Chapter 8:Viruses Higher Human Biology Unit1: Cell function and Inheritance.
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Transcript of Chapter 8:Viruses Higher Human Biology Unit1: Cell function and Inheritance.
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Chapter 8:Viruses
Higher Human Biology
Unit1: Cell function and Inheritance
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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).
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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
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Different types of viruses
• Influenza virus • AIDS virus
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COLD SORES CAUSES BY THE HERPES SIMPLES VIRUS
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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
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Different Shaped Viruses
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• 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.
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Structure of a Virus
• A virus contains nucleic acid (DNA or RNA)
• Surrounded by a protective coat (capsid)
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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!
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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.
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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
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Binding With the Host Cell
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Then the virus “injects” its DNA or RNA into the
cytoplasm
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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.
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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
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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.
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Before and After Lysis
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Budding
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TASK: Testing your knowledge
• Complete Torrance TYK questions on page 63.
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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.
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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 .
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Smallpox
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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!!
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Watch this - http://www.youtube.com/watch?v=jJwGNPRmyTI
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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.
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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.
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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.
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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.
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Electron micrograph (TEM) showing the cell membrane of a T-lymphocyte white blood cell (T-cell). The "bubbles" are budding Human Immunodeficiency Viruses (HIV).
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• Video
• And http://www.xvivo.net/zirus-antivirotics-condensed/
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Random but interesting…
• In the future:– Viruses might cure cancer – They can deliver gene therapy – They could help us fight antibiotic-resistant
bacteria
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TASK: Testing your knowledge
• Complete Torrance TYK questions on page 65.
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Essay question 2009
2B. Describe the nature and reproduction of viruses. (10)