VIRUSCHAPTER 17

VOCABULARY

• Antibody• Antigen• DNA• Host cell• Lymphocyte• Lysogenic cycle• Lytic cycle• Membranous

envelope• Mucous membrane

• Nucleic acid core• Phagocytic white blood

cell• Primary line of defense• Protein capsid• RNA• Secondary line of

defense• Tertiary line of defense• Viral specificity• White blood cell

READINGS

• Describe the basic structure of a virus – p. 356-357• Evaluate the evidence used to classify viruses as living or non-

living - p. 357-360• Compare and contrast the lytic cycle and lysogenic cycles – p.

357-359• Describe the body’s basic lines of defense against a viral attack –

p. 968-976• Give examples of ways to reduce the chance of contracting a viral

disease – p.973-975, p.980-981• Define and give examples of viral specificity CHAPTER 45 –

p.973-975, p.979-981• Evaluate the effects of virulence on human health CHAPTER 45

– p. 973-975, p.979-981

Viral characterisitics

• Not living organisms • Non-cellular • Consist of a nucleic acid core (DNA or RNA) and a

protein coat called the capsid • Capsid made of protein subunits called capsomeres • Cannot grow or replicate on their own (inactive

particles)

Viral characteristics cont.

• Can only reproduce inside of a living host cell using its raw materials & enzymes

• Lack ribosomes & enzymes needed for protein synthesis or metabolism

• Are extremely small particles ranging from 20 - 400 nanometers on average

• Largest virus is 1000 nanometers in dimension • Some can cause disease (smallpox, measles,

mononucleosis, influenza, colds, AIDS, Ebola

EBOLA VIRUS

Virus characteristics cont.

• Some may also cause cancers such as leukemias • Virus free cells are rare • Highly host specific (only infect certain cells) • Referred to as phages • Viruses are classified into 2 main groups by their

nucleic acid --- DNA or RNA Viruses •  DNA & RNA viruses are subdivided by capsid shape

& whether they do or don't have an envelope

Viral Structure

• DNA or RNA core surrounded by protein sheath called capsid

• Nucleocapsid  includes the viral nucleic acid & its capsid • Some form lipid rich covering around capsid called the

envelope • Envelope usually formed from host cell membrane • Envelope may have spikes to help chemically recognize

& attach to the host cell • Shaped determined by the arrangement of proteins

making up the capsid • TMV is rod shaped

• Adenovirus & polio viruses are icosohedral (20 sided)

• T -phages have a head & tail

Bacteriophages or T-Phages

• Among the most complex viruses • Attack bacterial cells • Composed of a icosohedral head,

tail, base plate, & tail fibers • Long DNA molecule is inside the

head  • Tail helps inject the viral DNA into

host cell • Tail fibers used to attach to host

RETROVIRUSES

• Contain RNA

• Have an enzyme called reverse transcriptase which helps use the RNA to make DNA

• Use the host cell's ribosomes & raw materials to make viral proteins

• Cause some cancers & AIDS

HIV VIRUS

VIROID

• Viroids• Smallest particle able to replicate • Made of a short, single strand of RNA with no capsid • Cause disease in plants

Viroid attack on potato

PRIONS

• No nucleic acid or capsids • Made of protein particles that have folded

incorrectly • Attacks the central nervous system • Cause animal diseases in cows (Mad Cow

disease), sheep, & humans

LYTIC CYCLE

• Viral replication that rapidly kills the host cell causing it to lyse or burst

• Involves 5 steps ----- Adsorption, Injection, Replication, Assembly, & Lysis

LYTIC CYCLE

• Adsorption --- phage attaches to cell membrane of host • Injection --- nucleic acid (DNA) of virus injected into

host cell • Replication --- viral DNA inactivates host cell's DNA &

uses host's raw materials & ribosomes to make viral DNA, capsids, tails, etc.

• Assembly --- new viral parts are combined to make new phages

• Lysis --- enzymes weaken & destroy the cell membrane causing it to lyse releasing new viruses that infect other cells

LYTIC CYCLE

• Phases of the Lytic Cycle of a Virulent Virus: 

• Absorption: Virus attaches itself to the cell. Entry: Enzymes weaken the cell wall and nucleic acid is injected into the cell, leaving the empty caspid outside the cell. Many viruses actually enter the host cell intact. Replication: Viral DNA takes control of cell activity. Assembly: All metabolic activity of the cell is directed to assemble new viruses. Release: Enzymes disintegrate the cell in a process called lysis, releasing the new

LYSOGENIC CYCLE

• Replication in which the virus stays inactive inside of the host cell & doesn't immediately kill it

• Viruses are called temperate phages

• Lysogenic steps include adsorption, injection, recombination, cell reproduction, activation, replication, assembly, & lysis

LYSOGENIC CYCLE

• Recombination ---Viral DNA joins with host cell DNA forming an inactive prophage

• Host cell reproduces  normally until activated by an external stimuli 

• External stimuli unknown, but could be ultraviolet radiation, carcinogens, etc.

• Once activated, prophage forms new viruses & destroys host cell

• The Lysogenic Cycle of a Temperate Virus: • The virus attaches itself and injects its DNA

into the cell. • The viral DNA attaches itself to the host DNA,

becoming a new set of cell genes called a prophage.

• When the host cell divides, this new gene is replicated and passed to new cells. This causes no harm to the cell, but may alter its traits.

• Now there are two possibilities: • The prophage survives as a permanent part

of the DNA of the host organism. • Some external stimuli can cause the

prophage to become active, using the cell to produce new viruses

DEFENSE

• Primary line of defense – e.g., skin, mucous membrane, tears

• Secondary line of defense – e.g., phagocytic white blood cells engulf viruses

• Tertiary line of defense – e.g., white blood cells called lymphocytes produce antibodies

VACCINE

• How Do Vaccines Work?• When a person is sick, the immune system makes

antibodies that have the ability to remember the pathogen. • With subsequent exposure to the pathogen, the immune

system quickly responds by producing white blood cells to fight the infection, which results in no or only minor symptoms of disease.

• This adaptive immune system response occurs because the immune system is capable of immunologic memory.

• Vaccines work the same way except they cause immunity to a pathogen without causing the symptoms and complications of disease.

HOW ARE VACCINES MADE

• How Are Vaccines Made?• Vaccines use altered versions of viruses or bacteria to

trigger an immune response without causing any symptoms of disease.

• There are four basic ways vaccines are made.

• 1. Weaken the virus: Viruses require cells to reproduce themselves. If they are grown repeatedly in cells other than those they are used to, they change and are no longer able to grow well in the original cells. This is known as cell culture adaptation. Examples of vaccines that are made this way include measles, mumps, rubella, chickenpox, shingles and rotavirus.

• 2. Inactivate the virus: The virus is killed so that it cannot replicate. Examples include polio shot, hepatitis A, influenza shot and rabies.

• 3. Use part of the virus: A piece of the virus is used to make the vaccine. Examples are HPV and hepatitis B.

• 4. Use part of the bacteria:• a. Toxoid vaccines: Some bacteria produce poisons, called

toxins, which cause illness. When the toxins are treated with a chemical, the toxins are no longer able to cause disease. Inactivated toxins are called toxoids. Examples include diphtheria, tetanus and pertussis.

• b. Polysaccharide vaccines: Some bacteria are covered with a sugar coating called a polysaccharide. In some cases, the polysaccharides used in vaccines are attached to a helper protein to make the vaccine. Examples include Hib, pneumococcal and meningococcal.

• http://www.youtube.com/watch?v=EqK1CYYQIug&list=PLFCE4D99C4124A27A&index=44&feature=plpp_video

• http://www.youtube.com/watch?v=L8oHs7G_syI&list=PL7A750281106CD067&index=22&feature=plpp_video