3.2: Bacteria and Viruses

1A: Prokaryotes and eukaryotes

Prokaryotes:

• Uni-cellular (single-celled)

• Prokaryote → organism

• Prokaryotic → cell

• Less complicated and smaller (0.1 to 5 micrometers (µm))

• No membrane-bound organelles

• No nucleus

Eukaryotes

• Uni or multicellular

• Eukaryote → organism

• Eukaryotic → cell

• Some have cell walls

• More complex and larger (10 to 100 micrometers (µm))

• Membrane-bound organelles

• nucleus

Similarities:

• DNA

• Ribosomes

• Cytoplasm

• Cell membrane

2B: Viruses and Bacteria

Viruses:

Viruses are different from living cells in many ways.  They cannot reproduce without a host, they do not contain cytoplasm, cell membranes or membrane-bound organelles.  When they are outside of a cell they lie dormant until a cell comes along that they can infect.  Rather than undergo mitosis, a virus will use a host cell to make multiple copies of itself.  In a typical reproductive cycle, a virus lands on a cell, injects its DNA, and the host cell replicates and assembles new viruses for it, until the host cannot fit any more viruses inside. They burst out of the cell wall into the extracellular space to infect more viruses.   This is called the lytic cycle.  Occasionally a virus will enter the lysogenic cycle when it's DNA actually becomes a part of one of the host's chromosomes, when this happens, the host is referred to as a provirus. In a provirus, the viral DNA is permanently a part of the host cell, and will not kill the cell unless it separates from the chromosome and enters the lytic cycle.  

It is also a good idea to know the basic structure of a simple virus type called a "bacteriophage".  Bacteriophages are viruses that attack bacteria. (see image right)

Domain Archaea and Domain Bacterias:

It is easy to think that all bacteria are alike, because we can't see them without an electron microscope.  However, bacteria in domain Archaea have unique traits that distinguish them from bacteria in Domain Bacteria.  One thing they do share however, is shape.  The most common bacterial shapes are rod (bacilli), sphere (cocci) or spiral (spirillum).  When bacteria form chains, the prefix "strept" is added, and when bacteria aggregate in groups, the prefix "staphylo" is added.  A common example is strep throat, which is caused by chains of cocci bacteria, therefore called "streptococci".  If there was a cluster of rod shaped bacteria, it would be called "staphylobacilli", and so on.

Kingdom Archaebacteria:

Kingdom Archaebacteria is the most primitive kingdom based on the fossil record. The oldest fossils of life on Earth are of Archaebacteria, found in Australia's stromatolites, dated to be approximately 3.8 billion years old. In order to exist on a primitive Earth, Archaebacteria evolved to be able to use methane as food in a process called methanogenesis. When they make their own food from methane, Archaebacteria are considered to be autotrophs, meaning they make their own food. Some members of Archaea can also obtain food from other sources, which makes them heterotrophs as well. 

Archaebacteria are able to survive in areas that are extremely low in oxygen. When an organism does not require oxygen to live, it is called "anerobic". Archaea are able to live in extreme environments where no other life forms are found such as in hot springs, or very salty water or low oxygen water. Due to their ability to live in extreme habitats, Archaebacteria are also referred to as "extremophiles". The word "phile" means to love. Therefore extremophiles love extreme environments. 

Archaebacteria reproduce asexually through a process called binary fission. During this process the cell creates a copy of it's single chromosome, and after the cell reaches a certain size, it begins to build a new cell wall between the two chromosomes, creating two smaller, identical bacteria cells. This is an asexual process, and creates two clones, which does not increase genetic diversity. When environmental conditions become less favourable, Archaebacteria will exchange DNA in a process called conjugation. During conjugation, one bacterial cell injects a part of it's chromosome into another bacterial cell. Unlike binary fission, conjugation does increase genetic diversity. 

The diagram left demonstrates the process of binary fission, which clones the bacteria and does not increase genetic variation.

The diagram above demonstrates the process of conjugation, during which a bacteria gives a part of it's DNA to another bacteria, and it does increase genetic diversity. 

Another way for scientists to identify Archaea is by using a technique called a gram stain.  Archaea contain a thick layer in their cell wall of peptidoglycan that Eubacteria do not have.  When a gram stain is applied, the peptidoglycan turns purple. Scientists call this a gram-positive bacteria. Archaebacteria are gram-positive.

Kingdom Eubacteria:

Kingdom Eubacteria are much more diverse than Archaea and are found almost everywhere from the air you breathe to the inside of your intestines. Your body is actually made up with more Eubacteria cells than human cells, which is why a good way to remember the name "Eubacteria" is that "you" is full of "You" bacteria. Due to this fact, most bacterial diseases that can infect humans are caused by Eubacteria. Examples include chlamydia and strep throat. Eubacteria can be autotrophic, but they are aerobic, so some species are able to perform photosynthesis rather than methanogenesis (which is only used by Archaea). During photosynthesis, Eubacteria use sunlight, water and carbon dioxide to produce glucose and oxygen. It is believed that a species of Eubacteria called Cyanobacteria were responsible for changing the early Earth's atmosphere from one with high methane concentrations, to one that has high oxygen concentrations. Life as we know it relies on this atmospheric composition.  

Eubacteria also reproduce asexually using binary fission which does not increase genetic diversity, and by conjugation, which does increase genetic diversity. When Eubacteria is given a gram test, it stains gram negative, having a pink colour. This means that the cell walls of Eubacteria do not contain peptidoglycan like the walls of Archaebacteria. 

Task:  Make a chart that compares Kingdom Archaebacteria and Kingdom Eubacteria by using the following categories:

• Heterotrophic or Autotrophic?  If Autotrophic, what process do they use to make their food?

• Habitat - Where are they typically found?

• Reproduction - How do they reproduce?  When do they increase or decrease genetic diversity?

• Gram Test - Are they gram positive or gram negative?