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The Living World
Fourth Edition
GEORGE B. JOHNSON
15
The First SingleCelled Creatures
PowerPoint® Lectures prepared by Johnny El-Rady
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15.1 Origin of Life
There are three possibilities for the appearance of
the first living organisms on Earth
1. Extraterrestrial origin
Life was transferred to Earth from a distant planet
2. Special creation
Life was created by supernatural or divine forces
3. Evolution
Life may have evolved from inanimate matter, with
selection as the driving force
Only the third possibility is scientifically testable
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Forming Life’s Building Blocks
Life originated 2.5 billion years ago
The Earth’s atmosphere then had no oxygen
It was rich with hydrogen-rich gases (NH3, CH4)
These simple
molecules combined
to form more
complex molecules
Lightning provided
the energy
Fig. 15.1
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Forming Life’s Building Blocks
Stanley Miller and Harold Urey reconstructed the
oxygen-free atmosphere of early Earth in their lab
Cellular building blocks form spontaneously, when
the system is subjected to lightning or UV light
They concluded that life may have evolved in a
“primordial soup” of biological molecules
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Forming Life’s Building Blocks
Concerns have been raised about the “primordial
soup” hypothesis
No oxygen => no protective ozone layer
Therefore, the UV light would have destroyed the
essential ammonia and methane gases
Louis Lerman, in 1986, proposed the bubble model
Key chemical life-building processes took place within
bubbles on the ocean’s surface
Inside the bubbles the essential gases would be
protected from UV light
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Fig. 15.2
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15.2 How Cells Arose
The first step may have been the formation of tiny
bubbles termed microspheres
These have cell-like properties
Those microspheres better able to incorporate
molecules and energy persisted longer than others
The first macromolecule produced was RNA
It provided a possible early mechanism of
inheritance
Later, RNA was replaced as hereditary material by
the much more stable double-stranded DNA
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Earth is formed 4.5
billion years ago
Fig. 15.3 A clock
of biological time
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15.3 The Simplest Organisms
The fossil record indicates that prokaryotes
appeared 2.5 billion years ago
Eukaryotes, on the other hand, appeared only
1.5 billion years ago
Today prokaryotes are the simplest and most
abundant form of life on earth
Prokaryotes occupy a very important place in the
web of life in earth
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The Structure of a Prokaryote
Prokaryotes are small, simply organized, single cells
that lack a nucleus
Spiral
Rod
Prokaryotes include
the bacteria and
archaebacteria
They come in three
main shapes
Rod-shaped (bacilli)
Spherical (cocci)
Spirally coiled (spirilli)
Fig. 4.9
Spherical
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The Structure of a Prokaryote
The cell membrane of prokaryotes is encased in a
cell wall
Bacterial cell walls are composed of peptidoglycan
Network of polysaccharides linked together by
peptide cross-links
Archaebacterial cell walls lack peptidoglycan
They are made of proteins, sugars or both
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Bacteria can be divided into two groups based on
their cell wall architecture
Gram-positive
Have a thick peptidoglycan layer
Lack outer membrane
Gram-negative
Have a thin peptidoglycan layer
Have an outer membrane containing lipopolysaccharide
The name refers to a differential stain developed by
Hans Christian Gram
Gram-positive cells retain the primary crystal violet stain
Gram-negative cells don’t and are stained by a safranin
(red) counterstain
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Fig. 15.4 The structure of bacterial cell walls
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Many bacteria have capsules
A gelatinous layer found external to the cell wall
Many bacteria possess threadlike flagella
Long external appendages used for locomotion
Some bacteria also possess pili
Short external appendages used for attachment
In harsh conditions, a few bacteria can form
endospores
Highly-resistant structures that may germinate into
active bacteria when conditions improve
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Prokaryotes
reproduce
by binary
fission
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Fig. 7.1
Some bacteria undergo a process called conjugation
The undirectional transfer of plasmid DNA following cell-tocell contact
Fig. 15.5
Pilus connecting
the two cells
together
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Both cells contain
a complete copy
of the plasmid
15.4 Comparing Prokaryotes to
Eukaryotes
Prokaryotes differ from eukaryotes in many respects
They have very little internal organization
They are unicellular and much smaller
They possess a single chromosome
They are far more metabolically diverse
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Prokaryotic Metabolism
Prokaryotes have evolved many more ways than
eukaryotes to acquire carbon and energy
Acquisition of Carbon
Autotrophs = Use CO2 as their only carbon source
Heterotrophs = Use preformed organic compounds as
carbon sources
Acquisition of Energy
Phototrophs = Use light as energy source
Chemotrophs = Use chemicals as energy source
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Prokaryotic Metabolism
Based on carbon and energy sources, prokaryotes
can be divided into four categories
1. Photoautotrophs
Use the energy of sunlight to build organic molecules
from CO2
Cyanobacteria
2. Chemoautotrophs
Obtain energy by oxidizing inorganic substances
Nitrifiers oxidize ammonia or nitrite
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Prokaryotic Metabolism
Based on carbon and energy sources, prokaryotes
can be divided into four categories
3. Photoheterotrophs
Use light as energy and pre-formed organic molecules
as carbon sources
Purple nonsulfur bacteria
4. Chemoheterotrophs
Use organic molecules as carbon and energy sources
Decomposers and most pathogens
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15.5 Importance of Prokaryotes
Prokaryotes were largely responsible for creating the
properties of the atmosphere and soil found on Earth
They are the principal decomposers
They are the only organisms capable of fixing
nitrogen, thus making it available for others
They cause major diseases in plants and animals,
including humans
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Prokaryotes are instrumental in genetic engineering
Nonpolluting insect control agents
Removing environmental pollutants
Fig. 15.7
Exxon Valdez oil spill
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15.6 Prokaryotic Lifestyles
Prokaryotes belong to two domains
Archaea
Members are termed archaebacteria
Bacteria
Members are termed bacteria
Includes the vast majority of prokaryotes described
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15.6 Prokaryotic Lifestyles
Archaeabacteria include
Methanogens
Hot spring in Yellowstone National Park
Use H2 to reduce CO2
to CH4
Live in swamps and
marshes
Extremophiles
Live in unusually harsh
environments
Thermoacidophiles
Fig. 15.8
Hot and acidic
environments
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15.6 Prokaryotic Lifestyles
Bacteria include
Cyanobacteria
Fig. 15.9
Individual cells adhere
in filaments
The most prominent
photosynthetic bacteria
Also capable of
nitrogen fixation
In specialized cells
called heterocysts
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Heterocysts
15.6 Prokaryotic Lifestyles
There are numerous phyla of non-photosynthetic
bacteria
Some are chemoautotrophs
But most are heterotrophs
Decomposers
Bacteria cause many diseases in humans
Among the most serious is tuberculosis (TB)
It is caused by Mycobacterium tuberculosis
The emergence of drug-resistant strains in the
1990s has raised serious medical concerns
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15.7 The Structure of Viruses
Viruses are not living organisms
Rather, they are “parasitic” chemicals that can only
reproduce within living cells
Viruses are very small
Range from 17 – 1,000 nm
Viruses occur in all organisms
In every case, the basic structure is the same
Segments of DNA or RNA wrapped in a protein coat
called the capsid
There is considerable difference, however, in the details
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Fig. 15.10 The structure of bacterial, plant and animal viruses
Membrane-like layer
Not found in all viruses
Capsid
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15.8 How Bacteriophages Enter
Prokaryotic Cells
Bacteriophages are
viruses that infect bacteria
The most studied are
double-stranded DNA
bacteriophages, including
Phage T4
Fig. 15.11b
Tail fibers
Phage lambda (l)
Same basic structure as T4
But only possesses one thin tail fiber
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15.8 How Bacteriophages Enter
Prokaryotic Cells
Bacteriophages undergo two types of reproductive
cycles
Lytic cycle
Example:
T4 bacteriophage
Lysogenic cycle
Example:
Bacteriophage l
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Lytic Cycle
Bacteriophage uses tail fibers to attach to bacterial
cell wall
Tail contracts and DNA is injected into host
cytoplasm
Viruses multiply within infected cell
They eventually lyse (rupture) the cell and spread to
other cells
Viruses that can only undergo a lytic cycle are
termed virulent
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Fig. 15.12 Lytic and lysogenic cycles of a bacteriophage
Uninfected cell
Lysis of cell
Assembly of new
viruses using bacterial
cell machinery
Replication
of virus
Bacterial chromosome
Lytic
cycle
Virus attaching
to cell wall
Viral DNA
injected into cell
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Lysogenic Cycle
Some bacteriophages do not immediately kill the
cells they infect
Rather, they integrate their DNA into the chromosome of
their host cell
This phenomenon is termed lysogeny
The viral DNA is called the prophage
At a later time, the prophage may exit the genome
This initiates viral replication and, ultimately, cell lysis
Viruses that can become stably integrated within
their host’s genome are termed temperate
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Fig. 15.12 Lytic and lysogenic cycles of a bacteriophage
Uninfected cell
Lysis of cell
Assembly of new
viruses using bacterial
cell machinery
Bacterial chromosome
Virus attaching
to cell wall
Lytic
cycle
Replication
of virus
Prophage exits the
bacterial chromosome
Viral DNA
injected into cell
Reduction to
prophage
Lysogenic
cycle
Viral DNA integrated
into bacterial chromosome
Reproduction of lysogenic bacteria
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Lysogenic Conversion
Expression of prophage genes may have an
important effect on the host cell
The bacterium Vibrio cholerae exists in two forms
A harmless form
A virulent form that causes the disease cholera
This form is infected with a temperate bacteriophage
which carries a gene encoding the cholera toxin
Lysogenic conversion is also responsible for the
presence of toxin genes in other bacteria
Corynebacterium diphtheriae  Diphtheria
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15.9 How Animal Viruses
Enter Cells
Plant viruses enter plant cells through tiny rips in
their cell wall at points of injury
Animal viruses enter their host cells through
endocytosis
A diverse array of viruses occur among animals
We will focus on the human immunodeficiency
virus (HIV)
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HIV
HIV causes Acquired Immunodeficiency Syndrome
(AIDS)
Fig. 15.13
AIDS was first reported in
the US in 1981
Clinical symptoms of AIDS
until generally 8-10 years
after infection with HIV
During this latent period,
carriers of HIV are infectious
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HIV
Attachment and entry
Virus circulates throughout the entire body but will only
infect macrophages
Involved in the uptake and recycling of organic material
Specificity is mediated by gp120 protein spikes found on
the HIV surface
gp120 binds to CD4 marker on the macrophages
It then binds a second receptor protein, called CCR5
This triggers endocytosis of the virus into the
macrophage
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HIV
Replication
Once inside macrophages, HIV sheds its protective coat,
releasing its RNA genome
The viral enzyme reverse transcriptase synthesizes
double-stranded DNA from the single-stranded RNA
The enzyme does not proofread so many mutations
are introduced
The double-stranded DNA is integrated into the host
cell’s genome
In all of this, no lasting damage is done to the host cell
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HIV
Starting AIDS
Eventually, by chance, HIV alters the gene for gp120
causing it to produce a new form of gp120 protein
This prefers to bind to a different co-receptor, called
CXCR4
The virus can now infect CD4+ T cells
New viruses exit T cells by bursting through the
plasma membrane
The destruction of the body’s T cells causes the onset of
AIDS
Cancers and opportunistic infections are free to
invade the defenseless body
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Fig. 15.14 How the HIV infection cycle works
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15.10 Disease Viruses
Influenza
Perhaps the most lethal virus in human history
Natural reservoirs ducks and pigs in central Asia
AIDS (HIV)
First entered humans from chimpanzees in Africa
The chimpanzee virus is called simian immunodeficiency
virus (SIV)
Ebola virus
Filamentous virus that attacks connective tissue
Natural host of the virus is unknown
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15.10 Disease Viruses
Hantavirus
Discovered in 1993
Natural host is the deer mice
SARS
Severe acute respiratory syndrome
Caused by a coronavirus
Natural host is most likely the civet
West Nile Virus
Mosquito-borne virus
Natural host is birds
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15.10 Disease Viruses
Ali Maow Maalin
of Somalia
Smallpox
Caused by the variola virus
15 million worldwide cases
in 1967
A world immunization
campaign eradicated the
disease in 1977
The virus still exists in
Atlanta and Moscow
“Weaponized” smallpox
remains a bioterrorist threat
Fig. 15.15 The last known
smallpox case
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