Cell Theory, Microscopes, Prokaryotes & Viruses

4.1-4.316.1-16.10

10.17-10.21

The Cell Theory• All living things are

composed of cells.• Cells are the basic units of

structure and function in living things.

• New cells are produced from pre-existing cells.

The Discovery of the Cell:Hooke

• In 1665, Robert Hooke used an early compound microscope to look at a thin slice of cork, which is a plant material.

• He noticed that cork looked like thousands of tiny, empty chambers which he called “cells”

• We now know cells are the basic units of life.

The Discovery of the Cell:Van Leeuwenhoek

• The existence of cells was unknown for many years, but this changed with the invention of the microscope.

• Anton von Leeuwenhoek used a single-lens microscope to observe pond water and other things.

• The microscope revealed a world of tiny living organisms.

Developing the Cell Theory• In 1838, Matthias Schleiden

concluded that all plants were made of cells.

• In 1839, Theodor Schwann stated that all animals were made of cells.

• In 1855, Rudolph Virchow concluded that new cells were created only from division of pre-existing cells.

• These discoveries led to the cell theory.

Compound Light Microscope

• Combination of lenses and light used to magnify small objects held on a slide

•Live specimen may be observed (ie, pond water)

•Max magnification = 1000x

Electron Microscopes• Image produced on a computer screen using a beam of

electrons rather than light• More powerful (300,000x or more) than light

microscopes, but specimen cannot be alive– Transmission

• Study of inner structure of a specimen• Samples are cut into thin slices for viewing• Images are 2-D

– Scanning• Allows study of specimen surface• Images are 3-D

Images from Electron Microscopes

Why are cells so small?➢ DNA Overload: the genetic material is only capable of

“managing” a limited amount of cellular activity

➢ Materials Exchange: As a cell get bigger, the volume increases faster than the surface area, making it difficult to get material in and wastes out at a pace fast enough to allow the cell to survive.

Surface Area to Volume Ratio

Two Types of Cells•All cells are surrounded by a barrier called a cell membrane and contain DNA

•Eukaryotic cells contain a nucleus & membrane organelles. (plants, animals, fungi & protists.)

•Prokaryotic cells do NOT contain a nucleus (still have DNA) and most organelles (do have ribsomes) and are classified as bacteria.

Early Earth

• Earth is about 4.6 billion years old

• Fossils resembling photosynthetic prokaryotes have been found in dome shaped rocks called stromatolites, date back to 3.5 billion year ago.

Origin of Organic Molecules• In 1953, Miller & Urey

designed an experiment to simulate conditions on early Earth

• Under many different conditions this basic set up has produced all 20 amino acids, several sugars, lipids, the nitrogenous bases found in DNA & RNA and ATP

• Solutions of amino acids and dropped them onto the surface of hot sand, clay or rocks has resulted in the formation of polypeptides

The RNA World • A characteristic of life is the process of inheritance, which

is based on molecules that can copy themselves.

DNA → RNA → Protein

• How did this information flow originate?

• A popular hypothesis is that genes were originally short strands of RNA capable of replicating without enzymes

–Scientists have observed RNA molecules copying themselves in solutions containing nucleotides without enzymes or cells present

– During the “RNA world”, RNA might have stored genetic information in addition to directing protein synthesis

Formation of Pre-Cells

• Experiments have shown that polypeptides can form microscopic fluid-filled spheres.

• If certain kinds of lipids are in the solution selectively permeable membranes will form.

• These “molecular packages” are referred to as pre-cells

Hypothetical 4-Stage Sequence for Origin of Life

Small organic molecules formed from simpler inorganic molecules

These small molecules joined into more complex ones

Molecules that could copy themselves provided a basis for inheritance of molecular information

These molecules became packaged within membranes and separated from their surroundings

Archaea v. Bacteria

• “archaea” is derived from the Greek work for ancient

• Exist in harsh habitats resembling conditions of early Earth

• They are referred to as “Extremophiles”– Thermophiles

– Halophiles

– anaerobic

• Believed to be as closely related to eukaryotes as they are to bacteria

• Contain different information in their nucleic acids

• RNA polymerases differ

• Lack introns

• Susceptible to antibiotics that do not affect archaea

• Contain peptidoglycan in their cell walls

Phylogenic Tree of the Three Domains

Structure & Function of Bacteria: Shape

• Cocci – spherical

• Bacilli – rod-shaped

• Spirilla – spiral shaped

Structure & Function of Bacteria: Cell Wall

• Gram + (purple stain): thick layer of peptidoglycan

• Gram – (pink stain): thinner layer of peptidglycan with outer membrane

Structure & Function of Bacteria: Motility

• Flagellum

• Pilli

• Slime secretion

Bacterial ReproductionBinary fission: DNA is copied and moved to opposite

ends of the cell as the cell divides; occurs almost continuously.•Rapid reproduction rate, many can divide within 20 minute

•Results in a colony of cells that are clones (unless mutations occur)

Genetic Variation• Despite reproducing

asexually, bacteria are able to obtain genetic variation through conjugation

• Two bacterial cells temporarily join and exchange plasmid DNA; does not need to occur between cells of the same species/strain

Endospores

• Allow bacteria to survive periods of very harsh conditions by going into a dormant endospore form

• After copying DNA, one copy is surrounded by a thick protective coat and the outer cell disintegrates

• Often able to survive for years in this state, when conditions are more favorable, the endospores will absorb water and grow again.

Modes of Nutrition

The Oxygen Revolution• The evolution of photosynthetic cyanobacteria

resulted in the presence of free oxygen in oceans, lakes & the atmosphere

• This oxygen was toxic to many existing organisms because it attacks the bonds of organic molecules & many went extinct

• Some were not exposed to oxygen and remained anaerobic; their descendants still exist in similar environments today

• A small number were able to use the oxygen in the extraction of energy from food, their descendents are the wide variety of aerobic organisms in existence today

Beneficial Uses of Bacteria• Chemical recycling:

– Decomposers: replenish soil nutrients and release CO2 back to the atmosphere

– Nitrogen fixing bacteria: convert nitrogen gas in the atmosphere to an organic form usable by other organisms for nucleotide and amino acid formation

• Bioremediation:– Sewage treatment: decompose organic matter in sewage sludge

– Oil spill clean-up: genetically modified digest oil

– Clean old mining sites: detoxify by extracting lead & mercury

• Medicine/Pharmaceuticals:– Produce desired gene products: insulin, HGH

– Probiotics: restore beneficical intestinal bacteria

• Food:– Cheese

– Yogurt

• All living things share 8 characteristics. Viruses do not meet all of these characteristics.

• Viruses attack eukaryotic cells and Bacteriophages attack prokaryotic cells.

• Viruses and phages are capable of reproducing at a very rapid rate, but only in a host cell.

• Viruses are responsible for many diseases and can be found everywhere.

Viruses & Bacteriophage: The Boundary of Life

Viral Structure

Viruses & Disease• Method of causing disease is very different from that of

bacteria

• Therefore methods for treatment & prevention differ from methods for bacteria

• Antibiotics will not work on viruses because they target specific not found in viruses or host cells– Damage or prevent formation of bacterial cell wall

• Some examples of viral diseases include:Influenza (RNA) Polio (RNA)

Common cold (RNA) Hepatitis (DNA)

Measles (RNA) Herpes (DNA)

Mumps (RNA) Smallpox (DNA)

AIDS (RNA) Rabies (RNA)

Viral Infection

• Viruses and bacteriophages invade cells and use the host cell's machinery to synthesize more of their own macromolecules.

• Once inside the host the bacteriophage or virus may go into two reproductive cycles:– Lytic cycle: destroying the

host cell during reproduction.

– Lysogenic Cycle – a parasitic type of partnership with the cell

Lytic Cycle & Lysogenic Cycle

Viruses are host specific – a protein on the surface of the virus has a shape that matches a molecule in the plasma membrane of its host, allowing the virus to lock onto the host cell.

ProvirusesA provirus is a DNA virus that has been inserted into a host

cell chromosome.

Retroviruses & HIV• Retroviruses reverse the

normal DNA to RNA to protein flow – In RNA viruses, the RNA

nucleotide will be translated

• Reverse transcriptase catalyzes the synthesis of DNA from an RNA template, the DNA intermingles with host DNA as a provirus making it difficult to detect

Prions • Proteins that cause several diseases of

the brain: Mad cow disease, Kuro, Creutzfeldt-Jacob disease (CJD) & Scrapie (in sheep)

• Only infectious agent that do not contain genetic material

• Normal form play important roles in brain function such as helping nerve cells communicate

• Abnormal prions destroy the brain

• Three ways to acquire abnormal prions:

– Infection with abnormal prions

– Inherited genes that give rise to abnormal prions

– Spontaneous genetic mutations that give rise to abnormal prions

Viroids• Small strands of RNA rather than strands of protein.

• Smaller than the strands of genetic info in viruses and contain no protein coat.

• Replicated using host cell machinery, like viruses

• Cause plant diseases: potato spindle tuber, avocado sunblotch, chrysanthemum stunt, and chrysanthemum chlorotic mottle

Immune Response

HIV doesn’t target just any cell, it goes right for the cells that want to kill it. “Helper" T cells are HIV's primary target. These cells help direct the immune system's response to various pathogens.

HIV undermines the body's ability to protect against disease by depleting T cells thus destroying the immune system. The virus can infect 10 billion cells a day, yet only about 1.8 billion can be replaced daily.

Defense Against Viral Diseases

From HIV to AIDS

• During first few years (7-10) after HIV infection, person is usually asymptomatic.

• During the symptomatic phase, the body has insufficient numbers of T-Cells (from normal 800-1200 /mm3 to 200/ mm3 ) to mount an immune response against infections. – Chronic diarrhea, minor mouth infections, night sweats, headache

& fatigue are common

• At the point when the body is unable to fight off infections, a person is said to have the disease AIDS. (Generally when count drops below 200 /mm3 )

• It is not the virus or the disease that ultimately kills a person; it is the inability to fight off something as minor as the common cold.

AIDS: The Global Epidemic• Around 2.6 million

people became infected with HIV in 2009.

• Sub-Saharan Africa has been the hardest hit by the epidemic. In 2009 over two-thirds of AIDS deaths were in this region