Bacteria and Viruses

Chapter 19

Biology – Miller • Levine

What is a Virus?

From the Latin word for “poison”Particles of nucleic acid (DNA or RNA),

protein and occasionally lipidsVarious sizes and structuresIn order to reproduce, they must infect a

living cell

Virus Structures

Virus Structure

So small they can only be seen with a powerful electron microscope

A typical virus:A core of either DNA or RNAA protein coat called a capsid that

surrounds the DNA or RNA

Viral Infection

Viruses are very specific to the organism they infect

Plant viruses do not infect animal cellsAnimal viruses infect only certain

species of animalBacteriophages – viruses that infect

bacteria (“bacteria eaters”)

Lytic Infection

1. The virus injects its DNA or RNA into the host cell.

2. The virus takes over the cell and makes thousands of copies of itself.

3. The cell lyses, or bursts, and releases the new viruses.

4. The new viruses go and infect more cells.

Lytic Infection

Lysogenic Infection

1. A virus injects its DNA or RNA into the host cell.

2. The viral DNA is inserted into the host cell’s DNA, called a prophage.

3. Each time the cell reproduces, the viral DNA is copied and passed on.

4. Eventually, the cell enters the lytic cycle.

Lysogenic Infection

Viruses and Disease

Cause human diseases such as polio, measles, AIDS, mumps, influenza, rabies and the common cold

Vaccine – a weakened or killed virus or viral proteins that provides immunity against a virus

Oncogenic viruses – viruses that cause cancer

Viruses and Disease

Retroviruses – viruses that contain RNA as their genetic information

They must produce DNA from their RNA and then insert it into the host cell’s DNA

HIV is a retrovirus

Are Viruses Alive?

Yes…Viruses reproduceThey have DNA or RNAThey evolve

No…They must reproduce inside a host cellThey are not made of cellsThey do not have the other characteristics

of living things

Prokaryotes

Single-celled organisms that lack a nucleus

Used to be in a single kingdom – MoneraAre now divided into two kingdoms:

EubacteriaLive almost anywhereCell walls contain peptidoglycan

ArchaebacteriaCell walls lack peptidoglycanLive in extremely harsh environments

Identifying Prokaryotes

ShapesBacilli – rod shapedCocci – sphericalSpirilla – spiral shaped

Cell WallsGram-positive – appear purpleGram-negative – appear red

Movement – flagella, wiggling, or don’t move at all

Shapes of Prokaryotes

Obtaining Energy

Photoautotrophs – carry out photosynthesis

Chemoautotrophs – carry out chemosynthesis

Photoheterotrophs – capture sunlight for energy, but feed on organic compounds for nutrition

Chemoheterotrophs – obtain their energy and nutrition from organic compounds

Releasing Energy

Obligate aerobes – organisms that require oxygen for respiration

Obligate anaerobes – organisms that do not require oxygen and may be poisoned by it

Facultative anaerobes – can survive with or without oxygen

Growth and Reproduction

Binary fission – a type of asexual reproduction where a bacterial cell divides in half, producing identical “daughter” cells

Conjugation – the exchange of genetic information between two bacterial cells

Endospore – when a bacterium forms a tough wall that encloses the DNA & some cytoplasm; allows it to survive harsh conditions

Conjugation

Bacteria in Nature

Decomposers – break down dead matter into reusable nutrients

Nitrogen fixers – convert nitrogen gas into a form that is usable by plants

Humans use bacteria to make certain foods – yogurt, cheese, sour cream

Can be used in industry to clean up waste products & oil spills

Bacteria and Disease

Pathogens – disease-causing agentsBacteria cause disease such as

tuberculosis, strep throat, pneumonia and cholera

Bacteria cause disease in one of two ways:Damage tissues by breaking them down for

foodRelease toxins (poisons) that harm the body

Antibiotics – chemicals that block the growth & reproduction of bacteria

Bacteria and Disease

Controlling Bacteria

Sterilization – destroys bacteria by subjecting them either to great heat or to chemicals

Refrigeration can stop food from spoiling due to bacteria

Canning can be done to preserve food for a long period of time