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Chapter 20
Viruses and Bacteria
Section 1: Viruses
Section 2: Bacteria
Section 1
Viruses
Objectives:
•Describe why a virus is not considered a living
organism.
•Summarize the discovery of the tobacco mosaic
virus.
•Describe the basic structure of a virus.
•Summarize the steps of viral replication.
•Explain how HIV infects immune system cells.
Section 1
Viruses
Is a Virus Alive?
•Discovery of Viruses Viruses do not have all the
characteristics of life and are therefore not
considered to be alive.
Section 1
Viruses
Viral Structure
•Characteristics of Viruses Viruses consist of
segments of a nucleic acid (DNA or RNA)
contained in a protein coat, and some have an
envelope.
Section 1
Viruses
Viral Reproduction
•Viral Replication Viruses replicate inside living
cells. They enter a cell by injecting their genetic
material into the cell, by slipping through tears in
the plant cell wall, or by binding to molecules on
the cell surface and triggering endocytosis.
•Lytic Cycle In bacteria viruses, the cycle of viral
infection, replication, and cell destruction is part of
the lytic cycle.
•Lysogenic Cycle In the lysogenic cycle, the viral
genome replicates without destroying the host cell.
Section 1
Viruses
Viral Reproduction continued
•Host Cell Specificity Viruses are often species
specific.
•Structure of HIV—an Enveloped Virus HIV
replicates inside only human immune system cells,
specifically white blood cells.
Section 1
Viruses
How HIV Infects Cells
•Attachment HIV binds to human immune cells that
contain CD4 receptors.
•Entry into Macrophages Macrophages contain
CD4 receptors and CCR5 co-receptors, and this
allows HIV to enter these cells.
•Replication HIV replicates inside the host
macrophage, but does not kill the cell. Instead, the
new viruses are released from the host by budding.
•AIDS HIV infection leads to the destruction of the
body’s T cells and weakens the immune system. This
can lead to AIDS.
Section 1
Viruses
Viral Diseases
•Emerging Viruses Emerging viruses are
geographically isolated viruses that cause disease
in humans.
•Prions and Viroids Viroids are infectious RNA
molecules that cause disease in plants, and prions
are infectious proteins that cause disease in certain
animals.
Section 2
Bacteria
Objectives:
•List seven differences between bacteria and
eukaryotic cells.
•Describe three different ways bacteria can obtain
energy.
•Describe the external and internal structure of
Escherichia coli.
•Distinguish two ways that bacteria cause disease.
•Identify three ways that bacteria benefit humans.
Section 2
Bacteria
Bacterial Structure
•Characteristics of Bacteria Bacteria differ from
eukaryotes in their cellular organization, cell
structures, and metabolic diversity. Bacteria are
grouped according to their ribosomal RNA
sequences and the way they obtain energy.
•Bacterial Cell Shapes A bacteria is one of three
shapes: rod-shaped (bacillus), round-shaped
(coccus), and spiral-shaped (spirillum).
Section 2
Bacteria
Bacterial Structure continued
•Gram Staining Bacteria can be classified
according to their cell wall structure. Gram staining
can be used to distinguish these two groups.
Section 2
Bacteria
Obtaining Energy
•Photosynthesizers Photosynthetic bacteria can be
classified into four major groups: purple nonsulfur
bacteria, green sulfur bacteria, purple sulfur
bacteria, and cyanobacteria.
•Chemoautotrophs Chemoautotrophs obtain
energy by removing electrons from inorganic
molecules such as ammonia, hydrogen sulfide, or
methane.
•Heterotrophs Most bacteria are heterotrophs and
many are aerobic.
Section 2
Bacteria
Pathogenic Bacteria
•Bacteria Can Metabolize Their Host Bacteria
can cause disease by metabolizing nutrients in their
host.
•Bacterial Toxins Bacteria can cause disease by
releasing toxins, which damage their host.
•Biowarfare Biowarfare is the deliberate exposure
of people to biological toxins or pathogens such as
bacteria or viruses.
Section 2
Bacteria
Antibiotics
•Fighting Bacteria Bacterial disease can be fought
with soap, chemicals, and antibiotics.
•Antibiotic-Resistant Bacteria Mutations that
confer resistance to antibiotics are strongly favored
in bacterial populations being treated with an
antibiotic.
Section 2
Bacteria
Importance of Bacteria
•Food and Chemical Production Bacteria are
used to make foods, antibiotics, and other useful
chemicals.
•Mining and Environmental Uses of Bacteria
Mining companies use bacteria to concentrate
desired elements from low-grade ore. Bacteria are
also used to clean the environment and cycle
important chemicals in the environment.