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Chapter 16Evolution of

Microbial Life

Viruses Reproduce in Living Cells

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16.1 Viruses have a simple structure The size of a virus is comparable to that of a

large protein macromolecule, ranging from 0.2 to 2 μm

All viruses possess the same basic anatomy An outer capsid, which is composed of protein An inner core of nucleic acid (DNA or RNA)

A viral genome has as few as three and as many as 100 genes

The covering of a virus contains the capsid, which may be surrounded by an outer membranous envelope If not, the virus is said to be naked. Naked viruses

can be transmitted by contact with inanimate objects, such as desktops 16-3

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Figure 16.1A Adenovirus, a naked virus, with a polyhedral capsid and a fiber at each corner

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Figure 16.1B Influenza virus, surrounded by an envelope with spikes

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16.2 Some viruses reproduce inside bacteria

All sorts of cells, whether prokaryotic or eukaryotic, are susceptible to a viral infection

Viruses are specific Specificity extends to the type of cell infected by the

virus Example: tobacco mosaic virus infects only tobacco leaves

Bacteriophages, or simply phages, are viruses that parasitize bacteria Two types of bacteriophage life cycles

Lytic cycle and the Lysogenic cycle

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Figure 16.2 The lytic and lysogenic cycles in prokaryotes

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APPLYING THE CONCEPTS—HOW SCIENCE PROGRESSES 16.3 Viruses are responsible for

a number of plant diseases Approximately 2,000 kinds of plant diseases

have been attributed to viruses Plant viruses are responsible for the loss of over

15 billion dollars annually by reducing the yield of important agricultural and horticultural crops

Once a plant is infected the virus spreads slowly throughout the plant

In some instances, plants have been purposefully infected with a virus in order to produce traits considered desirable by gardeners

Example: Some variegation in leaves and flowers can be brought about by viruses

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Figure 16.3A The tobacco mosaic virus (TMV) is responsible for discoloration in the leaves of tobacco plants

Figure 16.3B A virus is responsible for the variegation and streaking in Rembrandt tulips

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16.4 Viruses reproduce inside animal cells and cause diseases

Replication of an animal virus with a DNA genome involves certain steps Attachment: Glycoprotein spikes projecting through the

envelope allow the virus to bind to host cells Penetration: After the viral particle enters the host cell,

uncoating follows and viral DNA enters the host Biosynthesis: The capsid and other proteins are

synthesized by host cell ribosomes according to viral DNA instructions

Maturation: Viral proteins and DNA replicates are assembled to form new viral particles

Release: In an enveloped virus, budding occurs and the virus develops its envelope

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Figure 16.4 Replication of an animal virus

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16.5 The AIDS virus exemplifies RNA retroviruses

Genome for an HIV virus consists of RNA, instead of DNA

HIV is a retrovirus. It uses reverse transcription from RNA into DNA in order to insert a complementary copy of its genome into the host’s genome

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Figure 16.5 Reproduction of HIV

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APPLYING THE CONCEPTS—HOW BIOLOGY IMPACTS OUR LIVES

16.6 Humans suffer from emerging viral diseases

Emergent diseases - newly recognized as infectious International travel facilitates disease transmission

Viruses are constantly in a state of evolutionary flux A new pathogen can emerge through the acquisition of new

surface antigens Some viruses can easily move from animals to humans

Example: Rabies can be spread by the bite of a rabid animal, such as a skunk, raccoon, bat, cat or dog

Many viruses are transmitted by vectors, usually insects that carry pathogens from an infected individual or reservoir to a healthy individual Mosquitoes serve as a common vector for several viral diseases,

including West Nile virus and yellow fever 16-16

Figure 16.6A Surgical masks provide protection against the transmission of SARS

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Figure 16.6B Exterminating possibly infected chickens may protect against bird flu

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The First Cells Originated on Early Earth

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16.7 Experiments show how small organic molecules

may have first formed Two different hypotheses to explain how organic

molecules could have formed Prebiotic Soup Hypothesis

The early atmosphere contained no oxygen and was a reducing environment

In such an environment, methane, ammonia, hydrogen, and water could be reduced to a variety of amino acids and organic acids

Iron-Sulfur World Hypothesis At hydrothermal vents on the ocean floor, cool water is

heated to a temperature as high as 350°C, and when it spews back out, it contains various mixed iron and nickel sulfides that can change N2 to NH3

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Figure 16.7A Laboratory re-creation of chemical evolution in the atmosphere

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Figure 16.7B Chemical evolution at hydrothermal vents

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16.8 RNA may have been the first macromolecule

Two stages in origin of life Chemical Evolution

Organic monomers arise from inorganic compounds and polymers arise when monomers join together

Biological Evolution A plasma membrane surrounds polymers producing a protocell A true cell has arisen when the cell reproduces in the same manner

as today’s cells

RNA-First Hypothesis The first macromolecules need to have enzymatic functions, not

only to allow the genetic material to replicate, but also to perform any number of metabolic functions

This has led research to conclude it was an “RNA world” some 4 BYA and that RNA chains were the first forms of life

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Figure 16.8 The origin of the first cell(s) can be broken down into these steps

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16.9 Protocells preceded the first true cells

Origin of Plasma Membrane - First and foremost, the 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 lipidprotein outer membrane

Liposomes - Lipids naturally organize themselves into double-layered bubbles, roughly the size of a cell

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Figure 16.9A Microspheres, which are made of protein, could have acquired an outer lipid-protein membrane during the origin of the first cell

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Figure 16.9B Liposomes, which are composed of lipids, have a double-layered outer membrane

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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 RNA is unique in that it could have also synthesized

the proteins that took over most of the enzymatic functions in cells

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Origin of Metabolism to Acquire Energy

The cell would have had to carry on nutrition so that it could grow

Two theories If organic molecules formed in the atmosphere

Nutrition would have been no problem because simple organic molecules could have served as food

Thus the protocell was a heterotroph

If the protocell evolved at hydrothermal vents It may have carried out chemosynthesis Synthesizing organic molecules by oxidizing inorganic

compounds, such as hydrogen sulfide (H2S)

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Both Bacteria and Archaea Are Prokaryotes

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16.10 Prokaryotes have particular structural features

Prokaryotes are unicellular organisms Range in size from 1-10 μm in length and 0.7-1.5 μm in

width Prokaryote means “before a nucleus”

These organisms lack a eukaryotic nucleus, but have a dense area called a nucleoid, consisting of a circular strand of DNA

Three Basic Shapes of Prokaryotes Cocci (sing., coccus) - round or spherical Bacilli (sing., bacillus) - rod-shaped Spirilla (sing., spirillum) - spiral- or helical-shaped

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Figure 16.10A Anatomy of bacteria

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Figure 16.10B The three shapes of prokaryotes

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16.11 Prokaryotes have a common reproductive strategy

Figure 16.11A Binary fission results in two bacteria

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Formation of Endospores in Bacteria

When faced with unfavorable environmental conditions, some bacteria form endospores A portion of the cytoplasm and a copy of the

chromosome dehydrate and are then encased by a heavy, protective spore coat

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Figure 16.11B Endospores within Clostridium tetani, a bacterium

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16.12 How genes are transferred in bacteria

Transformation - a recipient bacterium picks up from its surroundings free pieces of DNA secreted by live prokaryotes or released by dead prokaryotes

Conjugation - the donor bacterium passes DNA to the recipient by way of a sex pilus, which temporarily joins the two bacteria

Transduction - bacteriophages carry portions of bacterial DNA from a donor cell to a recipient When a bacteriophage injects its DNA into the donor cell, the

phage DNA takes over the machinery of the cell and causes it to produce more phage particles

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Figure 16.12A Gene transfer by transformation

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Figure 16.12B Gene transfer by conjugation

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Figure 16.12C Gene transfer by transduction

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16.13 Prokaryotes have various means of nutrition

Obligate Anaerobes - unable to grow in the presence of free oxygen A few serious illnesses—such as botulism, gas

gangrene, and tetanus—are caused by anaerobic bacteria

Facultative anaerobes - able to grow in either presence or absence of oxygen

Most prokaryotes are aerobic and require a constant supply of oxygen

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Autotrophic Prokaryotes

Some prokaryotes produce their own organic nutrients

Photoautotrophs use solar energy to reduce carbon dioxide to organic compounds Two types of photoautotrophic bacteria

Those that evolved first and do not give off oxygen Those that evolved later and do give off oxygen

Chemoautotrophs remove electrons from inorganic compounds, such as hydrogen gas, hydrogen sulfide, and ammonia, and to reduce CO2 to an organic molecule

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Figure 16.13A Some anaerobic photosynthetic bacteria live in the muddy bottom of eutrophic lakes

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Figure 16.13B Some chemosynthetic prokaryotes live at hydrothermal vents

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Chemoheterotrophic Prokaryotes

Many prokaryotes are aerobic saprotrophs They secrete digestive enzymes into the environment to

breakdown large organic molecules to smaller ones to be absorbed

In ecosystems, saprotrophic bacteria are called decomposers They play a critical role in recycling matter and make

inorganic molecules available to photosynthesizers

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16.14 The cyanobacteria are ecologically important organisms Perform photosynthesis like plants and are likely

the first to have generated oxygen Some possess heterocysts for nitrogen fixation Common in aquatic habitats and some harsh

habitats Some are symbiotic with other organisms (e.g.

lichens are cyanobacteria and fungi)

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16.15 Some archaea live in extreme environments

Archaea are found in extreme environments hot springs, thermal vents, and salt basins

They may have diverged from a common ancestor relatively soon after life began

Structure and Function Plasma membranes of archaea contain unusual lipids

that allow them to function at high temperatures Some have unique forms of metabolism

Methanogens have the unique ability to form methane

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Types of Archaea

Figure 16.15A Methanogen habitat and structure 16-48

Figure 16.15B Halophile habitat and structure

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Figure 16.15C Thermoacidophile habitat and structure

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APPLYING THE CONCEPTS—HOW BIOLOGY IMPACTS OUR LIVES 16.16 Prokaryotes have environmental

and medical importance

Prokaryotes are everywhere Prokaryotes are most cosmopolitan of all life-forms and

are virtually everywhere Found in oceans, our intestines, hot springs, and soil

Prokaryotes were and are environmentally important Ancient photosynthetic cyanobacteria released copious

amounts of oxygen Prokaryotes play an essential role in the carbon

nitrogen, sulfur, and phosphorus environmental cycles16-51

Prokaryotes Are Medically Important

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APPLYING THE CONCEPTS—HOW BIOLOGY IMPACTS OUR LIVES 16.17 Disease-causing microbes can

be biological weapons Biological warfare is the use of viruses and bacteria

as weapons of war Likely agents to be used by bioterrorists

Anthrax - from the bacterium Bacillus anthracis Smallpox - caused by the variola virus Botulism - caused by the toxin of the anaerobic

bacterium Clostridium botulinum Plague - from the bacterium Yersinia pestis, has been

called the Black Death and bubonic plague Tularemia - caused by the bacterium Francisella

tularensis Hemorrhagic fevers - caused by several types of

viruses, are characterized by high fever and severe bleeding from several organs 16-53

Connecting the Concepts:Chapter 16

Viruses are noncellular, disease-causing agents Medical significance of viruses cannot be underestimated Humans use viruses for gene research and even for gene therapy

Prokaryotes are cellular, but their structure is simpler than eukaryotes They lack a nucleus and membranous organelles

Many prokaryotes live in environments that may resemble habitats available when Earth first formed We find prokaryotes in such hostile habitats as swamps, the Dead

Sea, and hot sulfur springs Cyanobacteria are believed to have introduced oxygen into the

Earth’s ancestral atmosphere Most bacteria are decomposers that recycle nutrients in both

aquatic and terrestrial environments16-54