTES AND VIRUSES
Microorganisms consist of organisms that are too small to be seen clearly without a microscope.
These microorganisms include:
-prokaryotes, which range in size from 0.2-1.0 micrometers in diameter.
and
-unicellular eukaryotic organisms of the kingdom Protista that are in the 10-100 micrometer range. (See fig 21.1)
How big (or small) is a micrometer? 1/1,000,000 of a meter. But just how small is that? The period at the end of this sentence is about 1 millimeter (1000 micrometers) in diameter.
There are groups of biological entities that are even smaller than bacteria.
These groups are, VIRUSES, VIROIDS, AND PRIONS
-some are things pathogens that can make organisms sick.
-these are not organisms, because they don’t have all the requirements for being alive. They are however derived from organisms.
THE PROKARYOTIC DOMAINS
The organisms that make up the two prokaryotic domains (Bacteria and Archaea) were the first organisms to arise on earth (about 3.5 billion years ago).
While only about 5000 prokaryotic species have been described, there may be as many as 100-1000 times that number.
Bacteria have evolved to live in almost every possible environmental condition found on Earth including some very extreme conditions.
While the range of conditions that the domains bacteria and archaea can in live in is large, the conditions a single species requires may be very specific.
In order for these domains to be able to occupy so many different ecological niches, they have had to develop the ability to use many different energy sources.
Some autotrophic prokaryotes, like cyanobacteria, are photoautotrophic and obtain their energy from sunlight via photosynthesis.
Other prokaryotic species are chemoautotrophs which derive their energy from inorganic chemicals (chemosynthesis).
There are also heterotrophic bacteria that must have an organic food source for either their energy or their carbon source.
The two domains are similar in many respects (See table 21.1):
- No nucleus
- Single chromosome (circular)
- Cell wall
- Reproduction by binary fission
One main distinction between the Bacteria and the Archaea is the type of a
semi-rigid permeable cell wall.
This bacterial cell wall is composed of peptidoglycan
-peptidoglycan is a complex of structural polysaccharides cross-linked by peptides.
The cell wall of a bacteria gives it the different types of characteristic shapes (See fig. 21.4)
-coccus are spherical
-bacillus are rod shaped
-spirillum are helical or corkscrew shaped
These are some shapes, but there are many variations of these basic shapes.
The peptidoglycan cell wall of bacteria can be stained by a specific stain known as a Gram stain (named for Hans Christian Gram, a Danish physician)
The Domain Bacteria can be divided into two groups based on the ability to be stained with Gram stain.
Gram positive bacteria have the cell wall exposed to the environment. Some bacteria are not stained by Gram stain (Gram negative).
These bacteria still have a peptidoglycan cell wall, but they also have an extra outer membrane.
Surrounding the cell walls of some bacteria is a glycocalyx.
This structure is composed of polysaccharides and/or proteins. It’s function to attach the bacteria to a surface.
About half the bacterial species are capable of motion using rotating flagella (See 21.4).
-flagella (singular flagellum) are filaments that extend from the membrane of a cell and are used for cellular locomotion.
The anchor of the bacterial flagellum is an “axle-and-wheel” arrangement which allows the flagellum to rotate like a propeller.
Many species can attach themselves to a surface (e.g., a host-cell or another bacterium) via pili (singular pilus).
-pili are hair-like structures on the surface of certain bacteria (See fig 21.3) that aid in attachment.
Reproduction in prokaryotes is asexual in the form of binary fission (See fig 21.7). The entire bacterial chromosome is replicated and passed on to each daughter cell.
Some bacteria are able to transfer genetic information between cells by bacterial conjugation.
-bacterial conjugation is the transfer of genetic information (a plasmid) between two bacteria via a conjugation tube know as a sex pilus. (See fig 21.8)
-plasmid is a small double-stranded ring of DNA that carries extrachromosomal genes in some bacteria.
While the prokaryotes occupy many diverse habitats some of the most extreme environments are the realms of Archaebacteria. Some examples include:
Extreme thermophiles (heat loving), including thermoacidophiles, which live in acidic hot springs or in the hot (105 oC) water surrounding
deep-sea volcanic vents.
Methanogens which are strict anaerobes and must live in environments
without oxygen.
Extreme halophiles (salt loving) that inhabit the extremely salty Dead Sea and Great Salt Lake.
Some prokaryotes have developed a differ mechanism for handling harh environments.
Eubacteria, especially the bacilli, can form a protective endospore in response to harsh environmental conditions.
-endospores are protective “resting” structures in which the bacterial chromosome is surrounded by a durable wall. (See fig 21.14)
These structures are resistant to dehydration, extreme heat or cold, and most poisons.
The endospore will persist until conditions are favorable and then develop into a bacterium.
>Viruses
A virus particle is small (0.05 - 0.2 micrometers) and comes in many different shapes. These particles all have two parts:
genome - the genetic information can be either DNA or RNA (ds or ss)
capsid - a protein coat that surrounds and protects the genome
Some viruses also have an envelope
- a membrane (phospholipid bilayer) that surrounds the capsid
The entire function of a virus is to take control of a host cell to produce more virus particles. A biological hijacking.
The genetic material of the virus “reprograms” the host cell to make more viruses (See fig 21.20). Because viruses require a host for their reproduction, they aren’t alive.
The cell type that a virus will infect is usually very specific and the host species, which the virus can infect, are often a narrow range.
Viruses can exhibit two reproductive pathways , the lytic and lysogenic pathways.
The lytic pathway (See fig 21.20)
---Lytic pathway includes the lysis of the host cell.
The lytic cycle proceeds rapidly through the following steps:
1. Virus attachment to the cell surface.
2. Penetration of the viral DNA into the cell.
3. Replication and synthesis of the viral components (genome and capsid).
4. New virus assembly.
5. Cell lysis and release of the virus.
Typically this cycle can take 20-30 minutes and produce 100's of virus particles per bacterial host cell infected.
>>>The lysogenic pathway (See fig 21.20)
Some viruses can coexist with their bacterial hosts.
These viruses can reproduce by the lysogenic pathway.
---Lysogenic pathway allows reproduction of the virus without destruction of the host cell.
In the lysogenic cycle the viral genome (DNA form) is incorporated into the bacterial host cell’s genome.
-The virus DNA reproduces along with the bacterial DNA
-If virus DNA leaves bacterial chromosome, then the lytic cycle begins
A lysogenic bacterial cell may express some viral genes. The expression of the viral genes is a lysogenic conversion and changes the host cell’s phenotype
Often this lysogenic conversion results in the production of a toxin
(e.g., botulism, diphtheria, scarlet fever)
>Viroids are even simpler/smaller biological entities.
-viroids are small fragments of ssRNA (either linear or circular) with no protein coat.
Like a virus, a viroid takes over the cell it infects. Viroids are plant pathogens that can infect citrus, potatoes, and other crop species.
>Prions are unusual infectious particles
-prions are protein particles with NO GENETIC MATERIAL.
Most prions have been linked to degenerative diseases of nervous tissue especially the brain. Include in these prion diseases are kuru or Creuztzfeldt-Jakob disease and more recently “mad cow disease”
The prion is an improperly folded version of a natural protein. The wrongly folded protein seems to cause other copies of the protein to covert from the correct folding pattern into the incorrect form. As the bad copies accumulate, they result in physical changes in the tissue.
Some of the most common human inflections and diseases are caused by viruses while others are the result of pathogenic bacteria and still others come from protists (single cell eukaryotes).
Because of mutations new emerging pathogens are a continuing threat.
The widespread use (and misuse) of antibiotics, some pathogenic bacteria have developed antibiotic resistance.