Prokaryotes & Viruses

Chapter 16

Early Earth• Earth is about 4.6 billion years old• Fossils resembling photosynthetic prokaryotes

have been found in dome shaped rocks called stromatolites, date back to 3.5 billion year ago.

Origin of Organic Molecules• In 1953, Miller & Urey

designed an experiment to simulate conditions on early Earth

• Under many different conditions this basic set up has produced all 20 amino acids, several sugars, lipids, the nitrogenous bases found in DNA & RNA and ATP

• Solutions of amino acids and dropped them onto the surface of hot sand, clay or rocks has resulted in the formation of polypeptides

The RNA World • A characteristic of life is the process of inheritance, which

is based on molecules that can copy themselves.

DNA RNA Protein

• How did this information flow originate?

• A popular hypothesis is that genes were originally short strands of RNA capable of replicating without enzymes

–Scientists have observed RNA molecules copying themselves in solutions containing nucleotides without enzymes or cells present

– During the “RNA world”, RNA might have stored genetic information in addition to directing protein synthesis

Formation of Pre-Cells • Experiments have

shown that polypeptides can form microscopic fluid-filled spheres.

• If certain kinds of lipids are in the solution selectively permeable membranes will form.

• These “molecular packages” are referred to as pre-cells

Hypothetical 4-Stage Sequence for Origin of Life

1. Small organic molecules formed from simpler inorganic molecules

2. These small molecules joined into more complex ones

3. Molecules that could copy themselves provided a basis for inheritance of molecular information

4. These molecules became packaged within membranes and separated from their surroundings

Archaea v. Bacteria• “archaea” is derived from

the Greek work for ancient• Exist in harsh habitats

resembling conditions of early Earth

• They are referred to as “Extremophiles”– Thermophiles– Halophiles– anaerobic

• Believed to be as closely related to eukaryotes as they are to bacteria

• Contain different information in their nucleic acids

• RNA polymerases differ• Lack introns• Susceptible to antibiotics

that do not affect archaea• Contain peptidoglycan in

their cell walls

Phylogenic Tree of the Three Domains

Structure & Function of Bacteria: Shape• Cocci – spherical• Bacilli – rod-shaped• Spirilla – spiral shaped

Structure & Function of Bacteria: Cell Wall

• Gram + (purple stain): thick layer of peptidoglycan

• Gram – (pink stain): thinner layer of peptidglycan with outer membrane

Structure & Function of Bacteria: Motility

• Flagellum• Pilli• Slime secretion

Bacterial ReproductionBinary fission: DNA is copied and moved to opposite ends of the cell as the cell divides; occurs almost continuously.•Rapid reproduction rate, many can divide within 20 minute

•Results in a colony of cells that are clones (unless mutations occur)

Genetic Variation• Despite reproducing

asexually, bacteria are able to obtain genetic variation through conjugation

• Two bacterial cells temporarily join and exchange plasmid DNA; does not need to occur between cells of the same species/strain

Endospores

• Allow bacteria to survive periods of very harsh conditions by going into a dormant endospore form

• After copying DNA, one copy is surrounded by a thick protective coat and the outer cell disintegrates

• Often able to survive for years in this state, when conditions are more favorable, the endospores will absorb water and grow again.

Modes of Nutrition

The Oxygen Revolution• The evolution of photosynthetic cyanobacteria resulted

in the presence of free oxygen in oceans, lakes & the atmosphere

• This oxygen was toxic to many existing organisms because it attacks the bonds of organic molecules & many went extinct

• Some were not exposed to oxygen and remained anaerobic; their descendants still exist in similar environments today

• A small number were able to use the oxygen in the extraction of energy from food, their descendents are the wide variety of aerobic organisms in existence today

Beneficial Uses of Bacteria• Chemical recycling:

– Decomposers: replenish soil nutrients and release CO2 back to the atmosphere

– Nitrogen fixing bacteria: convert nitrogen gas in the atmosphere to an organic form usable by other organisms for nucleotide and amino acid formation

• Bioremediation:– Sewage treatment: decompose organic matter in sewage sludge– Oil spill clean-up: genetically modified digest oil– Clean old mining sites: detoxify by extracting lead & mercury

• Medicine/Pharmaceuticals:– Produce desired gene products: insulin, HGH– Probiotics: restore beneficical intestinal bacteria

• Food:– Cheese– Yogurt

• All living things share 8 characteristics. Viruses do not meet all of these characteristics.

• Viruses attack eukaryotic cells and Bacteriophages attack prokaryotic cells.

• Viruses and phages are capable of reproducing at a very rapid rate, but only in a host cell.

• Viruses are responsible for many diseases and can be found everywhere.

Viruses & Bacteriophage: The Boundary of Life

Viral Structure

Viruses & Disease• Method of causing disease is very different from that of

bacteria• Therefore methods for treatment & prevention differ from

methods for bacteria• Antibiotics will not work on viruses because they target

specific not found in viruses or host cells– Damage or prevent formation of bacterial cell wall

• Some examples of viral diseases include:Influenza (RNA) Polio (RNA)

Common cold (RNA) Hepatitis (DNA)

Measles (RNA) Herpes (DNA)

Mumps (RNA) Smallpox (DNA)

AIDS (RNA) Rabies (RNA)

Viral InfectionViral Infection

• Viruses and bacteriophages invade cells and use the host cell's machinery to synthesize more of their own macromolecules.

• Once inside the host the bacteriophage or virus may go into two reproductive cycles:– Lytic cycle: destroying the host

cell during reproduction.– Lysogenic Cycle – a parasitic

type of partnership with the cell

Lytic Cycle & Lysogenic CycleLytic Cycle & Lysogenic Cycle

Viruses are host specific – a protein on the surface of the virus has a shape that matches a molecule in the plasma membrane of its host, allowing the virus to lock onto the host cell.

ProvirusesProvirusesA provirus is a DNA virus that has been inserted into a host cell chromosome.

Retroviruses & HIV• Retroviruses reverse the

normal DNA to RNA to protein flow – In RNA viruses, the RNA

nucleotide will be translated

• Reverse transcriptase catalyzes the synthesis of DNA from an RNA template, the DNA intermingles with host DNA as a provirus making it difficult to detect

Prions Prions • Proteins that cause several diseases of

the brain: Mad cow disease, Kuro, Creutzfeldt-Jacob disease (CJD) & Scrapie (in sheep)

• Only infectious agent that do not contain genetic material

• Normal form play important roles in brain function such as helping nerve cells communicate

• Abnormal prions destroy the brain• Three ways to acquire abnormal prions:

– Infection with abnormal prions– Inherited genes that give rise to abnormal

prions– Spontaneous genetic mutations that give rise

to abnormal prions

ViroidsViroids• Small strands of RNA rather than strands of protein. • Smaller than the strands of genetic info in viruses and contain no

protein coat. • Replicated using host cell machinery, like viruses• Cause plant diseases: potato spindle tuber, avocado sunblotch,

chrysanthemum stunt, and chrysanthemum chlorotic mottle

Immune Response

HIV doesn’t target just any cell, it goes right for the cells that want to kill it. “Helper" T cells are HIV's primary target. These cells help direct the immune system's response to various pathogens.

HIV undermines the body's ability to protect against disease by depleting T cells thus destroying the immune system. The virus can infect 10 billion cells a day, yet only about 1.8 billion can be replaced daily.

Defense Against Viral Diseases

From HIV to AIDSFrom HIV to AIDS• During first few years (7-10) after HIV infection, person is

usually asymptomatic.• During the symptomatic phase, the body has insufficient

numbers of T-Cells (from normal 800-1200 /mm3 to 200/ mm3 ) to mount an immune response against infections. – Chronic diarrhea, minor mouth infections, night sweats, headache &

fatigue are common

• At the point when the body is unable to fight off infections, a person is said to have the disease AIDS. (Generally when count drops below 200 /mm3 )

• It is not the virus or the disease that ultimately kills a person; it is the inability to fight off something as minor as the common cold.

AIDS: The Global EpidemicAIDS: The Global Epidemic• Around 2.6 million

people became infected with HIV in 2009.

• Sub-Saharan Africa has been the hardest hit by the epidemic. In 2009 over two-thirds of AIDS deaths were in this region