Chapter 20 Viruses and Bacteria

I. Viruses A. Is a Virus Alive? All living things are made of cells, are able to grow and reproduce, and are guided by information stored in their DNA Viruses do not grow, do not have homeostasis, and do not metabolize, therefore they are not considered to be living Viruses are pathogens, or agents that cause disease, and replicate by infecting cells and using the cell to make more viruses

B. Viral Structure Viruses are segments of nucleic acids contained in a protein coat, or capsid The nucleic acid may either be RNA or DNA, but not both - DNA viruses include those that cause warts, chickenpox, and mononucleosis

Many viruses have a membrane, or envelope, surrounding the capsid - helps virus enter cells - consists of proteins, lipids, and glycoproteins

Viruses exist in a variety of shapes 1. filaments 2. spherical 3. helical 4. polyhedral

Bacteriophages specifically infect bacteria - most bacteriophages consist of a polyhedron capsid attached to a helical tail

C. Viral Reproduction Viruses must rely on living cells for replication because they have no structures to make protein 1. Lytic Cycle The cycle of viral infection, replication, and cell destruction in bacterial viruses

Viral genes use the host cell to replicate viral genes and to make viral proteins, such as capsids The proteins are then assembled with the replicated viral genes to form complete viruses The host cell breaks open and releases newly made viruses

2. Lysogenic Cycle Viral genes are inserted into host chromosomes - the resulting virus is called a provirus Whenever the host cell divides, the provirus also divides, infecting all resulting host cells without destroying them Changes in the environment can cause the provirus to enter the lytic cycle

3. Host Cell Specificity Viruses are often restricted to certain kinds of cells 4. Enveloped Virus Many viruses that infect only animals have an external viral envelope ex. Human immunodeficiency virus (HIV) which causes acquired immune deficiency syndrome (AIDS)

The viral envelope is composed of a lipid bilayer derived from the membrane of the host cell Glycoproteins are embedded on the envelope The envelope holds the capsid, which in turn holds the viral genes

D. How animal viruses infect cells 1. Attachment Glycoproteins on the envelope of HIV precisely fits human cell surface receptors called CD4 receptors on macrophage (immune system) cells

2. Entry into Macrophage The glycoprotein must also activate a second co- receptor, called CCR5 to start endocytosis into a macrophage

3. Replication The HIV viral envelope is left outside of the cell The HIV particle sheds its capsid The particle then releases an enzyme called reverse transcriptase Reverse transcriptase copies the viral RNA into a complementary DNA

Translation of the viral DNA directs the production of many copies of the virus by the host cell’s machinery New virus capsids are released from the cell by budding and are thus covered with an envelope derived from the host’s cell membrane

4. AIDS Years after the initial infection, HIV’s glycoproteins change and start to recognize and bind to cell surface receptors on lymphocytes called T cells Unlike its activity in macrophages, HIV reproduces in T cells and then destroys them Upon destroying T cells, virus particles increase in number in the blood and infect more lymphocytes It is this destruction of the body’s lymphocytes that blocks the body’s immune response and signals the onset of AIDS

E. Viral Diseases Emerging viruses are viruses that evolve in geographically isolated areas and are pathogenic to humans Prions are composed of proteins but have no nucleic acid (cause mad cow disease) Viroids are composed of a single strand of RNA that has no capsid

II. Bacteria A. Bacterial Structure Prokaryotes that include organisms that compose the kingdoms Eubacteria and Archaebacteria Differ from eukaryotes in seven ways

1. Internal compartmentalization Lack a nucleus 2. Cell size One-tenth the size of eukaryotic cells 3. Multicellularity Single cells that are not specialized 4. Chromosomes Single circular piece of DNA

5. Reproduction Binary fission - one cell pinches into two cells 6. Flagella eukaryotic cell flagella are more complex 7. Metabolic diversity Many metabolic abilities including anaerobic and aerobic processes

B. Bacterial Cell Shapes 1. One of three basic shapes a. coccuss – a round-shaped cell b. bacillus – a rod-shaped cell c. spirillum – a spiral cell

2. Cell walls a. Eubacteria have cell walls made of peptidoglycan surrounded by a gel- like layer called a capsule - two types distinguished by a Gram stain; Gram- negative and Gram positive - important medically in determining their susceptibility to different antibiotics b. Archaebacteria often lack cell walls Gram – Gram +

3. Endospores Thick-walled surroundings that form during harsh conditions and enclose the chromosomes of some bacteria

4. Pili a. enable bacteria to adhere to surfaces b. enable bacteria to exchange genetic material through conjugation

C. Obtaining Energy 1. Photosynthesizers a. green sulfur bacteria and purple sulfur bacteria Grow in anaerobic (oxygen-free) environments - cannot use water as a source of electrons for photosynthesis

b. Purple nonsulfur bacteria Use organic compounds as a source of electrons for photosynthesis c. Cyanobacteria Often clump together in large mats of filaments Thought to have made the Earth’s oxygen atmosphere

2. Chemoautotrophs Obtain energy by removing electrons from inorganic molecules or organic molecules Manufacture their own amino acids and proteins using hydrogen-rich chemicals

3. Heterotrophs Many grow in aerobic (oxygen-rich) environments Break down the bodies of dead organisms and make nutrients available to other organisms Principal decomposers of the living world

D. Pathogenic Bacteria 1. Bacteria can metabolize their host Heterotrophic bacteria obtain nutrients by secreting enzymes that break down organic structures and then absorbing them The bacteria that causes tuberculosis uses human tissue as their source of nutrients

2. Bacteria Toxins Bacteria cause disease by secreting chemical toxins into their environment E.coli causes food poisoning by releasing a toxin

III. Antibiotics Different antibiotics interfere with different cellular processes These processes do not occur in viruses so antibiotics are not effective against them

A. Antibiotic- resistant bacteria Some bacteria have become resistant to antibiotics due to mutations arising spontaneously Susceptible bacteria are eliminated from the population Resistant bacteria survive and pass on their resistance traits

1. Antibiotic misuse If antibiotic treatment ends too soon, some of the bacteria may survive Usually the most resistant bacteria survive and develop antibiotic resistance

2. Multiple-antibiotic resistance Bacteria acquire several antibiotic-resistance genes Bacteria gain resistance to antibacterial agents in antibacterial soaps

IV. Importance of Bacteria A. Food and Chemical Production Many foods we eat are processed by specific kinds of bacteria Bacteria are used to produce different kinds of chemicals for industrial uses (acetone or butanol) Genetically engineered bacteria used for medicine

B. Mining and Environmental Uses of Bacteria Bacteria can convert elements into soluble compounds so they can be washed away from desired minerals in low-grade ore Bacteria metabolize organic chemicals and are used to help clean up environmental disasters