VIRUSES

WHAT IS A VIRUS? Very small

Non-living particle

Made up of nucleic acid and proteins

Can only reproduce by infecting living cells

Do not carry out respiration, grow or develop

STRUCTURE Viruses have an inner core which

contains either RNA or DNA

The core consists of the viruses genetic material.

Has outer coat called a capsid that is made up of proteins

STRUCTURE

Nucleic Acid: DNA or RNA

Capsid(Protein)

Viruses have different shapes and sizes.

The arrangement of the proteins in the capsid of a virus determines the virus’s shape

HOST CELL The cell in which a virus replicates Before a virus can replicate it must

enter a host cell

Before it can enter a host cell it must first recognize and attach to a receptor site on the plasma membrane of the host cell.

VIRAL INFECTION Most viruses are highly specific to the cells

they infect. Ex: Plant viruses only infect plants

Bacteriophages are viruses that infect bacteria Ex: bacteriophage T4

Viruses have two ways of infecting the host cell: -Lytic Infection -Lysogenic Infection

BACTERIAPHAGE T4

LYTIC INFECTION Lytic Cycle

virus injects its nucleic acid into the host takes over the host cells metabolismcauses host cell to start replicating the

viruses proteins and nucleic acids virus secretes enzymes that “lyse” the host

cell’s cell wall and releases new virus particles

these particles can attack other host cells

Lytic Infection causes host cell to be “lysed” and destroyed.

LYSOGENIC INFECTION Lysogenic cycle

Lysogenic virus injects its nucleic acid into a host cell

Viral DNA is integrated into the host cell’s chromosome→ provirus

Every time host cell reproduces, the provirus is replicated along with it → every cell that originates from the infected host cell has a copy of the provirus

The lysogenic cycle can take place for many years

At any time the provirus can be activated and enter a lytic cycle

↓ host cell is killed

VIRUSES AND DISEASES Viruses cause human diseases such as

polio, measles , influenza, and AIDS.

Viruses attack and destroy certain cells in the body → causing symptoms of the disease

The best way to prevent against viral diseases is to prevent infection through vaccination.

WHAT IS A VACCINE? A preparation of a weakened or killed

virus or viral protein

When injected into the body, it stimulates the immune system in an effort to create permanent immunity to the disease

For vaccines to be most effective it must be used before an infection begins.

Vaccines are used for the prevention of viral infections

Vaccines are NOT used for bacterial infections

Disease Symptom IncubationMeasles Rash, fever 9-11 daysShingles Pain, itching on

skinYears

Warts Bumpy areas on skin

Months

Coryza (cold) Sneezing, runny nose, fever

2-4 days

HIV Fatigue, weight loss

2-5 years

The symptoms and incubation time of a disease can indicate how the virus acts inside its host cell.

Which diseases may be caused by lytic viruses?Which diseases may be caused by lysogenic viruses?

RETROVIRUSES Viruses that contain RNA as their genetic

material

When retroviruses infect a cell they produce a DNA copy of their RNA.

Called retroviruses because their genetic information is copied backwards

From RNA to DNA instead of DNA to RNA

Retroviruses are responsible for some types of cancer and HIV

RETROVIRUSES AND CANCER Retroviruses can cause some cancers

Retroviruses that convert, or transform, normal cells → tumor viruses

These viruses carry genes that disrupt the normal controls over cell growth and division

RETROVIRUSES AND CANCER

HIV

PLANT VIRUSES

TOBACCO MOSAIC VIRUS Was the first

virus to be identified

Causes disease in tobacco plants

KNOWLEDGE CHECK

What characteristic do viruses share with all living organisms?

a. Respiration b. Metabolism c. Replication d. Movement

KNOWLEDGE CHECK

A lytic infection occurs when a

a. Virus infects a bacterium and kills it immediately

b. Virus embeds its genome into the DNA of the host cell

c. Virus enters a cell, makes copies of itself, and causes the cell to burst

d. Virus inserts its DNA into the DNA of the host cell and remains part of the host cell for many generations

KNOWLEDGE CHECKOne group of viruses that contain RNA as their genetic information are

a. Oncogenetic virusesb. Retrovirusesc. Capsidsd. Prophages

KNOWLEDGE CHECKFill in the blanks

The nucleic acid core of a virus contains ________ or __________

All viruses contain a coat of ________ and a core of _________

VIDEOS http://www.pbs.org/wnet/secrets/previou

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