biologist wendell stanley of the rockefeller institute purified tobacco mosaic virus (TMV)

• An animal virus enters its host cell by endocytosis.• A bacterial virus, or bacteriophage, punches a hole in the bacterial cell wall and injects its DNA into the cell... Viral Reproduc

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Chapter Presentation

Transparencies Standardized Test Prep

Visual Concepts

Table of Contents

Section 1 Viruses

Section 2 Bacteria

• Describe the basic structure of a virus.

• Summarize the steps of viral replication

• Explain how HIV infects immune system cells

• Viruses are pathogens—agents that cause disease.

• Viruses do not grow, do not have homeostasis, and

do not metabolize

Virus

Pathogen

Is a Virus Alive?, continued

Discovery of Viruses

• Near the end of the nineteenth century, scientists were trying to find the cause of tobacco mosaic

disease, which stunts the growth of tobacco plants

• In 1935, biologist Wendell Stanley of the Rockefeller Institute purified tobacco mosaic virus (TMV) and determined that the purified virus is a crystal

• Stanley concluded that TMV is a chemical rather

than an organism

Viral Structure

• The virus protein coat, or capsid, may contain either RNA or DNA, but not both

• Many viruses have a membrane, or envelope,

surrounding the capsid

• The envelope helps the virus enter cells It consists of proteins, lipids, and glycoproteins, which are

proteins with attached carbohydrate molecules that are derived from the host cell

Parts of a Virus

Viral Structure, continued

• Some viruses are long rods that form filaments

• Spherical viruses are typically studded with

Structures of

TMV and

Influenza Virus

Structures of

Adenovirus and

Bacteriophage

Bacteriophage

• An animal virus enters its host cell by endocytosis.

• A bacterial virus, or bacteriophage, punches a hole in the bacterial cell wall and injects its DNA into the cell

Viral Reproduction, continued

Lytic Cycle

• In bacterial viruses, the cycle of viral infection,

replication, and cell destruction is called the lytic

cycle

• After the viral genes have entered the cell, they 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 is broken open and releases newly made viruses

Lytic Cycle

Viral Reproduction, continued

• In this cycle, called the lysogenic cycle, the viral

genome replicates without destroying the host cell

Prophages and Proviruses

Lysogenic Cycle

Viral Replication in Bacteria

Lysogenic Cycles

Viral Reproduction, continued

Host Cell Specificity

• Viruses are often restricted to certain kinds of cells.

• Viruses may have originated when fragments of host

genes escaped or were expelled from cells

• The hypothesis that viruses originated from a variety

of host cells may explain why there are so many

different kinds of viruses Biologists think there are at least as many kinds of viruses as there are kinds of organisms

Viral Reproduction, continued

Structure of HIV—an Enveloped Virus

• The human immunodeficiency virus (HIV) causes

acquired immune deficiency syndrome (AIDS)

• Within HIV’s envelope lies the capsid, which in turn encloses the virus’s genetic material

• In the case of HIV, the genetic material is composed

of two molecules of single-stranded RNA

How HIV Infects Cells

How HIV Infects Cells, continued

Entry into Macrophages

• HIV cannot enter a cell merely by docking onto a CD4 receptor Rather, the glycoprotein must also activate a second co-receptor, called CCR5

• It is this event at CCR5 that starts endocytosis.

• Because human macrophages possess both CD4 and CCR5 receptors, HIV can enter macrophages

How HIV Infects Cells, continued

Replication

• Once inside a cell, the HIV particle sheds its capsid.The particle then releases an enzyme called reverse transcriptase

• Reverse transcriptase copies the naked viral RNA

into a complementary DNA version

• Translation of the viral DNA by the host cell’s

machinery directs the production of many copies of the virus

Infection of

Macrophage

by HIV

How HIV Infects Cells, continued

AIDS

• For years after the initial infection, HIV continues to replicate (and mutate) Eventually and by chance, HIV’s surface glycoproteins change to the point that they now recognize a new cell surface receptor This receptor is found on the subset of lymphocytes

called T cells.

• Unlike its activity in macrophages, HIV reproduces in T cells and

then destroys them.

• It is this destruction of the body’s T cells that blocks the body’s

immune response and signals the onset of AIDS.

Syndrome)

Viral Diseases

• Perhaps the most lethal virus in human history is the

influenza virus

• Certain viruses can also cause some types of cancer.

• Viruses associated with human cancers include

hepatitis B (liver cancer), Epstein-Barr virus (Burkitt’s lymphoma), and human papilloma virus (cervical

cancer)

Important Viral Diseases

Viral Diseases, continued

Emerging Viruses

• Viruses that evolve in geographically isolated areas and are pathogenic to humans are called emerging viruses

• These new pathogens are dangerous to public

health People become infected when they have

contact with the normal hosts of these viruses

• Examples of emerging viruses include West Nile virus and hantavirus

Viral Diseases, continued

Prions and Viroids

• Prions are composed of proteins but have no nucleic acid A disease-causing prion is folded into a shape that does not allow the prion to function

• Contact with a misfolded prion will cause a normal prion to misfold, too In this way the misfolding

spreads.

• A viroid is a single strand of RNA that has no capsid Viroids are important infectious disease agents in

plants.

• Distinguish two ways that bacteria cause disease.

• Identify three ways that bacteria benefit humans

Bacteria

Characteristics of Bacteria

Bacterial Structure

• Bacteria differ from eukaryotes in at least seven

ways

• Bacteria are prokaryotes. Unlike eukaryotes,

prokaryotes lack a cell nucleus

• Most bacterial cells are about 1 µm in diameter; most eukaryotic cells are more than 10 times that size

Bacterial Structure, continued

• All bacteria are single cells.

• Bacterial chromosomes consist of a single circular

piece of DNA Eukaryotic chromosomes are linear pieces of DNA that are associated with proteins

• Bacteria reproduce by binary fission, a process in

which one cell pinches into two cells

Bacterial Structure, continued

• Bacterial flagella are simple structures composed of a single fiber of protein that spins like a corkscrew to move the cell

• Some bacteria also have shorter, thicker outgrowths

called pili.

• Bacteria have many metabolic abilities that

eukaryotes lack For example, bacteria perform

several different kinds of anaerobic and aerobic

processes, while eukaryotes are mostly aerobic

organisms

Structure of Cilia and Flagella

Pilus

Structural Characteristics of a Bacterial Cell

Parts of a Prokaryotic Cell

and Multicellular

Bacterial Cell Shapes

• A bacterial cell is usually one of three basic shapes:

bacillus, a rod-shaped cell; coccus, a round-shaped cell; or spirillum, a spiral cell

• Members of the kingdom Eubacteria have a cell wall

made of peptidoglycan, a network of polysaccharide molecules linked together with chains of amino acids

• Outside the cell wall and membrane, many bacteria

have a gel-like layer called a capsule.

Three Bacterial Cell Shapes

Bacterial Capsule

Bacterial Cell Shapes, continued

• Eubacteria can have two types of cell walls,

distinguished by a dye staining technique called the

Gram Staining

Gram Stain

Bacterial Cell Shapes, continued

• Some bacteria form thick-walled endospores around

their chromosomes and a small bit of cytoplasm

when they are exposed to harsh conditions

• Pili enable bacteria to adhere to the surface of

sources of nutrition, such as your skin Some kinds of pili enable bacteria to exchange genetic material

through a process called conjugation

• Conjugation is a process in which two organisms

exchange genetic material

Conjugation

Escherichia coli

Obtaining Energy

Photosynthesis

• Photosynthetic bacteria can be classified into four major groups based on the photosynthetic pigments they contain: purple nonsulfur bacteria, green sulfur bacteria, purple sulfur bacteria, and cyanobacteria

• Green sulfur bacteria and purple sulfur bacteria grow

in anaerobic environments

• Cyanobacteria are thought to have made the Earth’s oxygen atmosphere

Obtaining Energy, continued

Chemoautotrophs

• Bacteria called chemoautotrophs obtain energy by

removing electrons from inorganic molecules such as ammonia and hydrogen sulfide or from organic

molecules such as methane

• In the presence of one of these hydrogen-rich

chemicals, chemoautotrophic bacteria can

manufacture all their own amino acids and proteins

Chemoautotroph

Obtaining Energy, continued

Heterotrophs

• Most bacteria are heterotrophs.

• Many are aerobic, that is, they live in the presence of

oxygen Some other bacteria can live without oxygen

• Together with fungi, heterotrophic bacteria are the

principal decomposers of the living world; they break down the bodies of dead organisms and make the

nutrients available to other organisms

Pathogenic Bacteria

Bacteria Can Metabolize Their Host

• Heterotrophic bacteria obtain nutrients by secreting

enzymes that break down complex organic structures

in their environment and then absorbing them If that environment is your throat or lungs, this can cause serious problems

• Several common bacterial diseases include dental

cavities, strep throat, tuberculosis, and acne

Important Bacterial Diseases

Pathogenic Bacteria, continued

Bacterial Toxins

• The second way bacteria cause disease is by

secreting chemical compounds into their environment These chemicals, called toxins, are poisonous to

eukaryotic cells.

• When bacteria grow in food and produce toxins, the

toxins can cause illness in humans who eat those

contaminated foods.

• Most bacteria can be killed by boiling water or various chemicals.

Pathogenic Bacteria, continued

Biowarfare

• Biowarfare is the deliberate exposure of people to

biological toxins or pathogens such as bacteria or

viruses

• Biologists are working on new approaches to

recognize the onset of an attack with a bioweapon, to treat infected people, and to slow the spread of any outbreak of disease

• In 1928, the British bacteriologist Alexander Fleming

discovered the antibiotic penicillin

• Today different antibiotics are used to interfere with

different cellular processes

• Because these processes do not occur in viruses,

antibiotics are not effective against them

Antibiotics, continued

Antibiotic-Resistant Bacteria

• Some bacteria have become resistant to antibiotics.

• Susceptible bacteria are eliminated from the population, and

resistant bacteria survive and reproduce, thus passing on their resistance traits.

• Usually, if the full course of the antibiotic is administered, all the

targeted bacteria are killed and there is no chance for a

resistant strain to develop If antibiotic treatment ends

prematurely, some of the more-resistant bacteria may survive and reproduce.

Importance of Bacteria

Food and Chemical Production

cheese, sauerkraut, olives, vinegar, and sourdough

different kinds of chemicals for industrial uses

• Genetic engineering companies use genetically

engineered bacteria to produce their many products, such as drugs for medicine and complex chemicals for research

Bacteria and Food

Importance of Bacteria, continued

Mining and Environmental Uses of Bacteria

• Mining companies can use bacteria to concentrate

desired elements from low-grade ore

• Bacteria metabolize different organic chemicals and

are therefore used to help clean up environmental disasters such as petroleum and chemical spills

• Powders containing petroleum-metabolizing bacteria

are used to help clean oil spills

Multiple Choice

The diagram below illustrates viral replication in

bacteria Use the diagram to answer questions 1–3

Multiple Choice, continued

1 Which type of cycle is represented in the diagram?

A aerobic

B anaerobic

C lysogenic

D lytic

Multiple Choice, continued

1 Which type of cycle is represented in the diagram?

A aerobic

B anaerobic

C lysogenic

D lytic

Multiple Choice, continued

2 What is the virus doing to the bacterium in step 1?

F injecting its capsid

G injecting its DNA

H withdrawing proteins

J withdrawing DNA

Multiple Choice, continued

2 What is the virus doing to the bacterium in step 1?

F injecting its capsid

G injecting its DNA

H withdrawing proteins

J withdrawing DNA

Multiple Choice, continued

3 What is happening in step 4?

A Viruses are entering the bacterium through a

hole in the cell membrane

B Viruses are repairing the cell membrane using viral proteins

C Newly formed viruses are being released from the bacterium

D The bacterium is ejecting its own chromosome inside a capsid

Multiple Choice, continued

3 What is happening in step 4?

A Viruses are entering the bacterium through a

hole in the cell membrane

B Viruses are repairing the cell membrane using viral proteins

C Newly formed viruses are being released from the bacterium

D The bacterium is ejecting its own chromosome inside a capsid