essentials of biology sylvia s. mader chapter 17 lecture outline prepared by: dr. stephen ebbs...

Essentials of BiologySylvia S. Mader

Chapter 17Lecture Outline

Prepared by: Dr. Stephen EbbsSouthern Illinois University Carbondale

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

17.1 The Viruses

• Our understanding of disease, genetics, and some of the characteristics of life has come from the study of viruses.

• Viruses are extremely small and noncellular and are not included in the classification of living organisms.

• Viruses consist of two primary structures.– An outer capsid– An inner core of either DNA or RNA

17.1 The Viruses (cont.)

• The outer membrane of some animal viruses may include an outer membrane envelope with spike-shaped glycoproteins.

• This outer membrane is a piece of the host’s plasma membrane which also contains viral proteins.

• The interior of the virus also contains a variety of proteins.


• The debate over whether viruses are living organisms has focused on several facts.– Viruses are obligate intracellular parasites,

meaning that they can only reproduce inside a living cell.

– Viruses can be synthesized chemically in the laboratory.

– Viruses have a genome that is subject to mutation and controls viral reproduction.

Viral Reproduction

• Viruses are specific to a particular host cell.

• Once inside the host, the viral genome takes over the cell and uses the host’s enzymes, ribosomes, tRNA, and ATP to synthesize new viral particles.

Reproduction of Bacteriophages

• A bacteriophage (or phage) is a virus that reproduces in a bacterium.

• There are two possible cycles of phage reproduction.– A lytic cycle– A lysogenic cycle

Reproduction of Bacteriophages (cont.)

• The lytic cycle has five stages.– Attachment– Penetration– Biosynthesis– Maturation– Release

• The attachment stage involves the attachment of the capsid molecule to the receptor on the surface of the host cell.


• Penetration is the stage during which a viral enzyme breaches the bacterial cell wall so that the viral DNA can be injected.

• During the biosynthesis stage, the virus deactivates all host genes not needed for viral reproduction and initiates the synthesis of viral components.


• The assembly of new viral particles occurs during maturation.

• During the release stage, viral lysozymes rupture the bacterium to release the new virus particles.

• The lytic cycle causes the death of the host bacteria.


• In the lysogenic cycle, the host cell may not immediately lyse because the phage is latent.

• After attachment and penetration, integration occurs as the viral DNA becomes incorporated into the host DNA.

• This latent viral DNA is called a prophage.


• The prophage is replicated along with the host DNA and is passed along to all daughter cells.

• Daughter cells with a prophage are lysogenic cells.

• These lysogenic cells can be triggered to enter and complete the lytic cycle.

Plant Viruses

• Plant viruses infect plants through damaged tissues.

• Plant viruses spread through the plant by migrating through the plasmodesmata which interconnect plant cells.

Plant Viruses (cont.)

Plant Viruses (cont.)

• Plant viruses are transmitted by several mechanisms.– Insects– Pruning and propagating tools– Seeds and pollen

• Viral diseases cannot be controlled with chemicals, but may be with biotechnology.

Animal Viruses

• The reproduction of animal viruses is similar to that of bacteriophages, but with some differences.

• When animal viruses undergo attachment, the envelope fuses with the plasma membrane so the virus can enter by endocytosis.

Animal Viruses (cont.)

• A virus that enters by endocytosis is uncoated as the capsid is removed.

• The uncoated viral genome then begins the biosynthesis stage.

• The new viral particles are released from the host cell as they bud, picking up a viral capsid.

Retroviruses

• RNA animal viruses that have a DNA stage (like HIV) are called retroviruses.

• Retroviruses have an enzyme called reverse transcriptase which carries out transcription to form a cDNA from RNA.

• The term cDNA indicates that the DNA is a copy of the viral genome.

Retroviruses (cont.)

• The single stranded cDNA replicates to become double-stranded DNA that integrates into the host genome.

• New retroviruses are produced as the viral DNA is transcribed.

Animal Viruses (cont.)

Emerging Viruses

• Besides HIV, there are other viruses that are becoming problematic worldwide.– West Nile virus– The SARS virus– Hantavirus– Ebola virus

• One reason that these viruses are becoming more important is that their range has changed recently.

Emerging Viruses (cont.)

• Several other factors increase the incidence of viral disease. – Viruses have high rates of mutation.– Some viruses can jump from one host species

to another.– The mode by which the virus is transmitted

can also change.

Emerging Viruses (cont.)

Drug Control of Human Viral Diseases

• The available antiviral drugs control viral infections in several ways.– Some antiviral compounds are structurally

similar to nucleotides and interfere with viral reproduction.

– Some drugs block reverse transcriptase.– Protease inhibitors are used to block

maturation of viral proteins.

17.2 Viroids and Prions

• Naked RNA strands that cause disease in crops like plant viruses are called viroids.

• Protein particles that cause disease are called prions.

• Prions were discovered when a cannibalistic tribe developed a disease after consuming human brain tissue.

• Prions cause fatal neurodegenerative disorders.

17.3 Prokaryotes

• The first cellular organisms on the planet were single cell prokaryotes.

• There are two types of prokaryotes.– Bacteria– Archaea

General Biology of Bacteria

• Bacteria can have several shapes.– Rod-shaped, called bacilli– Spherical, called cocci– A curved rod, called vibrio– Spiral-shaped, called spirillium or a spirochete

• Some bacteria can form doublets, and are therefore called diplococci or diplobacilli.

General Biology of Bacteria (cont.)


• Bacteria have a simple structure.– A single, closed circle chromosome contained

within the nucleoid– Additional circular DNA molecules (plasmids)– Ribosomes for protein synthesis– An outer cell wall reinforced with

peptidoglycan– For some bacteria, a flagella for locomotion.


• Bacteria and archaea reproduce by binary fission.– The circular chromosome replicates and

separates.– The enlarged cell is partitioned by the plasma

membrane and cell wall, forming two identical cells.

• Binary fission is not a mitotic process.


• Some bacteria can undergo conjugation using sex pili.

• Conjugation allows for a form of sexual recombination as genetic information is exchanged between bacteria.

• Bacteria can undergo transformation when they incorporate DNA from the environment into their own genome.


• When bacteriophages transmit viral DNA from one bacterial cell to another the process is called transduction.

• When bacteria experience unfavorable environmental conditions, they can form endospores.

• These endospores can remain dormant for thousands of years and still remain virulent.


• Like plants, cyanobacteria are photoautotrophs, using solar energy to synthesize carbohydrates.

• Some photosynthetic bacteria use H2S instead of water (H2O) during photosynthesis.


• Some bacteria are chemoautotrophs because they do not use solar energy to reduce CO2 to form carbohydrates.

• The electrons required for this are obtained from minerals such as iron.


• Like animals, most bacteria are chemoheterotrophs obtaining the necessary nutrients from external sources.

• However bacteria are saprotrophs, secreting enzymes to the external environment to help acquire nutrients.


• Bacteria can be free-living or symbiotic.

• The bacteria that reside in the nodules of legumes are symbiotic and assist the plant in acquiring nitrogen.

Environmental and Medical Importance of Bacteria

• One reason bacteria are important is that their activity helps nutrients cycle in the environment.

• Bacteria have long been used to process human wastes such as sewage.

• More recently, bacteria have been used to degrade pollutants in the environment, a process called bioremediation.

Environmental and Medical Importance of Bacteria (cont.)


• Bacteria are used to produce several products.– Alcoholic beverages– Cheese– Vitamins and

antibiotics

• Biotechnology can be used to engineer bacteria that produce chemicals and drugs.


• Bacteria that cause disease are called pathogens.

• Pathogens create disease in two ways.– Pathogens can produce toxins that cause

disease.– Pathogens can adhere to surfaces and/or

invade organs or cells, causing disease.

Archaea

• The archaea are one of the three domains of life.

• The archaea are bacteria that live in extreme environments.

• Eukaryotes are believed to be derived from the archaea.

Structure and Function

• Archaea have cellular modifications that allow them to survive extreme conditions.– The plasma membrane has unusual lipids that

allow the cell to function at high temperature.– The cell walls are very diverse, consisting in

some cases mostly of carbohydrates or protein.

• Archaea are chemoautotrophic.

Types of Archaea

• Methanogens are methane-synthesizing archaea found in anaerobic environments like swamps.

Types of Archaea (cont.)

• Halophiles are archaea that require high salt concentrations for proper growth.

Types of Archaea (cont.)

• The thermoacidophiles live in hot, acidic environments such as hot springs and thermal vents.

17.4 Protists

• Protists are aquatic organisms that show remarkable morphological variability.

• Protists are eukaryotic organisms that may be unicellular or multicellular.

General Biology of Protists

• Algae are photosynthesizing protists considered part of the aquatic phytoplankton.

• Algae can also be found on the surface of soils, rocks, and trees.

• Algae can form symbiotic relationships with other organisms.– Lichens– Coral

General Biology of Protists (cont.)

• Protozoans are unicellular chemoheterotrophs.

• Protozoans also have locomotory structures such as cilia, flagella, or pseudopods.

• Protozoans are part of the aquatic zooplankton.

• Some protozoans are human pathogens.

Algae

• As photosynthetic protists, algae have chloroplasts similar to those in plants.

• Algae have other organelles that are similar to those in plants.– Mitochondria– Vacuole– Cell wall

• Algae also have organelles called pyrenoids involved in starch storage and metabolism.

Algae (cont.)

Algae (cont.)

• The mechanisms of reproduction vary by algal species.– Algae can reproduce sexually.– Algae can reproduce via binary fission. – Algae can break up to form fragments called

zoospores that develop into algal cells.

Algae (cont.)

• There are four main categories of algae.– Green algae, such as Chlamydomonas,

Volvox, and Spirogyra.– Red algae, which are involved in the

formation of coral.– The diatoms are the golden-brown algae. – The brown algae, along with the green and

red, are seaweeds.

Algae (cont.)

Protozoans

• Protozoans are complex unicellular organisms.

• Protozoan organelles function in a manner analogous to animal organ systems.

• The nuclear structure varies in protozoans.– Some protozoa are multinucleated.– Some have a macro- and a micronucleus.

Protozoans (cont.)

• Protozoans reproduce by binary fission.

• Protozoans feed by phagocytosis, with the phagocytic vacuoles acting like a stomach.

• Contractile vacuoles act to maintain water balance (osmoregulation).

Protozoans (cont.)

Protozoans (cont.)

• The apicomplexa are immobile, spore-forming protozoans called sporozoans.

• Plasmodium is a protozoan transmitted by mosquitoes that is responsible for malaria.

• Ciliates, like Paramecium, use cilia for movement.

Protozoans (cont.)

• The amoeboids use pseudopods for movement.

• Marine ameobas such as the radiolarians and foraminiferans contributed to the formation of limestone formations.

• The ameoba Entamoeba histolytica is the cause of dysentery.

Protozoans (cont.)

• The zooflagellates are flagellated and may also cause human disease.– The trypanosome of one group is the cause of

African sleeping sickness.– Giardia can cause severe diarrhea.

Protozoans (cont.)

Slime Molds and Water Molds

• Slime molds are protists that contribute to the decomposition of plant material.

• Slime molds also feed on bacteria.

• Water molds are decomposers but are also animal and plant parasites.

Slime Molds and Water Molds (cont.)

• Water molds have a cell wall composed of chitin rather than cellulose.

• Slime and water molds both form spores.

• Slime and water molds are closely related to ameoboids and feed by phagocytosis.


• Plasmodial slime molds exist as a plasmodium, a diploid multi-nucleated cytoplasmic mass enclosed by a slime sheath.

• These slime molds decompose dead plant material.

• Under unfavorable conditions, the plasmodial slime mold develops spore-producing sporangia.


• The spores produced germinate when favorable conditions return to release a haploid flagellated or ameoboid cell.

• Two of these haploid cells can fuse to form a zygote that grows to form the plasmodium.

essentials of biology sylvia s. mader chapter 17 lecture outline prepared by: dr. stephen ebbs...

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