Chapter 8:Viruses

Higher Human Biology

Unit1: Cell function and Inheritance

Lesson Aims

• To describe the structure of a virus

• The examine the nature of viruses and their invasion of cells

• To discuss the alteration of cell instructions to produce more viruses ( the process of viral replication).

Viral diseases an introduction

• MUMPS, MEASLES, GERMAN MEASLES, and CHICKEN POX are common childhood illnesses caused by viruses.

• The common cold is usually due to the rhinovirus. The "flu" is caused by INFLUENZA viruses.

• HEPATITIS, an inflammation of the liver, may be the result of one of several viruses. Rabies and yellow fever are highly lethal viral diseases.

• HIV - Human Immunodeficiency Virus

Different types of viruses

• Influenza virus • AIDS virus

COLD SORES CAUSES BY THE HERPES SIMPLES VIRUS

Size

• Viruses are a lot smaller than cells.

• Smaller than bacteria• 20-300 nanometres (nm)• Viruses can only be seen

using an electron microscope.

• 200 nm 0.2 µm 0.0002 mm 0.0000002 m

Different Shaped Viruses

• They are not cellular – no nucleus, membrane, other organelles to sustain it’s own life.

• However, they can reproduce – but they rely on living cells to do so.

• Therefore they possess the blue print for life – Nucleic acid (DNA or RNA)

It has long been debated whether viruses are living organisms or not.

Structure of a Virus

• A virus contains nucleic acid (DNA or RNA)

• Surrounded by a protective coat (capsid)

Spread of the virus - also known as obligate parasites

• Viruses can be spread by– touching, – body fluids, HIV – aerosols (sneezing, coughing

etc.), e.g. Pneumonia– Bites, e.g. Rabies, Malaria

Viruses cause disease therefore they are called __________.

Remember Viruses also attack plants, seeds and bulbs!

SPECIFICITY

• Viruses are often specific with respect to their chosen type of host cell.

• The poliomyelitis virus attacks nerve cells

• The Hepatitis virus attacks liver cells.

INVASION: Entering the Host Cell

• A virus will attach itself to a cell• Different viruses employ different methods of

entering the host cell (antigen dependant – HIV-wbc’s, flu-throat cells etc.)

• Process depends on antigenic sites on the virus binding with certain complementary molecules in the membrane or the specific host cell

• Following binding the nucleic acid is introduced into the host cell

Binding With the Host Cell

Then the virus “injects” its DNA or RNA into the

cytoplasm

Viral DNA replication and

protein synthesis• Once in the host cell the virus takes over the cell’s

biochemical machinery (cell metabolism).

• The nucleus then follows the viral DNA (blueprint) .

• Host cell supplies energy (ATP), nucleotides, enzymes and amino acids

• Host then makes many identical copies of viral nucleic acid (Viral DNA) and protein coats.

ASSEMBLY

• Many viral protein coats are then made using the cell’s free amino acids and protein synthesis factory (ribosomes, enzymes codons and anti-codons).

EACH VIRAL DNA BECOMES ENCLOSED IN A PROTEIN COAT

Release

• The protein coats house the NA and the new viruses is released out of the cell either by– LYSIS (bursting of the host cell

membrane)

– Or BUDDING (the cell membrane pinches off containing the new viral particle)

•

The new viruses are ready to repeat the process on more cells.When the virus bursts out the cell, the cell dies.

Before and After Lysis

Budding

TASK: Testing your knowledge

• Complete Torrance TYK questions on page 63.

History of viral diseases

• Humans have been known to be plagued by viruses since the beginning of recorded time.

• An ancient Egyptian relic from about 1500BC shows a priest with a shrivelled leg thought to be due to poliomyelitis.

History of viral diseases

• The conquest of the Mexican Aztec nation in the 1500s by a small force of Spaniards is thought to be the reason that the smallpox virus transfer from Europe to Mexico causing a lethal epidemic .

Smallpox

Smallpox – led to the 1st vaccine

• Up until the end of the 18c smallpox was wide spread in Britain.

• It causes severe fever and was fatal in 1:5 cases.

• Survivors were left badly scarred

• In 1796 a British doctor Edward Jenner injected a small boy with a milder strain called cowpox. Jenner then injected the boy with the deadly strain. Fortunately he survived.

THE SCIENCE OF ARTIFICIALLY ACQUIRED ACTIVE IMMUNITY BEGAN!!

Watch this - http://www.youtube.com/watch?v=jJwGNPRmyTI

Safe vaccines – Polio prevention

• The first vaccine used to cause immunity against poliomyelitis virus contained DEAD VIRUS.

• After this vaccines became ATTENUATED – this means the NA has been rendered harmless but the antigenic properties of its protein coat remain unaltered.

• Use of this vaccine almost caused the complete eradication of polio.

Retrovirus

• A retrovirus which contains RNA.

• It also contains reverse transcriptase an enzyme which produces viral DNA from viral RNA.

• The virus can thus replicate itself.

Acquired Immune Deficiency Syndrome (AIDS)

• AIDS is a retrovirus • It attacks helper T-

lymphocytes. • The AIDS virus

attaches itself by glycoprotein on its surface to receptors on the helper T cell surface.

The mechanism of HIV

• The envelope surrounding the HIV particle fuses with the membrane of the helper T-cell and the virus enters the host cell.

• Viral DNA becomes incorporated into the host cell’s DNA where it can remain dormant for many years.

• Viral mRNA is transcribed and it directs synthesis of new viral particles inside the host cells

• These escape from the infected helper T cell by budding.

• The original T cell’s membrane is left perforated which causes destruction of the cell.

Electron micrograph (TEM) showing the cell membrane of a T-lymphocyte white blood cell (T-cell). The "bubbles" are budding Human Immunodeficiency Viruses (HIV).

• Video

• And http://www.xvivo.net/zirus-antivirotics-condensed/

Random but interesting…

• In the future:– Viruses might cure cancer – They can deliver gene therapy – They could help us fight antibiotic-resistant

bacteria

TASK: Testing your knowledge

• Complete Torrance TYK questions on page 65.

Essay question 2009

2B. Describe the nature and reproduction of viruses. (10)