LECTURE 7VIRUSES & PROKARYOTES

SBU3023SEM. 1 2011/2012

VIRUS CHARACTERISTICS Nucleic acids enclosed in

a protein coat. Smaller than the smallest

bacterium. Nonliving particles. Shape determines the cell

the virus attacks.This picture shows the three major virus shapes.

NAMING OF VIRUSNamed after diseases

that they cause or organs that they attack.− Ex.: poliovirus, smallpox virus, HIV etc.

A virus that infects a bacterium is called a bacteriophage or phage for short.

VIRAL METABOLISMViral cannot grow,

reproduce or develop without a host cell.

Viruses share parasitic relationships with their hosts.

VIRAL STRUCTURECapsid

Nucleic acid (DNA or RNA)Envelope:

Layer that surrounds the capsid, found mostly in larger viruses (such as those that affect humans).

VIRAL REPLICATION (LYTIC)1. Attach to a host cell with matching

receptors.2. The viral genetic material enters the

cell either through injection or through endocytosis that forms a vesicle.

3. The host cell copies the viral material.4. The viral DNA is transcribed,

translated and reassembled into new virus.

5. The host cell bursts open, releasing new viruses.

LYTIC CYCLELytic cycle: rapid

replication and assembly, followed with lysis (bursting).

VIRAL REPLICATION (LYSOGENIC)

Some viruses go through a lysogenic cycle, a replication cycle in which the virus’s nucleic acid is integrated into the host cell’s chromosome, followed by a lytic cycle.

1. Attachment2. Infection3. Genetic material inserted into host DNA

(now called provirus)4. Replication

LYSOGENIC CYCLEA. Attachment and

Entry

Bacterial host

chromosomeA lysogenic virus injects its nucleic

acid into a bacterium.

The viral nucleic acid is called

a provirus when it becomespart of the host’s chromosome.

B. Provirus FormationProvirus

C. Cell Division

Althoughthe provirusis inactive,it replicatesalong withthe host cell’schromosome.

LYSOGENIC CYCLELYTIC CYCLE The provirus

leaves the chromosome.Viral nucleic

acid and proteins are made. The cell breaks

open releasing viruses.

LYSOGENIC VIRAL DISEASESHerpes

simplex IHerpes

simplex II that causes genital herpes.

Hepatitis B virus that causes hepatitis B.

Chickenpox virus but may become lytic and cause shingles (itchy rash and red spots or blisters [pox] all over the body).

Herpes virus

HIVHuman immunodeficiency

virus (HIV) causes a condition called AIDS.

HIV destroys the T cells that are part of the human immune system.

When the T cell counts drop to a particular level, a person is said to have AIDS.

HIV

HIVHIV is a retrovirus. Retroviruses are viruses

that have RNA instead of DNA as the genetic material.

HIV injects its RNA into the host cell.

An enzyme (reverse transcriptase) makes a DNA complement of the RNA.

The viral DNA is embedded into the host cell, which the virus then goes through a lysogenic and lytic cycle.

HIV LIFE CYCLERetrovirus

Reverse transcripta

se

RNA

Enteringcell

RNA

DNA

DNA is made from the viral RNA.

Provirus in host chromosome mRNA

New virus partsNew virus forming

Exiting cell

Retrovirus Cycle

CANCER & VIRUSES Some viruses cause

cancers.These viruses cause the

cells to divide abnormally, creating tumors.−Human papilloma viruses (HPVs) are a

group of over 100 related viruses that can cause warts on the skin, mouth, genital organs, and larynx.

−Hepatitis B virus (HBV) and hepatitis C virus (HCV) - cause liver cancer.

−Epstein-Barr virus (EBV) – causes nasopharyngeal cancer (cancer of the area in the back of the nose).

PRIONS & VIRIODSPrions are composed of proteins

but have no nucleic acid to carry genetic information.−They cause other proteins to fold incorrectly, resulting in improper functioning proteins.

− Prions cause mad cow disease.Viroids are composed of a single

circular strand of RNA with no protein coat.−Viroids have been shown to cause infectious diseases in several plants.

PLANT VIRUSThe first virus to be identified

was a plant virus, called tobacco mosaic virus, that causes disease in tobacco plants.

Plant viruses enter the plant through wounds or insect bites.

Tobacco mosaic virus causes yellow spots on tobacco leaves, making them unmarketable. A. Infected & B. Healthy leaf.

A. B.

NEUTRAL PLANT VIRUSESSome mosaic viruses cause

striking patterns of color in the flowers of plants.

Rembrandt tulips

WHERE DO VIRUSES COME FROM?

Some scientists suggest that viruses are nucleic acids that break free from their host cells while maintaining an ability to replicate parastically within the host cells.

PROKARYOTEProkaryotes are much more

diverse in both habitat and metabolism than the eukaryotes.

However, prokaryotes are not very diverse in body shape or size. Much of their classification into different species is done by examining their internal biochemistry and their DNA.

Nearly all prokaryotes are single-celled. Differentiation into different cell types almost never occurs in prokaryotes.

Two major groups: the Eubacteria (sometimes just called Bacteria) and the Archaea (or Archaebacteria). Very different genetically.

PROKARYOTE Bacteria were first

discovered in the late 1600’s by Antony van Leeuwenhoek, using the microscope he invented.

The first recorded observation were of the bacteria found in the dental plaque of two old men who never cleaned their teeth.

METABOLIC DIVERSITY

Requires oxygen (obligate aerobe).Requires oxygen-free

environment (obligate anaerobe).

Can live with or without oxygen, able to perform cellular respiration or fermentation.

DOMAINS EUBACTERIA AND ARCHAEA

Used to be lumped into one group, bacteria. Eubacteria have cell walls with peptidoglycan. Archaea have chemically unique cell membranes.

Also have a large number of unique genes.

DOMAIN EUBACTERIA Identified by;

− Their shape• Can be rods, spheres or spiral-shaped

− Metabolic reactions− Chemistry of their cell walls− The number and placement of flagella− Their ability to form spores

BACTERIAL CELLS STRUCTURE

PROKARYOTIC METABOLISM

PROKARYOTIC CELL WALL

Functions to protect the cell and maintain cell shape

May be composed of peptidoglycan−May be Gram positive or Gram negative

Archaean cell walls lack peptidoglican

THE GRAM STAINWhich divides most clinically

significant bacteria into two main groups, is the first step in bacterial identification.

Bacteria stained purple are Gram positive – their cell walls have thick peptidoglycan and teichoic acid.

Bacteria stained pink are Gram negative – their cell walls have thin peptidoglycan and lipopolysaccharides with no teichoic acid.

THE GRAM STAIN

SHAPES OF BACTERIA

DOMAIN EUBACTERIA Occupy several different ecological niches. Decomposers

− Heterotrophs that break down organic matter.− Called saprophytes.

• Can live on dead organic matter.− Some are anaerobic.

−Others are aerobic and break down organic matter into carbon dioxide and water.

−Important in recycling carbon, nitrogen, phosphorus, etc.

−Could be used to break down sewage, clean up oil spills.

−Used in food production (Swiss cheese, yogurt, vinegar, etc.)

−Are responsible for food spoilage.

DECOMPOSERS IN SEWAGE

DOMAIN EUBACTERIA Commensal bacteria

−Bacteria that live on or in organisms without benefiting or harming the host.

− Normal microflora• Bacteria on our skin.• E. coli in our large intestine.

CYANOBACTERIA Photosynthetic eubacteria

−Cyanobacteria performs photosynthesis like plants.• Use carbon dioxide and water;

release oxygen; make glucose.• Commonly found in many aquatic

environments.Can reproduce rapidly and can

cause dangerous blooms in certain conditions.

CYANOBACTERIA

DOMAIN EUBACTERIA Mutualistic bacteria

− E. coli in our large intestine.• Produce antibiotics and compete for resources. Reducing the growth of pathogenic bacteria.

• They produce and release vitamin K.− Cyanobacteria and fungi in lichens.− Plants and nitrogen-fixing bacteria in their roots.

DOMAIN EUBACTERIA Bacteria and mineral cycles

− Nitrogen cycle• Nitrogen-fixing bacteria transform

atmospheric nitrogen into ammonia.Ammonia can be used by plants to make amino acids.

Other bacteria convert ammonia waste to nitrate or nitrite.

Other bacteria convert nitrite to atmospheric nitrogen.

− Bacteria also cycle sulfur, iron, manganese, etc.

DOMAIN EUBACTERIA Disease-causing bacteria− Also called pathogens.−Some commensalistic bacteria can

become pathogenic.•If they are given the opportunity to increase in number.

•If they have the opportunity to invade new tissue.

• Example: Streptococcus pneumoniae

DOMAIN EUBACTERIA Pathogenic bacteria cause disease by

−Altering the normal physiology of the tissue they invade.

−Releasing enzymes that destroy the tissue they invade.

−Releasing toxins or poisons into the hosts tissue.• Causes tissue damage, fever, aches and pains.

Infections are usually stopped by the host’s immune system.

Some pathogenic bacteria invade plants.− Plant blights, wilts, cankers and soft rots.

A BACTERIAL PLANT DISEASE

Citrus canker is a disease affecting citrus species that is caused by the bacterium Xanthomonas axonopodis.

DOMAIN EUBACTERIA Control of bacterial populations

− Bacterial populations grow rapidly because

• Their generation time is so short. About every 20 minutes.1 bacteria can lead to millions in a matter of days.

• They can generate resistant forms or stages. Spores

• They mutate rapidly. Can produce antibiotic resistant strains.

DOMAIN EUBACTERIA Control of bacterial populations

− Antibiotics • Kill bacteria by

Inhibiting important metabolic pathways.

Prohibiting the bacterium from dividing.

Most can be taken orally.

DOMAIN EUBACTERIA Antiseptics

− Kill bacteria or stop their growth.− Used on surfaces, not taken orally.

Other strategies for killing bacteria− Boiling

• Won’t inactivate spores.− Pressure and heat will destroy spores.

• Autoclaving

DOMAIN ARCHAEA Most archaea are considered extremophiles. − Can survive in extreme conditions.

Methanogens− Strict anaerobes− Release methane− Live in a variety of environments.

• In mud at the bottom of lakes, swamps.

• Intestinal tracts of animals.• Used to manage human waste.

DOMAIN ARCHAEA Extreme halophiles

− Live in extremely salty environments• Great Salt Lake• Dead Sea• Salt ponds

− Some are aerobic heterotrophs.− Some are photosynthetic autotrophs.

• Use bacteriorhodopsin that contains carotene.

• Appear reddish

DOMAIN ARCHAEA Thermophiles

−Live in extremely hot environments (above 50°C).• Host springs• Hydrothermal vents in the ocean floor.

− Metabolically diverse• Some are aerobic; some are anaerobic.

•Some can reduce sulfur and release sulfide gas.

• Some live in very acidic conditions.

HYDROTHERMAL VENTS

CONCLUSION