Chapter 17
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Chapter 17. Viruses & prokaryotes. 17-1 Viruses. What is a virus? How do viral life cycles differ? What is the relationship between viruses and their hosts?. What is a virus?. A virus is a noncellular particle made up of genetic material and protein that can invade living cells

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Chapter 17

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Chapter 17

Viruses & prokaryotes

17-1 Viruses

  • What is a virus?

  • How do viral life cycles differ?

  • What is the relationship between viruses and their hosts?

What is a virus?

  • A virus is a noncellular particle made up of genetic material and protein that can invade living cells

  • First discovered in 1935 by an American scientist named Wendell Stanley

  • It was called the tobacco mosaic virus or TMV

TMV on tobacco plant

Structure of a virus

  • Composed of a core of DNA or RNA surrounded by a protein coat called a capsid

  • Capsid protects the genetic material

  • The core contains several genes to several hundred genes

  • More complex structures are in viruses called bacteriophages – viruses that invade bacteria

  • Has a head region (capsid), a nucleic acid core, and a tail

  • Some have tail fibers that allow them to attach to the bacteria

  • Viruses are rod-shaped, tadpole-shaped, helical and cubelike shaped

  • Vary in size from 20 to 400 nanometers

  • A nanometer is one billionth of a meter

Specificity of a virus

  • Specific viruses infect specific organisms

  • Plant virus cannot infect an animal

  • Only mammal viruses that do not infect other animals and vise versa

  • Viruses for every type of organism

Life cycle of a Lytic Virus

  • In order to reproduce, viruses must invade, or infect, a living host cell

  • They also invade in different ways…

  • One way is done by lytic viruses where when they invade the cell bursts or lyses

Life cycle of a lytic virus


  • A virus is activated by contact with the right host cell (chance)

  • It then injects its DNA into the cell

Life cycle of a lytic virus


  • The RNA polymerase of the host cell creates messenger RNA of the virus DNA

  • This mRNA then takes over the host cell

  • Some proteins turn off the creation of molecules for the cell

Life cycle of a lytic virus


  • It then uses the host cell to make thousands of copies of its own protein coat and DNA

  • The host cell is then filled with viral DNA molecules

  • These three steps can happen in 25 minutes

Life cycle of a lytic virus

  • The infected cell then lyses (bursts) and releases hundreds of virus particles

  • These particles than infect other cells

  • The host cell is lysed and destroyed so this process is called lytic infection

Lysogenic Infection

  • Lysogenic infection-the virus does not reproduce and lyse its host cell

  • The DNA of the virus enters the cell and is inserted into the DNA of the host cell

  • The viral DNA is then known as a prophage

Prophage activity

  • Blocks entry of other viruses and may even add useful DNA to the host cell’s DNA

  • Eventually it will remove itself from the DNA and create new virus particles


  • Retroviruses contain RNA as their genetic information

  • When they enter the cell they produce a DNA copy

  • This then enters into the host cells DNA

Viruses and Living Cells

  • Viruses must infect living cells to carry out their functions

  • Viruses are parasites-an organism that depends entirely upon another living organism for its existence in such a way that it harms that organism

Are viruses alive?

  • Viruses are not made of cells

  • They can grow, reproduce, regulate gene expression, an evolve

  • It is up for debate

Virus video

17-2 Prokaryotic Cells

Bacteria Intro video


  • Cells that do not have a nucleus




  • Methanogens – archaeabacteria that produces methane gas


Bacteria (E. Coli)

  • One-celled prokaryotes

They do not contain the complex range of membrane enclosed organelles that are found in most eukaryotic cells

Eubacteria Structure

  • generally surrounded by a cell wall made of carbohydrates

  • there is a cell membrane which surrounds the cytoplasm

  • long whip like flagella protrude from the membrane through the cell wall


They use photosynthesis to get energy

Fresh and saltwater, land, hot water, arctic, grow on snow

Methanogens are archaebactria that produce methane gas

prochlorobacteriamore related to chloroplasts[1].jpg

Bacteria Identification

  • cell shape

  • Cell Wall

  • Bacterial Movement

  • How the obtain energy

Bacterial shape

Gram Staining

  • There are two types of dye, The bacterial cells with only one thick layer of carbohydrate and protein molecules outside the cell membrane took up the crystal violet. The bacterial cells that have lipid and carbohydrate molecules appear red under the microscope and are gram negative

Gram Staining

Bacterial movement

  • propelled by flagella

  • spiral forward

  • glide slowly

  • Phototrophic autotrophs-prokaryotes that trap the energy of sunlight

  • Chemotrophic autotrophs-prokaryotes that live in harsh environments and obtain E from inorganic molecules

  • Chemotrophic heterotrophs-bacteria that obtain energy by taking in organic molecules and then breaking them down and absorbing them

  • Phototrophic heterotrophs- use sunlight for Energy but also need organic compounds for nutrients

Obligate aerobes (tuberculosis)

  • Require oxygen

Obligate anaerobes (Clostridium botulinum)

  • Bacteria that do not require oxygen


  • Poisons that can cause botulism which is a rare form of food poisoning (paralysis)

Facultative anaerobes (Gardnerella)

  • Bacteria that can survive with or without oxygen

Binary Fission

  • Bacterial cell replicates its DNA and divides in half (asexual reproduction)


  • Genetic information from one bacteria is transferred to the other (sexual reproduction)


  • Spore formed when a bacterium produces a thick internal wall that encloses its DNA and portion of its cytoplasm (Not reproduction)

Bacteria food production

Symbiotic relationship (E. Coli)

  • Both organisms benefit

They are able to break down cellulose due to the bacteria that make enzymes that allow animals to digest cellulose

They recycle and decompose, or break down dead material

Saprophyte bacteria

  • Bacteria that digest organic matter and break it down to simpler substances

Bacteria sewage decomposition

Nitrogen Fixation (Rhizobium)

17-3 Diseases Caused by Viruses and Prokaryotes

  • Only a small number of viruses and prokaryotes are capable of disease in humans

  • Pathogens-disease producing agents

  • Its basically a battle for nutrients between the host cell and the microorganism


Viruses and Disease

  • Viruses are the cause of such human diseases as small pox, polio, measles, AIDS, mumps, influenza, yellow fever, rabies, and the common cold

  • Caused from the virus life cycle

  • The only successful protection against them is preventing their infection

  • The immune system must be stimulated to prevent the infection

  • They stimulate them with vaccines which inject a weakened or killed disease causing virus

  • Once a viral infection starts, there is little science can do to stop the disease

  • They mainly treat the symptoms



  • These are small proteins that are produced by the body’s cells when the cells are infected by viruses

  • Seem to make it more difficult for the virus to spread

  • Could be used to fight viruses



  • Some viruses cause cancer in animals called oncogenic viruses

  • Virus adds certain genes to the cell to make it a cancer cell

  • Not very common

  • The Rous Sarcoma Virus (RSV) is an example in chickens

Rous Sarcoma Virus

Bacteria and Disease

  • Only a few bacteria produce diseases

  • Some of the diseases caused by pathogenic bacteria include diphtheria, tuberculosis, typhoid fever, tetanus, leprosy, syphilis, cholera, and bubonic plague

Bacterial Diseases

  • Cause disease in two general ways:

    • May damage the cells and tissues of infected organisms directly by breaking down its living cells to use for food

    • May release toxins that travel throughout the body, interfering with the host

  • Many diseases can be prevented by stimulating the immune system as well

  • There are also many other ways to fight against bacteria once infected

  • One is drugs and natural compounds, known as antibiotics, that attack and destroy bacteria

  • This has increased life expectancy over the last two centuries


Controlling Bacteria: Sterilization

  • Sterilization destroys living bacteria with great heat or chemicals

  • Bacteria cannot survive high temps so most can be killed boiling water

  • Disinfectants are another good way to sterilize many places


Food Processing

  • Bacteria are everywhere including in our food (if we don’t eat it they will which spoils it)

  • Refrigeration causes bacteria to not grow as fast

  • Many foods are sterilized by boiling, frying, or steaming (kills bacteria

Food Spoilage

  • Food that is properly canned will last pretty much indefinitely

  • Chemical treatments such as salt, vinegar, or sugar also preserve the food

  • Examples: salted meat, pickled vegetables, and jam


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