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Learning Outcome E1 & E2

Viruses & the Effects Viruses have on Human Health

Learning Outcome E1 & E2

Evaluate the evidence used to classify

viruses as living or non-living

Evaluate the effects of viruses on human

health

Student Achievement

Indicators (E1)

Students who have fully met this learning outcome will be able to:

Identify criteria for classifying organisms as living

Describe the basic structure of a virus, including the antigens, the membranous envelope, the protein capsid, and the nucleic acid core (DNA or RNA)

Identify the role of the host cell in viral reproduction

Compare the lytic and lysogenic cycles

Student Achievement

Indicators (E2)

Students who have fully met this learning outcome will be able to:

Define and give examples of viral specificity

Describe the body’s basic lines of defence against a viral attack, including

• primary line of defence

• secondary line of defence

• tertiary line of defence

Give examples of ways to reduce the spread of viral diseases

Origin of Living Things The cell is the basic unit of life. Different cell are specialized for different

functions Example – muscles cells, red blood cells,

nerve cells

Vary in shape, size and appearance. All cells digest nutrients, excrete wastes,

synthesize necessary chemical and reproduce.

Units of Life: atoms/molecules → cells → tissues → organs → organs systems → organism

Origin of Living Things Abiogensis is a theory proposed by Aristotle

This theory proposed that non-living things can be transformed into living things spontaneously, this process is also known as spontaneous generation.

Example - Francesco Redi wanted to test hypothesis that rotting meat can be transformed into flies. He learned that flies come from other flies not the rotting meat. This was demonstrated when he placed rotting meat in a sealed container and flies

did not appear.

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Characteristics of Living Things

In order to be classified as living an organism must do or need to obtain the following:

1. Requires energy

May obtain energy through eating plants, animals or photosynthesizing

2. Grow Undergo the process of mitosis and grow and

develop

3. Reproduce Produce similar offspring – inherit traits from their

parents.

Characteristics of Living Things

4. Respond to stimuli

Example – move towards or away from light

5. Ability to move

All living organisms show movement of one kind

or another.

All living organisms have internal movement,

which means that they have the ability of moving substances from one part of their body

to another.

Some living organisms show external movement

as well - they can move from place to place by

walking, flying or swimming.

Characteristics of Living Things 6. Breathing or respiration

All living things exchange gases with their environment.

Animals take in oxygen and breathe out carbon dioxide.

7. Excretion Excretion is the removal of waste from the body.

If this waste was allowed to remain in the body it could be poisonous.

Humans produce liquid waste called urine.

We also excrete waste when we breathe out.

All living things need to remove waste from their bodies.

Cell Theory

All living thing are composed of cells.

The cell is the basic living unit

All cells arise from pre-existing cells

Cells do not come from non-living things

A Model for the Origin of Life

A primitive atmosphere contained gases including water vapour that escaped from volcanoes.

As the water vapour cooled, some gases were washed into the oceans by rain.

The availability of energy from volcanic eruptions and lightening allowed gases to form simple

organic molecules.

Organic molecules contain carbon

Simple organic compounds have joined to form

proteins and nucleic acids, which became

incorporated into membrane-bound spheres.

A Model for the Origin of Life

These sphere became the first cells and are

called protocells.

Eventually various types of prokaryotes then

eukarytoes evolved.

Prokaryotic cells are primitive cells that do not

have a true nucleus or a nuclear membrane.

Example - bacteria and other single celled organisms.

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A Model for the Origin of Life

Eukarytotic cells have a true nucleus

containing DNA and chromosomes, and membrane bound organelles such as

mitochondria.

Examples - all plants and animal

Some prokaryotes were oxygen producing photosynthesizers.

This was vital for other organisms because

oxygen in the atmosphere is necessary for aerobic cellular respiration to occur.

Viruses

In 1898 Friedrick Loeffler and Paul Frosch found evidence that foot-and-mouth disease in livestock as caused by an infectious particle smaller than bacteria.

Viruses are small biological particles that do not display most of the characteristics of living cells.

Often viruses are often the subject of debate in terms of being living and non-living.

Derived from the Latin word for poison

A Virus A T4 bacteriophage injecting DNA into a cell. Providing an effective mechanism for delivering human genetic therapy is one of the ways these stigmatised parasites are proving their true value to humanity.

Credit: Wikimedia

Basic Structure of

a Virus

Viruses

Classification

Viruses do not fit into the 5 or 6 kingdom system of classification

Viruses require a host in order to carry our all life functions (non-living characteristic)

Viruses Basic Characteristics Very small 20-400 nm in diameter

Simple organism: contains an inner nucleic core that contains genetic information surrounded by an out protective coat known as a capsid.

The capsid accounts for 95% of the viruses structure, and gives viruses a unique geometrical shape.

Does not have any membranes, cytoplasm, ribosomes or any other cellular components

Can not move or grow

Can only reproduce within a host

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Viruses

Viral Diversity

There are many different types of viruses

Bacteriophages are a type of virus that eats bacteria

Not all viruses cause disease

Example – tobacco mosaic virus infects the leaves of the tobacco plant and does not appear to destroy the plant tissue

Viruses are selective and only invade a specific host or hosts

Viruses Viral Specificity

Exhibit highly specialized relationships with their host; only infects bacteria, animals or plants.

Bacteriophages have a restricted host range, if two different phages infect the same type of bacteria this usually indicated the bacteria are very closely related to one another.

Many plants and animal viruses are capable of infecting several hosts

Example – swine flu can infect both humans and hogs

Example – Rabies can infect rodents, dogs and humans

Some viruses are so specific that they only invade certain cells

Example – the human cold virus only infects the cells of the upper respiratory tract.

Viruses

Viral Replication

DNA replication occurs in viruses in a variety of different ways.

There are four basic steps to viral replication:

1. Attachment and Entrance

The virus chemically recognizes the host cell and attaches to it.

Either the whole virus or its DNA or RNA material enters the host cell’s cytoplasm.

Viruses 2. Synthesis of proteins and nucleic acid Information in the viral DNA/RNA directs the host

cell in replicating nucleic acid, enzymes, capsid proteins and other viral units.

3. Brings the units together The viral nucleic acid, enzymes and proteins are

assembled in new viral particles.

4. Release of new virus particle Newly formed virus particles are released from the

infected cell, so the virus can infect other cells

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Viruses

The Lytic Cycle

Takes place quickly; the host cell ruptures (lysis) and dies after its contents are released.

A bacteriophage that causes lysis of a host cell is known as a virulent phage.

Viruses

The Lysogenic Cycle

The virus does not kill the host right away; the virus may coexist with the host through many

generations.

Retroviruses are RNA viruses that infect

animal cells and follow the lysogenic pathway of replication.

Example of a retrovirus - HIV

Bacteriophages that do not cause lysis of their hosts are called temperate phages

Viruses Temperate phages inject nucleic acid into the host

bacteria, similar to the way in which the virulent

phage acts but it does not take control of the cell.

The nucleic acid becomes integrated into the host DNA and acts as another set of genes in the

bacterial chromosome.

This genetic information is replicated along with the

host’ DNA and so it is passed onto the daughter cells.

Sometimes damage to the DNA cycle may activate

the virus and cause it to enter the lytic pathway.

The bacteriophage now becomes a virulent phage

and will kill its host.

Related to Viruses

Viroids - even smaller than viruses and consist

of RNA strands that lack a protein coat

Prions - infectious agents that are believed to

be the cause of Mad Cow Disease

Not much is known about prions

A Plant Viroid Coarse yellow blotches or

"measles" are caused by the

chrysanthemum stunt viroid.

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Viral Diseases Causes many diseases in humans: smallpox, common cold,

chickenpox. influenza, shingles, herpes, polio, rabies, Ebola virus, Hanta virus and HIV.

Viruses have the ability to transfer genetic material between different species of host

Example – the rabies virus can transfer from a dog to a human

This characteristic allows viruses to be used in genetic engineering

Also contribute to natural genetic engineering by incorporating some genetic material from its host as its replicating and transfers this genetic material to a new host.

This is known as transduction and is thought to drive some evolutionary change.

The Immune System

Animal Defenses

Two categories:

Innate Immunity - which does not distinguish

one infectious agent from another

Acquired Immunity - responds in specific ways

to particular toxins, microorganisms, aberrant and other substances marked as foreign

molecules

The Immune System Innate Immunity

There are two types of innate immunity: 1. Physical Defenses- First Line of Defense

a. Physical Barriers – the skin

Cannot normally be penetrated by bacteria and viruses

But, even minute abrasions allow their passage Secretions from sebaceous and sweat glands keep

the skin in a pH range of 3 to 5 (acidic)

Kills most microbes

Microbial colonization is also inhibited by saliva, tears, and mucus secretions that continually bathe exposed epithelium

The Immune System All of these secretions contain antimicrobial proteins

An example is a lysozyme, an enzyme that digests the cell walls of many bacteria. b. Mucus Membranes

Line digestive, respiratory, and genital area

Also prevents entry of harmful microbes

Traps microbes and particles In the trachea, ciliated epithelial cells sweep out mucus:

trap microbes Prevents microbes from entering the lungs Exposes them to the acidic environment of the stomach Kills most microbes

Hepatitis A is an exception (can survive this)

The Immune System

2. Internal Defenses - Second Line of Defense

This line of defence includes phagocytic cells

and antimicrobial proteins.

Comes into play if the first line of defense have

been compromised

Specific defensive proteins called antibodies

are produced by lymphocytes

Depends mainly on phagocytosis

Phogocytes are a specialized white blood cell

that ingest invading

Phagocyte Specialized white blood cell

engulfing a invader

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White Blood Cell

The Immune System

a. Phagocytic Cells

I. Macrophages

Large, long-lived phagocytes

Cells extend long pseudopodia, engulf the microbe into a vacuole which fuses with a

lysosome.

II. Lysosomes

• Kill in two ways:

By generating toxins such as nitric oxide

By digesting microbes with lysozyme

The Immune System

III. Esinophils

Help fight large parasitic invaders

Example - Schistosoma mansoni (blood

fluke)

They position themselves alongside the

parasite and discharge destructive enzymes from cytoplasmic granules

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The Immune System

IV. Neutrophils

Usually the first to arrive at site of invasion or infection

Attracted to chemical signals released by infected tissue

Self-destruct while destroying invaders

Life Span is short

The Immune System

V. Natural Killer Cells

Do not attack microbes directly, but destroy infected cells

Also attack abnormal body cells that could become cancerous

They attack the cell’s membrane and cause the cell to lyse (burst)

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The Immune System b. Antimicrobial Proteins

A variety of proteins

They function in nonspecific defense

Attack microbes directly Or impede microbe reproduction

c. Interferons

Another set of proteins

Nonspecific defense

Secreted by virus-infected cells Do not benefit the infected cell but induce

neighboring cells to produce chemicals that inhibit viral reproduction

The Immune System

d. Inflammatory Response

Tissue damages leads to a localized inflammatory response

Could be injury

Could be invasion by microbes

Capillaries respond by allowing increased

dilation, which causes:

Increased permeability

Leads to increased redness, heat, and swelling

Immune Response Example of inflammation

The Immune System

e. Histamine Release

Inflammation sets off histamine release

Histamine is released by special leukocytes

called basophils and by mast cells in connective tissue

Histamine triggers both increased dilation and permeability of nearby capillaries

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Immune System

f. Enhanced Blood Flow

In addition to histamine, damaged tissues also release prostaglandins and other substances the

also promote blood flow to the injured site

Enhances delivery of clotting elements

Blocks spread of microbes

Enhances migration of phagocytic cells

The Immune System

Acquired Immunity

The body’s third line of defense

The key cells of the third line of defense are

lymphocytes

There are two main types of lymphocytes

1. B Cells (aka Memory B Cells)

Mature in bone marrow

The Immune System

2. T Cells (aka Killer T cells)

Mature in the thymus (specialized organ of the immune system)

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The Immune System

Both cell types circulate through the blood

Both are concentrated in the spleen, liver and other lymphatic tissues

Lymphocytes recognize and respond to particular microbes and foreign

molecules (antigens)

Antigens are non-living foreign invaders, such

as dust

The Immune System

a. Antibodies

Proteins produced by B cells

Each antigen has a unique molecular shape

and stimulates certain B cells to secrete antibodies to interact specifically with it

B and T cells recognize specific antigens through their plasma membrane-bound

antigen receptors

The Immune System

Immune Response

After first exposure primary immune response takes about 10 to 17 days

Secondary immune response is faster and occurs between 2 and 7 days

Strength is of attack is greater and lasts longer

Secondary immune response is very specific