Transcript Essentials of Biology Sylvia S. Mader

Essentials of Biology
Sylvia S. Mader
Chapter 17
Lecture Outline
Prepared by: Dr. Stephen Ebbs
Southern 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.
17.1 The Viruses (cont.)
17.1 The Viruses (cont.)
• 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.
Reproduction of Bacteriophages
(cont.)
• 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.
Reproduction of Bacteriophages
(cont.)
• 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.
Reproduction of Bacteriophages
(cont.)
• 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.
Reproduction of Bacteriophages
(cont.)
• 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.
Reproduction of Bacteriophages
(cont.)
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.)
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.
General Biology of Bacteria
(cont.)
General Biology of Bacteria
(cont.)
• 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.
General Biology of Bacteria
(cont.)
General Biology of Bacteria
(cont.)
• 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.
General Biology of Bacteria
(cont.)
• 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.
General Biology of Bacteria
(cont.)
General Biology of Bacteria
(cont.)
• Like plants,
cyanobacteria are
photoautotrophs, using
solar energy to
synthesize
carbohydrates.
• Some photosynthetic
bacteria use H2S instead
of water (H2O) during
photosynthesis.
General Biology of Bacteria
(cont.)
• 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.
General Biology of Bacteria
(cont.)
• 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.
General Biology of Bacteria
(cont.)
• Bacteria can be freeliving 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.)
Environmental and Medical
Importance of Bacteria (cont.)
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.
Environmental and Medical
Importance of Bacteria (cont.)
• 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.)
Algae (cont.)
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, sporeforming 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.)
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.
Slime Molds and Water Molds
(cont.)
• 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.
Slime Molds and Water Molds
(cont.)
• 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.
Slime Molds and Water Molds
(cont.)
Slime Molds and Water Molds
(cont.)