Biology Exam 4

Prokaryotes

The first singles celled creatures, the simplest and most abundant organisms on earth.

Prokaryotes origin of Life

No one knows for sure where the first organisms (thought to be like today’s bacteria) came from.

There are several possibilities for the origin of life on earth,
such as:
• Extraterrestrial origin- another planet
• Special creation- intelligent design/religion
• Evolution- stable molecules
Only evolution permits testable hypotheses and is the only
scientific explanation.

Stanley Miller and Harold Urey experiment

Reconstructed the oxygen- free atmosphere of the early
earth in their laboratory.

They subjected it to the lightning and UV radiation that
it would have experienced then

Stanley Miller and Harold Urey experiment results

• They found that many of the building blocks of organisms
formed spontaneously.
• life may have evolved in molecules formed in the early
earth’s oceans.

How Did Cells Arose?

We don’t know how first cells formed, but most scientists assume the first cells formed spontaneously- from bubble enclosed molecules (microspheres)

Microspheres

It is when organic molecules are present in water, they tend to cluster together in structures of Bubble enclosed molecules,

It is assumed first celles formed spontaneosuly from

These microspheres have many cell-like properties

The first cells could have formed in a way similar to how microspheres form.

When was earth formed?

4.5 billion yrs ago

When was first life originated?

2.5 billion years ago.

A clock of biological time

A clock of biological time

Two types of prokaryotes

bacteria and archaea

Characterstics of the simplest organisms (prokaryotes)

• Small
• Simply organized
• Lack an organized nucleus
• Single celled

Prokaryotes differ from eukaryotes in

• Internal compartmentalization
• Cell size
• Unicellularity
• Chromosomes
• Cell division
• Flagella
• Metabolic diversity

Prokaryotes vs Eukaryotes diagram

Prokaryotic: Archaea includes what?

include methanogens and extremophiles

Methanogens

are anaerobes and cannot survive in oxygen. Found in swamps, marshes, cow guts

Extremophiles

live in harsh environments where mostorganisms cannot survive

Importance of Prokaryotes

• Decomposers-break down dead/decaying matter
• Commercial uses, such as genetically-modified bacteria
that break down oil
• Cause diseases in humans and other organisms- plague, pneumonia, tuberculosis

Prokaryotes come in many shapes pictures

oldest prokaryotes

Bacteria

Bacterial cells are simple in form

• Rod-shaped (bacilli)
• Spherical (cocci)
• Spirally coiled (spirilla)

The prokaryotic cell’s plasma membrane

is encased within a cell wall.

• The cell wall of bacteria is different than that of
archaea and those found in eukaryotes.

cell wall is made of peptidoglycan.

Gram-negative bacteria

-a thinner cell wall is surrounded by an outer membrane

The outer membrane prevents the cell wall from taking up a type of stain called a Gram stain.

Gram-positive bacteria

-no outer membrane and the cell wall is much thicker

Without the outer membrane, these bacteria take up the
Gram stain

Additional features of some bacteria include

-Capsule

-Flagella

-Pili

-Endospores

Capsule

gelatinous layer outside the cell wall

Flagella

long strands of protein used in swimming

Pili

shorter strands that help with attachment

Endospores

thick-walled enclosures of DNA and a small bit of cytoplasm that are extremely resistant to environmental stress

All prokaryotes can reproduce via

Binary fission

• After replicating DNA, the plasma membrane and cell wall
grow inward and eventually divide the cell.

Conjugation

Some bacteria can exchange genetic information via
plasmids (fragment of DNA) passed from one cell to another.

occurs through a special connection that forms between bacterial cells pili called a conjugation bridge.

Prokaryotic Lifestyles: Bacteria

• Heterotrophs-consume organic molecules
• Cyanobacteria were the first photosynthesizers (sun energy)
• Many bacteria are decomposers.
• Some bacteria form colonies called biofilms.
• Bacteria cause many human diseases. Tuberculosis is a
very serious disease fornhumans and other organisms.

Important Human Bacterial Diseases

Viruses

are parasitic chemicals, segments of DNA (or sometimes RNA) wrapped in a protein sheath called a capsid.

• They are not alive because they possess only a portion of
the properties of organisms and cannot reproduce on their
own.
• They are very small in size.
• They infect all organisms.
• There are structural differences among types of viruses.

The structure of bacterial, plant and animal viruses

Bacteriophage

are viruses that infect bacteria.

Structurally and functionally diverse

Process of infection by a bacteriophage

The tail tube pierces the bacterial cell wall.
• The contents of the head, mostly DNA, are injected into the bacterial cytoplasm.
• The viral DNA is transcribed and translated by the bacterial cell.
• New viral components are assembled.
• The host cell ruptures, releasing new viral cells to infect other bacterial cells.

How Bacteriophages Enter Prokaryotic Cells

During a viral infection, viruses cycle between lytic and
lysogenic cycles

The lytic cycle

The viral reproductive cycle

• The virus kills the infected host cell in which its replicating.

lysogenic cycle

Includes integration of the viral nucleic acid into the genome of the infected host cell, called lysogeny

Gene conversion

The expression of viral genes integrated into the host genome

Lytic and lysogenic cycles of a bacteriophage digram

Animal viruses

• Enter animal cell by membrane fusion or endocytosis

• On the surface of animal viruses are spikes which match
animal cell surface markers and trigger membrane fusion.

• Inside the animal cell, the virus sheds its protective coat
and replicates its DNA or RNA in the cytoplasm.

• New viruses produced in the animal cell exit by bursting
through the plasma membrane and killing the animal cell.

The HIV infection cycle

Emerging viruses

Arise in one species and pass to another, causing a new disease

Influenza

virus has been one of the most lethal viruses in human history

AIDS (HIV)

Is derived from a virus that originated in Central Africa in chimpanzees and monkeys

Ebola

virus also arose in Central Africa and attack human connective tissues

Zika

Virus is transmitted by mosquitos and causes microcephaly in unborn babies.

SARS

severe acute respiratory syndrome, originated from a
virus that infects the Chinese horseshoe bat

COVID-19

A close relative of SARS, attacks the upper and lower respiratory tracks

Important Human Viral Diseases