+
Evolution of Microbial Life
Chapter 16
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Characteristics of Microbes
 Prokaryotic cells

Smaller

Lack special structures such as a nucleus and organelles

All prokaryotic cells are microorganisms
 Some
microorganisms are eukaryotic
 Viruses?
+
“Micro”organisms
Microorganisms are diverse and widespread



Pathogens
 causing disease
Normal / indigenous flora
 decomposing dead skin cells
 supplying essential vitamins,
 guarding against pathogenic
organisms
Saprobes
 In soil
 decompose dead organisms,
sustaining chemical cycles
+ Microbiology


The study of organisms (microorganisms or microbes)
too small to be seen without magnification
This includes:
1.
Bacteria
2.
Viruses
3.
Fungi
4.
Protozoa
5.
Algae
6.
Helminthes
+
Viruses
+
16-7
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Capsid (protein)
Covering
Envelope (membranous)
Virus
Nucleic acid (DNA or RNA)
Inner core
Various proteins (enzymes)
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16-8
Viruses have a simple structure
Viruses
1.
2.
3.
4.
are classified:
Their type of nucleic acid – DNA or RNA
Whether nucleic acid is single-stranded
or double-stranded
Size and shape
Presence or absence of an outer
membrane
+

General Structure of Viruses
Capsids

All viruses possess

Constructed from identical subunits called
capsomers


made of protein
Structural types:
 helical


Continuous helix of capsomers forming a
cylindrical nucleocapsid
icosahedral

20-sided with 12 corners

vary in the number of capsomers
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General Structure of Viruses
 Viral envelope
 mostly animal viruses
 acquired when virus leaves
host cell
 Protects the nucleic acid when the virion is
outside the host cell
 spikes
 exposed proteins on the outside of the
envelope
 essential for attachment of the virus to
the host cell
+ Nucleic acids

Viral genome


Number of genes varies for each type of virus


few to hundreds
DNA viruses




either DNA or RNA but never both
usually double stranded (ds)
may be single stranded (ss)
circular or linear
RNA viruses



usually single stranded
may be double stranded
Segmented versus nonsegmented
+6 Steps in Viral Replication
1.
adsorption
2.
penetration
3.
replication
4.
assembly
5.
maturation
6.
release
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Emergent Viral Diseases

H1N1 virus




Severe acute respiratory syndrome (SARS)


Causes high fever, body aches, and pneumonia
Avian influenza (or bird flu)


Usually found in pigs
humans it causes the symptoms of flu
Named after spikes H1 and N1
Disease does not often spread from chickens to humans, nor is
it efficiently transmitted among humans
Ebola



1 of a number of viruses that cause hemorrhagic fever
Highly contagious and fatal
Vector and animal reservoir unknown
1613
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1614
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1615
HIV (the AIDS virus) exemplifies
retroviruses

Genome consists of RNA, instead of DNA

Retrovirus


HIV provirus



Uses reverse transcription
 Reverse transcriptase
 from RNA into DNA
Viral DNA integrated into host DNA
Usually transmitted to another person by means of cells that
contain proviruses
Emergent viral disease that jumped from chimpanzees to
humans
1616
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The First Cells Originated
on Early Earth
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
1617
Biological Evolution
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cell
DNA
RNA
genetic material
(DNA)
protocell
genetic material
(RNA?)
plasma
membrane
macromolecules
Chemical Evolution
polymerization
small organic molecules
energy
capture
abiotic
synthesis
inorganic chemicals
outgasing
from
volcanoes
early Earth
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1618
RNA may have been
the first polymer
 RNA-First
Hypothesis
 Which macromolecule lead to first protocell?
 RNA also acts as an enzyme (called a ribozyme)
 RNA genome could have possibly replicated on
its own
 Ribozymes can join amino acids into proteins
 Only the most efficient RNA molecules survived
and became the genetic material for the protocell
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1619
Protocells preceded the
first true cells

Microspheres may or may not be a type of cell resembling
early cells

Origin of Plasma Membrane

Protocell would have had an outer membrane

Two hypotheses on origin of first plasma membrane

If lipids are made available to microspheres, which are protein,
they acquire a lipid-protein outer membrane

Liposomes – Lipids naturally organize themselves into doublelayered bubbles, roughly the size of a cell
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1620
Protocells preceded the
first true cells

Origin of DNA Information System
 A protocell became a cell when it contained a DNA
information system
 DNA to RNA to proteins
 To make DNA, a ribozyme could have acted in the
same manner as the enzyme reverse transcriptase
 DNA took over the function of storing genetic
information
+
Prokaryotes
+
Bacteria & Archae
–
–
Single-celled organisms
Various shapes
•
•
•
–
–
–
Cocci
Bacilli
Spiral shapes
Cellular
Lack membrane-enclosed
cellular structures
Widely distributed in nature
Generalized structure of a prokaryotic cell
+ Appendages

Flagella
 rotates 360o
 1-2 or many distributed over entire cell

Fimbriae
 adhesion

Pili
 made of pilin protein
 found only in Gram negative cells
 Functions
 joins bacterial cells for DNA transfer
(conjugation)
 Adhesion
 to form biofilms and microcolonies
+
The Cell Envelope

External covering outside the cytoplasm

Composed of few basic layers:

glycocalyx

cell wall



Gram positive

Gram negative
cell membrane
Maintains cell integrity
+
Chromosome
 single,
circular, doublestranded DNA molecule
 contains
all the genetic
information required by a
cell
 DNA
is tightly coiled
around a protein

dense area called the
nucleoid
+
Plasmids
 small
circular, double-stranded
DNA
 stable
extrachromosomal DNA
elements that carry
nonessential genetic
information
 duplicated
and passed on to
offspring

replicate independently from the
chromosome
+ Endospores
 resting,
dormant cells
 produced
by some G+ genera
 resistance
linked to high levels of
calcium & certain acids
 longevity

verges on immortality
25 to 250 million years
steam at 120oC for
20-30 minutes will destroy
 pressurized
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1629
How genes are
transferred between bacteria
Transformation
1.

Recipient picks up free DNA from its surroundings
Conjugation
2.

Donor bacterium passes DNA to the recipient by way of a
conjugation pilus

Plasmid – small circle of DNA
Transduction
3.

Bacteriophages carry portions of bacterial DNA from a donor cell to
a recipient
1630
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donor cell
recipient cell
Lysis of
donor cell
releases
DNA.
DNA
Donor DNA is
taken up by recipient.
Gene transfer by
transformation
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1631
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
donor cell
recipient cell
donor cell
plasmid
DNA
Donor DNA is transferred directly to
recipient through a conjugation pilus.
Gene transfer by
conjugation
+
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
donor cell
recipient cell
Bacteriophage
infects a cell.
Gene transfer
by transduction
Donor DNA
is picked up by
bacteriophage
Donor DNA
transferred when bacteriophage
infects recipient.
16-32
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1633
Some archaea live
in extreme environments

Eukarya are believed to have split off from the archaeal line of
descent

Structure and Function

Plasma membranes of archaea contain unusual lipids that allow
them to function at high temperatures

No peptidoglycan in cell wall

Some have unique forms of metabolism


Methanogens have the unique ability to form methane
Archaea are found in extreme environments

Halophiles, thermoacidophiles
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Prokaryotes have medical and
environmental importance

1634
Vast majority of bacterial species are not pathogenic to humans
 Several have had a tremendous impact on human health
 May produce powerful toxins (poisons)
 Clostridium botulinum
 More different types of human disease are caused by
bacteria from the genus Streptococcus than any other type of
bacteria
 Tuberculosis (TB) kills more people worldwide than any other
infectious disease
 Mycobacterium tuberculosis
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1635