Chapter 20 Viruses and Bacteria Section 1: Viruses Section 2: Bacteria

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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?•Viruses do not have all the characteristics of life and are therefore not considered to be alive.•Viruses do not grow, do not have homeostasis and do not metabolize.•Viruses are pathogens-agents that cause disease.



Section 1

Viruses

Viral Structure•Viruses consist of segments of a nucleic acid (DNA or RNA) contained in a protein coat, or capsid, and some have an envelope. It consists of lipids, and glycoproteins, which are proteins attached to carbohydrate molecules



Section 1

Viruses

Viral Reproduction•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 are geographically isolated viruses that cause disease in humans.•Other classes of pathogens:

•Prions are infectious proteins that cause disease in certain animals. They are composed of proteins but have no nucleic acid. They cause normal proteins to misfold.•Viroids are infectious RNA molecules that cause disease in plants. They are a single strand of RNA that has no capsid.



Section 2

BacteriaObjectives:•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

BacteriaBacterial Structure•Characteristics of Bacteria Bacteria differ from eukaryotes in their

•Internal compartmentalization-no nucleus•Cell size-typically much smaller •Multicellularity-all bacteria are single cells•Chromosomes-circular in shape•Reproduction-binary fission•Flagella-simple structures composed a single protein fiber

•Pili-shorter, thicker outgrowths that allow bacteria to attach to surfaces

•Metabolic diversity-Anaerobic and Aerobic processes

•Bacterial Cell Shapes A bacteria is one of three shapes: rod-shaped (bacillus), round-shaped (coccus), and spiral-shaped (spirillum)



Section 2

BacteriaBacterial Structure continued•Bacteria can be classified according to their cell wall structure. Gram staining can be used to distinguish these two groups.

•Gram positive•Gram negative

•Antibiotics are chemicals that interfere with the life processes in bacteria. Gram staining determines which antibiotics are most effective.

Key Terms:

•Capsule: a gel-like layer found outside the cell wall and membrane.

•Endospores: a thick walled protective spore that forms inside a bacterial cell and resists harsh conditions.

•Conjugation: a process in which two organisms exchange genetic material.



Section 2

BacteriaObtaining Energy•Photosynthetic bacteria can be classified into four major groups: purple nonsulfur bacteria, green sulfur bacteria, purple sulfur bacteria, and cyanobacteria.

•Green sulfur and Purple sulfur grow in anaerobic conditions.

•Chemoautotrophs obtain energy by removing electrons from inorganic molecules such as ammonia, hydrogen sulfide, or methane.•Most bacteria are heterotrophs and many are aerobic.



Section 2

BacteriaPathogenic Bacteria•Bacteria can cause disease by metabolizing nutrients in their host. They secrete enzymes that break down organic structures.

•Ex. Mycobacterium tuberculosis •Bacteria can cause disease by releasing toxins, which damage their host.

•Ex. Stapylococcus aureus•Biowarfare is the deliberate exposure of people to biological toxins or pathogens such as bacteria or viruses.



Section 2

BacteriaAntibiotics•Bacterial disease can be fought with soap, chemicals, and antibiotics.•Mutations that confer resistance to antibiotics are strongly favored in bacterial populations being treated with an antibiotic.



Section 2

BacteriaImportance of Bacteria•Bacteria are used to make foods, antibiotics, and other useful chemicals.•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.

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Chapter 20 Viruses and Bacteria Section 1: Viruses Section 2: Bacteria