VIRUSES;SECTION 18-1

By, Mackenzie Pabst

What is a Virus?

disease-causing, nonliving particle composed of an inner core of nucleic acids surrounded by a capsid

A virus replicates inside living cells called host cells

Why are Viruses Non-Living? Viruses are nonliving because:

They don't exhibit all the criteria for lifeCriteria for life includes:

Respiration, growth and development All viruses can do is replicate

Replication is making exact copies of themselves with the help of living cells

How are Viruses Named?

Some Viruses are named after the disease they cause Example: Rabies

Virus Other viruses were

named for the organ or tissue they infect. Example: The

Adenovirus effects the adenoid tissue

Rabies Virus

How does a Virus attach to a Host Cell?

For a virus to replicate, it must enter a host cell

When a virus’ proteins interlock with the receptor site, the virus is attached to its host cell

The attachment process is like two pieces of a jigsaw puzzle fitting together

Virus Attachment

Each virus has a specifically shaped attachment protein

Viruses are species specific Some Viruses are cell-type specific

Example: polio viruses normally infect only intestinal and nerve cells.

Lytic Cycle:

A. Attachment:The Virus attaches to the Host Cell

B. Entry:The Bacteriophage injects its nucleic acid into the bacterial cell

C. Replication :The host’s metabolic machinery makes the virus’ nucleic acid and proteins

A Viral Replication Process

Lytic Cycle:

D. Assembly:New Virus Parts are made

E. Lysis and Release:The host cell breaks open and releases new viruses• This results in the

killing of the host cell

A Viral Replication Process

Another Viral Replication Process…

Of an Enveloped Virus: After attachment, the

membrane of the host cell surrounds the virus

This produces a virus-filled vacuole inside the host cell's cytoplasm.

The virus bursts out of the vacuole and releases its nucleic acid into the cell.

The Lysogenic Cycle

The Lysogenic Cycle:

Begins the same way as the Lytic Cycle

The virus attaches to the host cell and its nucleic acid is injected into the cell.

Then the viral DNA is integrated into the host cell's chromosomes This is done instead of immediately

taking over the host's genetic material (As in the Lytic Cycle)

This Cycle does not kill its host cell

RNA Viruses:

Many Viruses, including HIV, are RNA viruses This means their only nucleic acid is RNA

The RNA virus with the most complex replication cycle is the Retrovirus

The Retrovirus - uses reverse transcriptase to make DNA from viral RNA the retroviral DNA is then integrated into

the host cell's chromosome

Retrovirus Cycle:

Retrovirus Cycle:

Once inside a host cell, the retrovirus makes DNA from its RNA with reverse transcriptase

Reverse transriptase produces double-stranded DNA from the viral RNA. The double-stranded viral DNA

becomes part of the host cell’s chromosome and it becomes a provirus

Human Immunodeficiency Virus(HIV) HIV infects white blood cells in

humans The infected cells still function

normally Because the viral genetic material is a

provirus that only produces a small number of new viruses at a time

An infected person will not appear sick Can infect others with the virus through body

fluids

Human Immunodeficiency Virus(HIV) Most people with HIV

will get AIDS Over time, more white

blood cells are infected and killed

White Blood cells are essential to our immune system Their destruction

reduces the body’s ability to protect itself from other viruses

Virus’ link to Cancer:

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

Viruses can disrupt the normal growth and division of cells in a host This causes abnormal

growth and creates tumors

Prions: Are composed of

proteins but have no nucleic acid to carry genetic information

Are thought to act by causing other proteins to fold themselves incorrectly, resulting in improper functioning

Responsible for many animal diseases Example: Mad Cow

Disease

Viroids:

Composed of a single circular strand of RNA with no protein coat

Shown to cause infectious diseases in several plants.

Have less RNA than most viruses

Plant Viruses: There are more than

400 plant viruses Plant Viruses can

cause stunted growth and yield losses in their host plants.

The first virus to be identified as a plant virus was the tobacco mosaic virus

Plant Viruses:

Do not undergo lytic or lysogenic phases

Plant Viruses are named according to their host plant.

Not all viral plant diseases are fatal or even harmful.

The Mosaic virus, as seen in the following picture, cause patterns of color in the flowers of plants.