Viruses Chapter 39

Objectives

Define virusesConsider whether they are living or non-livingDescribe the structure of a virusList the names of the virus shapesDescribe how replication occurs in virusesExplain the medical importance of viruses to

humans, plants and animals.

Viruses

Viruses

Viruses are tiny. They are much smaller (50 times) than a bacterium. An electron microscope is needed to see them.

They are not made of cells and cannot reproduce on their own.

Therefore they are not alive according to our rules.

Viruses are not living organisms?

Living Non-living

Possess genetic material either DNA or RNA

Are non-cellular

Possess a protein coat Cannot reproduce by themselves

Can replicate (inside a living cell)

Only have one type of nucleic acid (living things have both DNA and RNA)

• Viruses do not respire.• They cannot reproduce

outside a host cell.• They are obligate parasites

Structure of Viruses

A virus consists of:A strand of genetic

material either DNA or RNA.

A protein shell called a capsid.

RNA or DNA?

Viruses with RNA– Human immunodeficiency virus (HIV)– Influenza viruses– Rabies

Viruses with DNA– Warts– Chickenpox– mononucleosis

Shapes

Viruses can be:1.Spherical2.Rod shaped 3.Complex shaped

Bacteriophage

A bacteriophage or phage is a virus that infects bacteria

Bacteriophages are among the best studied viruses. T phage takes about

30 minutes to replicate.

The chart below shows the replication of a bacteriophage. This virus infects bacteria. It takes about 30 minutes to complete its replication cycle.

Virus ReplicationAttachment Proteins on the virus match up with the wall of the host cellEntry The virus forms a hole and the viral DNA is injected into the bacteriaSynthesis The host DNA is switched off and the viral DNA takes over.It makes virus parts.Assembly The virus parts are put together and new viruses are made.Release The host cell bursts and releases thousands of the viruses.These move to other cells.

Disadvantages of Viruses

1. Diseases Viruses cause many diseases in humans, animals

and plants.e.g. colds, measles, cold sores, mosaic virus in

potato plants

Benefits of Viruses

1. Genetic EngineeringViruses can be used to transfer genes from one

organism to another in genetic engineering. Such viruses are called vectors

e.g. human insulin gene transferred into E. coli2. Control of infectionsBacteriophages may be used to control bacterial

infections

Viruses can be beneficial…Bacteriophages – attack & destroy bacteriaBaculovirus – ebola-like virus that attacks insects

– Could use for pest control in crops• Cabbage loopers eat

cabbage crops• Virus can kill pests in days

– (it’s really gross)

… and then there are those that are not so good….

Control & Immunity

• Viruses are controlled by body’s general defence system e.g. skin

• Immunity to many viral infections can be produced -by vaccinations or by injecting anti-biodies into the body.

• Antibiotics kill bacteria and fungi but have no effect on viruses.

Interferon

Interferon is a range of substances produced by virus infected cells to protect healthy cells.

Interferon can be made artificially now and can be used to treat colds etc.

Viruses Enter Living Cells

Viruses enter bacterial cells by punching a hole in the cells wall and injecting its DNA

Mutating virusesViruses can mutate when they copy the genetic material

Copy something wrongMistake proves usefulMore powerful virus (more infectious)

Viruses don’t mutate often, except…

Influenza & HIV

Retroviruses

Retroviruses contain RNA instead of DNAThey have an enzyme that converts the Virus

RNA to DNA

HIV is a Retrovirus

Plant Diseases Caused by Viruses

Potato Mosaic Virus

HIV VirusAIDS: Acquired Immune Deficiency Syndrome is caused by the HIV virus

Effects: The virus damages the lymphocyte white blood cells making them unable to make antibnodies to fight against other diseases e.g. colds.

A person suffering from AIDS is unable to fight infections and may die from diseases e.g. cancer or pneumonia.

Spread: Spread through body fluids e.g. blood

Unprotected sex with an infected person or drug users sharing needles

Treatment: Drugs are given to slow down the progression of AIDS but there is no cure.

HIV Virus

• Sexual contact• Sharing contaminated needles• Blood transfusions• Breast feeding (mother to baby)• Mother to baby during pregnancy or birth

How Is HIV Spread?

FACTS• In the US, there is better than a 1/1000 chance of contracting

HIV during unprotected sex• A person can be contagious for more than 10 years before

any sign of the disease is apparent• HIV becomes AIDS when the number of immune cells drops

below a predetermined number• No one dies from HIV or AIDS; people die from secondary

infections (ranging from the common cold to cancer)• More than 3 million people (size of Chicago) die each year

• There are approx. 14,000 new cases of HIV worldwide every day

2012 Q8 (Higher Level)

Solution to 2012 Q8

2012 Section C Q14C

2012 Section C Q14C Solutions

2011 Paper Section C Question 15 Part c

2011 Paper Section C Question 15 Part cSolutions