• Identify the different kinds of viruses and their structures.

Section Objectives:

• Compare and contrast the replication cycles of viruses.

• A vaccine is an injection of particles of viruses or bacteria that provide the human body with a defense against disease.

• You’ve probably had the flu—influenza—at some time during your life.

• Viruses are composed of nucleic acids enclosed in a protein coat and are smaller than the smallest

bacterium.

• Nonliving particles called viruses cause influenza.

What is a Virus?

Most biologists consider viruses to be nonliving because:

• They don’t carry out respiration, grow, or develop.

What is a Virus?

All viruses can do is replicate—make copies of themselves—and they can’t even do that without

the help of living cells.

• A cell in which a virus replicates inside of is called the host cell.

What is a Host Cell?

• Viruses, such as rabies viruses and polioviruses, were named after the diseases they cause.

• Other viruses were named for the organ or tissue they infect.

What are viruses named after?

• A virus that infects a bacterium is called a bacteriophage or “phage” for short.

What is a bacteriophage?

• A virus has an inner core of nucleic acid, either RNA or DNA

and an outer protein coat called a capsid.

Capsid

Nucleic acid

Viral Structure

• Some relatively large viruses, such as human flu viruses, may have an additional layer, called an envelope, surrounding their capsids.

Capsid

Nucleic acid

Envelope

Viral Structure

• Envelopes are composed primarily of the same materials found in the plasma membranes of all cells.

Polyhedral virus shape

Human Papilloma Virus

Envelope studded with projections

(flu and AIDS)

VIRUS SHAPES

Long narrow helical shape (Tobacco Mosaic

Virus)

Polyhedral-shaped head attached to a cylindrical tail

with leg-like fibers

VIRUS SHAPES

Before a virus can replicate, it must enter a host cell.

• A virus recognizes and attaches to a host cell when one of its proteins interlocks with a molecular shape that is the receptor site on the host cell’s plasma membrane.

Virus

• Each virus has a specifically shaped attachment protein. Therefore, each virus can usually attach to only a few kinds of cells.

• In general, viruses are species specific, and some also are cell-type specific. For example, polio viruses normally infect only intestinal and nerve cells.

Viral Attachment to Host

• The species specific characteristic of viruses is significant for controlling the spread of viral diseases.

For example, smallpox was easier to eradicate because it only affects humans (unlike the flu and West Nile that affect several types of animals.)

• Once attached to the plasma membrane of the host cell, the virus enters the cell and takes over its

metabolism.

Only then can the virus replicate.

Viruses have two ways of getting into host cells.

Viral Replication

• The virus may inject its nucleic acid into the host cell like a syringe injects a vaccine into your arm.

• The capsid of the virus stays attached to the outside of the host cell.

Viral Replication

• An enveloped virus (larger viruses) enter a host cell in a different way.

Viral Replication

• After attachment, the plasma membrane of the host cell surrounds the virus and produces a virus-filled vacuole inside the host cell’s cytoplasm.

• Then, the virus bursts out of the vacuole and releases its nucleic acid into the cell.

• The lytic cycle is a viral replication cycle in which a virus:

Click image to play movie

The Lytic Cycle

1. takes over a host cell’s genetic material

2. uses the host cell’s structures and energy to replicate the virus

3. then the host cell bursts, dies, and releases the replicated viruses.

Nucleic acid

Bacterial host cell

Bacteriophage Bacterial DNA

B. Entry

The bacteriophage injects its nucleic acid into the bacterial cell.

A. Attachment

C. ReplicationD. Assembly

E. Lysis and Release

The host’s metabolic machinery makes viral nucleic acid and proteins.

New virus particles are assembled.

The host cell breaks open and releases new virus particles.

The Lytic Cycle

• A lysongenic cycle begins in the same way as a lytic cycle.

• However, in a lysogenic cycle, instead of immediately taking over the host’s genetic material, the viral DNA is integrated into the host cell’s chromosome.

The Lysogenic Cycle

LYSOGENIC CYCLE

LYTIC CYCLE The provirus leaves the chromosome.

Viral nucleic acid and proteins are made.

The cell breaks open releasing viruses.

A lysogenic virus injects its nucleic acid into a bacterium.

Bacterial host chromosome

A. Attachment and Entry

B. Provirus Formation

Provirus

The viral nucleic acid is calleda provirus when it becomespart of the host’s chromosome.

C. Cell Division

Althoughthe provirusis inactive,it replicatesalong withthe host cell’schromosome.

The Lysogenic Cycle

• Viral DNA that is integrated into the host cell’s chromosomes is called a provirus.

• A provirus may not affect the functioning of its host cell, which continues to carry out its own metabolic activity.

• However, every time the host cell reproduces, the provirus is replicated along with the host cell’s

chromosome.

The Lysogenic Cycle

• Therefore, every cell that originates from an infected host cell has a copy of the provirus.

• The lysogenic phase can continue for many years. However, at any time, the provirus can be activated and enter a lytic cycle.

The Lysogenic Cycle

• Many disease-causing viruses have lysogenic cycles.

• Three examples of these viruses are herpes simplex I, herpes simplex II that causes genital herpes, and the hepatitis B virus that causes hepatitis B.

Viruses with Lysogenic Cycles

• Another lysogenic virus is the one that causes chicken pox.

• Having chicken pox, which usually occurs before age ten, gives lifelong protection from another infection by the virus.

• However, some chicken pox viruses may remain as proviruses in some of your body’s nerve cells.

• Later in your life, these proviruses may enter a lytic cycle and cause a disease called shingles—a painful infection of some nerve cells.

• Either lysis, the bursting of a cell, or exocytosis, the active transport process by which materials are expelled from a cell, release new viruses from the host cell.

Releasing new viruses from host cells

• The RNA virus with the most complex replication cycle is the retrovirus

• Many viruses have only RNA as their nucleic acid.

What is a retrovirus?

• Once inside a host cell, the retrovirus makes DNA from its RNA.

• To do this, it uses reverse transcriptase, an enzyme it carries inside its capsid.

What is a retrovirus?

• This enzyme helps produce double-stranded DNA from the viral RNA.

• Then the double-stranded viral DNA is integrated into the host cell’s chromosome and becomes a provirus.

Retrovirus Cycle

Exiting cell

Reverse transcriptase

Retrovirus

Enteringcell

RNA

RNA

DNA

DNA is made from the viral RNA.

mRNA

New virus parts

New virus forming

Provirus in host chromosome

Retrovirus

• Once inside a human host, HIV infects white blood cells.

Normal white blood cell

HIV

• Newly made viruses are released into the blood stream by exocytosis and infect other white blood cells.

HIV

• Infected host cells still function normally because the viral genetic material is a provirus that produces only a small number of new viruses at a time.

• Because the infected cells are still able to function normally, an infected person may not appear sick, but they can still transmit the virus in their body fluids.

HIV

• Some viruses have been linked to certain cancers in humans and animals.

• These viruses disrupt the normal growth and division of cells in a host, causing abnormal growth and

creating tumors.

Cancer and Viruses

HPV

• Another example is Hepatitis B, which has been shown to play a role in developing liver cancer.

Cancer and Viruses

• this woman is not pregnant• she has hepatitis B and is suffering from liver cancer• photo taken in a Thailand refugee camp• this woman was a Cambodian refugee• she died 4 months after she arrived in the camp (average life  expectancy after diagnosis of liver cancer is 6 months)

• Prions are thought to act by causing other proteins to fold themselves incorrectly, resulting in improper functioning.

Prions

• Prions are proteins that behave like viruses, but do not carry genetic information.

• Prions are responsible for many animal diseases, such as mad cow disease and its human equivalent, Creutzfeldt-Jakob disease.

Prions

• Viroids are composed of a single circular strand of RNA with no protein coat.

• Viroids have been shown to cause infectious diseases in several plants.

• The amount of viroid RNA is much less than the amount found in viruses.

Viroids

• The first virus to be identified was a plant virus, called tobacco mosaic virus, that causes disease in tobacco plants.

Tobacco mosaic virus causes yellow spots on tobacco leaves, making them unmarketable.

• Not all viral plant diseases are fatal or even harmful.

• Some mosaic viruses cause striking patterns of color in the flowers of plants.

Rembrandt tulips

Plant Viruses

Question 1Which of the following is NOT a reason that viruses are considered to be nonliving?

D. Viruses don’t develop.C. Viruses don’t grow.B. Viruses don’t respire.A. Viruses don’t replicate.

The answer is A.

Question 2Which is NOT a component of a virus?

D. phageC. DNA B. capsid A. RNA

The answer is D.

Question 3What two ways do viruses have of getting into host cells?

AnswerThe virus can inject its nucleic acid into the host cell, or attach to the host cell’s membrane and become surrounded by the membrane and placed in a vacuole. The virus then bursts out of the vacuole and releases its nucleic acid into the cell.

Question 4In the lytic cycle, after the host’s metabolic machinery makes viral nucleic acid and proteins the next phase is _______.

D. attachment

C. assembly

B. replication

A. lysis and release

The answer is C. In the assembly phase, the new virus particles are assembled.

Question 5With lysogenic viruses, what two phases of the lytic cycle are replaced by the lysogenic cycle?

D. attachment and entry

C. assembly and lysis and release

B. replication and assemble

A. entry and replication

The answer is D.

LYSOGENIC CYCLE

LYTIC CYCLE

A. Attachment and Entry

Question 6Explain why you can be infected with a virus but may have no symptoms of disease for years after the initial infection.

AnswerThe virus enters a lysogenic phase remaining inactive but replicating along with the host cell’s chromosomes. Eventually, the virus enters a lytic phase where it destroys its host cells and causes symptoms of disease.

Question 7What is the difference between lysis and exocytosis with respect to host cells that contain viruses? (TX Obj 3; 4C)

AnswerLysis, the bursting of the host cell, is caused when viruses break out of it. In exocytosis, the virus is enclosed in a vacuole that then fuses with the host cell’s plasma membrane. The virus is then released to the outside.

Question 8

What is the importance of reverse transcriptase to a retrovirus? (TX Obj 3; 4C)

Answer

The enzyme reverse transcriptase allows the retrovirus to make DNA from its RNA so the DNA may attach to the chromosomes of the host cell and divide with the host cell.

Question 9Particles that are composed of proteins but have no nucleic acid to carry genetic information are _______.

D. retroviruses C. viroids

B. prionsA. proviruses

The answer is B.