Ch. 19 Biology. Bacteria are prokaryotes w ith cells much smaller and more simply organized than...

Ch. 19Biology

Bacteria are prokaryotes with cells much smaller and more simply organized than those of eukaryotes

Viruses are smaller and simpler than bacteria

Virus Bacterium

Animalcell

Animal cell nucleus0.25 µm

Tobacco mosaic disease stunts growth of tobacco plants and gives their leaves a mosaic coloration

In the late 1800s, researchers hypothesized that a particle smaller than bacteria caused the disease

In 1935, Wendell Stanley confirmed this hypothesis by crystallizing the infectious particle, now known as tobacco mosaic virus (TMV)

Viruses are particles of nucleic acid, protein, and in some cases, lipids.

Viruses can replicate only by infecting living cells.

Copyright Pearson Prentice Hall

Viruses differ widely in terms of size and structure.

All viruses enter living cells and use the infected cell to produce more viruses.

• A typical virus is composed of a core of DNA or RNA surrounded by a protein coat.

• A capsid is the virus’s protein coat.

DNA viruses• Genome is DNA

RNA viruses• Genome is RNA• Smaller than DNA viruses• Lack of proofreading—leads to evolution

Capsid proteins bind to receptors on the cell surface and “trick” the cell into allowing it inside.

Once inside, viral genes are expressed and the cell transcribes and translates them into viral capsid proteins.

The host cell may makes copies of the virus, and be destroyed.

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Most viruses are highly specific to the cells they infect.

Viruses that infect bacteria are called bacteriophages.

Viruses that infect bacteria

Set in motion a genetic takeover of bacteria, such as Escherichia coli

Once the virus is inside the host cell, two different processes may occur. • Some viruses replicate immediately, killing

the host cell (lytic cycle). • Others replicate, but do not kill the host cell

immediately (lysogenic cycle).

Lytic Infection• In a lytic infection, a virus enters a cell,

makes copies of itself, and causes the cell to burst.

First, the bacteriophage injects DNA into a bacterium.

The bacteriophage DNA forms a circle.

Phage DNA takes control and has cell make more phage DNA and proteins

Phage DNA and proteins are assembled into new bacteriophages

The cell lyses (bursts) and releases bacteriophages

Lysogenic Infection • Other viruses cause lysogenic infections in

which a host cell makes copies of the virus indefinitely.

• In a lysogenic infection, a virus integrates its DNA into the DNA of the host cell, and the viral genetic information replicates along with the host cell's DNA.

• A lysogenic infection begins the same way as a lytic infection.

• The bacteriophage injects DNA into a bacterium.

• The bacteriophage DNA forms a circle.

• The viral DNA embedded in the host's DNA is called a prophage.

• Bacterial cell can divide normally through binary fission (copying the phage DNA as well).

• Phage DNA can exit the bacterial chromosome and then enter the lytic cycle.

Retroviruses contain RNA as their genetic information.

When retroviruses infect cells, they make a DNA copy of their RNA.

This DNA is inserted into the DNA of the host cell.

A retrovirus’ genetic information is copied backward—from RNA to DNA.

The virus that causes AIDS is a retrovirus.• Human

Immunodeficiency Virus (HIV)

Viruses are NOT living things.• Viruses are not made of cells.• Viruses must infect a living cell in order to

grow and replicate.• They have no metabolism--use host’s

respiration, nutrition, and all other functions of living things.

Viruses have many of the characteristics of living things.

After infecting living cells, viruses can replicate, regulate gene expression, and even evolve.

Because viruses are dependent on living things, it seems likely that viruses developed after living cells.

The first viruses may have evolved from genetic material of living cells.

Viruses have continued to evolve over billions of years.

Viruses that contain RNA as their genetic information are known as

a) prions.b) oncoviruses.c) retroviruses.d) bacteriophage.

The first type of virus to be studied was the a) bacteriophage.b) tobacco mosaic virus.c) influenza virus.d) AIDS virus.

Which of the following statements about viruses is true?a) Viruses appear similar to bacteria when studied

with a light microscope.b) Viruses display the essential characteristics of

living things.c) Viruses can reproduce independently if they

contain DNA.d) Viruses cannot reproduce unless they infect a

living cell.

A virus integrates its DNA into the DNA of the host cell but remains inactive for a while in

a) a lytic infection.b) a lysogenic infection.c) neither a lytic nor a lysogenic infection.d) retroviral infection.

Retroviruses are considered unique becausea) they have RNA in their capsid and not DNA.b) they have DNA in their capsid and not RNA.c) after infection of a host cell, their RNA makes

DNA.d) after infection of a host cell, their DNA makes

Ch. 19Biology

Bacteria and viruses are everywhere in nature, but only a few cause disease.

Disease-causing agents are called pathogens.

Bacterial Disease in Humans Growth of pathogenic bacteria disrupts the

body’s equilibrium by interfering with its normal activities and producing disease.

Bacteria produce disease in one of two general ways.• Some bacteria damage the cells and tissues of the infected organism directly by breaking down the cells for food.

• Other bacteria release toxins (poisons) that travel throughout the body interfering with the normal activity of the host.

Many bacterial diseases can be prevented by vaccines.

A vaccine is a preparation of weakened or killed pathogens.

When injected into the body, a bacteria/virus may prompt the body’s immunity to the disease.

Immunity is the body's ability to destroy new pathogens.

• If infection occurs, drugs can be used to destroy bacteria.

• These drugs include antibiotics, which are compounds that block the growth and reproduction of bacteria.

• A reason for increased human life expectancy is an increased understanding of how to prevent and cure bacterial infections.

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• There are various methods used to control bacterial growth, including: sterilization disinfectants food processing

• Sterilization by Heat Sterilization destroys bacteria by subjecting

them to great heat. Most bacteria are killed by prolonged high

temperatures.

• Disinfectants Disinfectants are chemical solutions that kill

pathogenic bacteria. They are used to clean rooms where bacteria

may flourish.

• Food Storage and Processing Bacteria can cause food to spoil. Refrigerated food stays fresh longer because the

bacteria will take longer to multiply. Boiling, frying, or steaming can sterilize certain

foods. Heat kills the bacteria

Viruses produce disease by disrupting the body's normal equilibrium.

Viruses can attack and destroy certain cells in the body, causing symptoms of the disease.

Other viruses cause infected cells to change patterns of growth and development.

Viral diseases cannot be treated with antibiotics.

Vaccines are often the best protection against most diseases.

Most vaccines work only if used before an infection begins.

Symptoms may be treated with over-the-counter medicines.

Viral Disease in Plants and Animals Viruses produce serious animal diseases

including foot-and-mouth disease. Many viruses infect plants. These viruses pose a serious threat to many

crops.

Antibiotics don’t work—no metabolic reactions to interfere with

Vaccines—harmless derivatives of pathogenic microbes that stimulate the immune system to mount defenses against the actual pathogen• Parts of viruses, modified or killed viruses

are injected into the body• Allows immune system to make antibodies

against specific markers on the viral coat HIV mutates too fast for immune system to keep up

Influenza, also known as the flu, is a contagious disease that is caused by the influenza virus. It attacks the respiratory tract in humans (nose, throat, and lungs). The flu is different from a cold. Influenza usually comes on suddenly and may include these symptoms: • Fever • Headache • Tiredness (can be

extreme) • Dry cough • Sore throat • Nasal congestion • Body aches

Emerging viruses are those that appear suddenly or suddenly come to the attention of scientists

Severe acute respiratory syndrome (SARS) appeared in China

Outbreaks of “new” viral diseases in humans are usually caused by existing viruses that expand their host territory

Young ballet students in HongKong wear face masks toprotect themselves from thevirus causing SARS.

The SARS-causing agent is acoronavirus like this one(colorized TEM), so named forthe “corona” of glyco-proteinspikes protruding form theenvelope.

Biologists know that bacteria can cause human disease bya) entering cells and using the cell to make new

bacteria.b) producing toxic substances that interfere with

normal cell function.c) decomposing the remains of dead organisms.

d) changing atmospheric nitrogen into nitrogen compounds.

A process that destroys bacteria by subjecting them to great heat is known as a) refrigeration.b) sterilization.c) pickling.d) boiling.

Which of the following diseases is transmitted by a mosquito bite?

a) influenzab) measlesc) West Nile virusd) chickenpox

The best way to combat viral diseases isa) to use antibiotics.b) to treat individual symptoms.c) to use preventive vaccines.d) to let the disease “cure itself.”

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