Biology in Focus - Chapter 24

CAMPBELL BIOLOGY IN FOCUS

Urry • Cain • Wasserman • Minorsky • Jackson • Reece

Lecture Presentations by Kathleen Fitzpatrick and Nicole Tunbridge

24Early Life and the Diversification of Prokaryotes

Earth formed 4.6 billion years ago The oldest fossil organisms are prokaryotes dating

back to 3.5 billion years ago Prokaryotes are single-celled organisms in the

domains Bacteria and Archaea Some of the earliest prokaryotic cells lived in dense

mats that resembled stepping stones

Overview: The First Cells

Figure 24.1

Prokaryotes are the most abundant organisms on Earth

There are more in a handful of fertile soil than the number of people who have ever lived

Prokaryotes thrive almost everywhere, including places too acidic, salty, cold, or hot for most other organisms

Some prokaryotes colonize the bodies of other organisms

Figure 24.2

Concept 24.1: Conditions on early Earth made the origin of life possible

Chemical and physical processes on early Earth may have produced very simple cells through a sequence of stages

1. Abiotic synthesis of small organic molecules

2. Joining of these small molecules into macromolecules

3. Packaging of molecules into protocells, membrane-bound droplets that maintain a consistent internal chemistry

4. Origin of self-replicating molecules

Synthesis of Organic Compounds on Early Earth

Earth’s early atmosphere likely contained water vapor and chemicals released by volcanic eruptions (nitrogen, nitrogen oxides, carbon dioxide, methane, ammonia, and hydrogen)

As Earth cooled, water vapor condensed into oceans, and most of the hydrogen escaped into space

In the 1920s, A. I. Oparin and J. B. S. Haldane hypothesized that the early atmosphere was a reducing environment

In 1953, Stanley Miller and Harold Urey conducted lab experiments that showed that the abiotic synthesis of organic molecules in a reducing atmosphere is possible

However, the evidence is not yet convincing that the early atmosphere was in fact reducing

Instead of forming in the atmosphere, the first organic compounds may have been synthesized near volcanoes or deep-sea vents

Miller-Urey-type experiments demonstrate that organic molecules could have formed with various possible atmospheres

Organic molecules have also been found in meteorites

Video: Hydrothermal Vent Video: Tubeworms

Figure 24.3

1953 19532008

Figure 24.3a

Abiotic Synthesis of Macromolecules

RNA monomers have been produced spontaneously from simple molecules

Small organic molecules polymerize when they are concentrated on hot sand, clay, or rock

Protocells

Replication and metabolism are key properties of life and may have appeared together

Protocells may have been fluid-filled vesicles with a membrane-like structure

In water, lipids and other organic molecules can spontaneously form vesicles with a lipid bilayer

Adding clay can increase the rate of vesicle formation Vesicles exhibit simple reproduction and metabolism

and maintain an internal chemical environment

Figure 24.4

Vesicleboundary

Precursormolecules only

ity, a

(a) Self-assembly

Time (minutes)

00 20 40 60

Precursor molecules plusmontmorillonite clay

20 m(c) Absorption of RNA(b) Reproduction

Figure 24.4a

Precursormolecules only

ity, a

Time (minutes)

00 20 40 60

Precursor molecules plusmontmorillonite clay

(a) Self-assembly

Figure 24.4b

20 m(b) Reproduction

Figure 24.4c

Vesicleboundary

(c) Absorption of RNA

Self-Replicating RNA

The first genetic material was probably RNA, not DNA

RNA molecules called ribozymes have been found to catalyze many different reactions For example, ribozymes can make complementary

copies of short stretches of RNA

Natural selection has produced self-replicating RNA molecules

RNA molecules that were more stable or replicated more quickly would have left the most descendant RNA molecules

The early genetic material might have formed an “RNA world”

Vesicles with RNA capable of replication would have been protocells

RNA could have provided the template for DNA, a more stable genetic material

Fossil Evidence of Early Life

Many of the oldest fossils are stromatolites, layered rocks that formed from the activities of prokaryotes up to 3.5 billion years ago

Ancient fossils of individual prokaryotic cells have also been discovered For example, fossilized prokaryotic cells have been

found in 3.4-billion-year-old rocks from Australia

Figure 24.5

Time (billions of years ago)

10 m30

Nonphotosynthetic bacteria

Cyanobacteria

Stromatolites

Possibleearliestappearancein fossil record

4 3 2 1 0

Figure 24.5a

Time (billions of years ago)

Nonphotosynthetic bacteria

Cyanobacteria

Stromatolites

Possibleearliestappearancein fossil record

4 3 2 1 0

Figure 24.5b

3-billion-year-oldfossil of a cluster ofnonphotosyntheticprokaryote cells

Figure 24.5c

5 cm1.1-billion-year-oldfossilized stromatolite

Figure 24.5d

1.5-billion-year-old fossilof a cyanobacterium

The cyanobacteria that form stromatolites were the main photosynthetic organisms for over a billion years

Early cyanobacteria began the release of oxygen into Earth’s atmosphere

Surviving prokaryote lineages either avoided or adapted to the newly aerobic environment

Concept 24.2: Diverse structural and metabolic adaptations have evolved in prokaryotes

Most prokaryotes are unicellular, although some species form colonies

Most prokaryotic cells have diameters of 0.5–5 µm, much smaller than the 10–100 µm diameter of many eukaryotic cells

Prokaryotic cells have a variety of shapes The three most common shapes are spheres (cocci),

rods (bacilli), and spirals

Figure 24.6

(a) Spherical (b) Rod-shaped (c) Spiral

Figure 24.6a

(a) Spherical

Figure 24.6b

(b) Rod-shaped

Figure 24.6c

(c) Spiral

Cell-Surface Structures

A key feature of nearly all prokaryotic cells is their cell wall, which maintains cell shape, protects the cell, and prevents it from bursting in a hypotonic environment

Eukaryote cell walls are made of cellulose or chitin Bacterial cell walls contain peptidoglycan, a network

of modified sugars cross-linked by polypeptides

Archaeal cell walls contain polysaccharides and proteins but lack peptidoglycan

Scientists use the Gram stain to classify bacteria by cell wall composition

Gram-positive bacteria have simpler walls with a large amount of peptidoglycan

Gram-negative bacteria have less peptidoglycan and an outer membrane that can be toxic

Figure 24.7

Peptido-glycanlayer

Cellwall

Gram-negativebacteria

Gram-positivebacteria

(b) Gram-negativebacteria

(a) Gram-positivebacteria

Plasmamembrane Plasma membrane

Peptidoglycanlayer

Cellwall

Outermembrane

Carbohydrate portionof lipopolysaccharide

Figure 24.7a

Peptido-glycanlayer

Cellwall

(a) Gram-positivebacteria

Plasmamembrane

Figure 24.7b

(b) Gram-negativebacteria

Plasma membrane

Peptidoglycanlayer

Cellwall

Outermembrane

Carbohydrate portionof lipopolysaccharide

Figure 24.7c

Gram-negativebacteria

Many antibiotics target peptidoglycan and damage bacterial cell walls

Gram-negative bacteria are more likely to be antibiotic resistant

A polysaccharide or protein layer called a capsule covers many prokaryotes

Figure 24.8

Bacterialcell wall

Bacterialcapsule

Tonsilcell

200 nm

Some bacteria develop resistant cells called endospores when they lack an essential nutrient

Other bacteria have fimbriae, which allow them to stick to their substrate or other individuals in a colony

Pili (or sex pili) are longer than fimbriae and allow prokaryotes to exchange DNA

Figure 24.9

Fimbriae

Motility

In a heterogeneous environment, many bacteria exhibit taxis, the ability to move toward or away from a stimulus

Chemotaxis is the movement toward or away from a chemical stimulus

Most motile bacteria propel themselves by flagella scattered about the surface or concentrated at one or both ends

Flagella of bacteria, archaea, and eukaryotes are composed of different proteins and likely evolved independently

Figure 24.10

Flagellum

Filament 20 nm

HookMotorCell wall

RodPeptidoglycanlayer

Plasmamembrane

Figure 24.10a

Evolutionary Origins of Bacterial Flagella

Bacterial flagella are composed of a motor, hook, and filament

Many of the flagella’s proteins are modified versions of proteins that perform other tasks in bacteria

Flagella likely evolved as existing proteins were added to an ancestral secretory system

This is an example of exaptation, where existing structures take on new functions through descent with modification

Internal Organization and DNA

Prokaryotic cells usually lack complex compartmentalization

Some prokaryotes do have specialized membranes that perform metabolic functions

These are usually infoldings of the plasma membrane

Figure 24.11

Respiratorymembrane

0.2 m 1 m

Thylakoidmembranes

(a) Aerobic prokaryote (b) Photosynthetic prokaryote

Figure 24.11a

Respiratorymembrane

(a) Aerobic prokaryote

Figure 24.11b

Thylakoidmembranes

(b) Photosynthetic prokaryote

The prokaryotic genome has less DNA than the eukaryotic genome

Most of the genome consists of a circular chromosome

The chromosome is not surrounded by a membrane; it is located in the nucleoid region

Some species of bacteria also have smaller rings of DNA called plasmids

Figure 24.12

Plasmids

Chromosome

There are some differences between prokaryotes and eukaryotes in DNA replication, transcription, and translation

These allow people to use some antibiotics to inhibit bacterial growth without harming themselves

Nutritional and Metabolic Adaptations

Prokaryotes can be categorized by how they obtain energy and carbon Phototrophs obtain energy from light Chemotrophs obtain energy from chemicals

Autotrophs require CO2 as a carbon source

Heterotrophs require an organic nutrient to make organic compounds

Energy and carbon sources are combined to give four major modes of nutrition Photoautotrophy Chemoautotrophy Photoheterotrophy Chemoheterotrophy

Table 24.1

The Role of Oxygen in Metabolism

Prokaryotic metabolism varies with respect to O2

Obligate aerobes require O2 for cellular respiration

Obligate anaerobes are poisoned by O2 and use fermentation or anaerobic respiration, in which substances other than O2 act as electron acceptors

Facultative anaerobes can survive with or without O2

Nitrogen Metabolism

Nitrogen is essential for the production of amino acids and nucleic acids

Prokaryotes can metabolize nitrogen in a variety of ways

In nitrogen fixation, some prokaryotes convert atmospheric nitrogen (N2) to ammonia (NH3)

Metabolic Cooperation

Cooperation between prokaryotes allows them to use environmental resources they could not use as individual cells

In the cyanobacterium Anabaena, photosynthetic cells and nitrogen-fixing cells called heterocysts (or heterocytes) exchange metabolic products

Figure 24.13

Heterocyst

Photosyntheticcells

In some prokaryotic species, metabolic cooperation occurs in surface-coating colonies called biofilms

Reproduction

Prokaryotes reproduce quickly by binary fission and can divide every 1–3 hours

Key features of prokaryotic biology allow them to divide quickly They are small They reproduce by binary fission They have short generation times

Adaptations of Prokaryotes: A Summary

The ongoing success of prokaryotes is an extraordinary example of physiological and metabolic diversification

Prokaryotic diversification can be viewed as a first great wave of adaptive radiation in the evolutionary history of life

Prokaryotes have considerable genetic variation Three factors contribute to this genetic diversity

Rapid reproduction Mutation Genetic recombination

Concept 24.3: Rapid reproduction, mutation, and genetic recombination promote genetic diversity in prokaryotes

Rapid Reproduction and Mutation

Prokaryotes reproduce by binary fission, and offspring cells are generally identical

Mutation rates during binary fission are low, but because of rapid reproduction, mutations can accumulate rapidly in a population

High diversity from mutations allows for rapid evolution

Prokaryotes are not “primitive” but are highly evolved

Figure 24.14 Experiment

0.1 mL(population sample)

Results

Daily serial transfer

Old tube(discardedaftertransfer)

New tube(9.9 mLgrowthmedium)

Generation10,000 20,00015,0005,0000

Figure 24.14a

ResultsPo

Generation10,000 20,00015,0005,0000

Genetic Recombination

Genetic recombination, the combining of DNA from two sources, contributes to diversity

Prokaryotic DNA from different individuals can be brought together by transformation, transduction, and conjugation

Movement of genes among individuals from different species is called horizontal gene transfer

Transformation and Transduction

A prokaryotic cell can take up and incorporate foreign DNA from the surrounding environment in a process called transformation

Transduction is the movement of genes between bacteria by bacteriophages (viruses that infect bacteria)

Figure 24.15-1

1 Phage infects bacterialdonor cell with A andB alleles.

Donor cell

Phage DNA

Figure 24.15-2

Phage DNA isreplicated andproteins synthesized.

Donor cell

Phage DNA

Figure 24.15-3

Fragment of DNA withA allele is packagedwithin a phage capsid.

Donor cell

Phage DNA

Phage with A alleleinfects bacterialrecipient cell.

Recipientcell

Crossing over

Donor cell

A− B−

Phage DNAFigure 24.15-4

Figure 24.15-5

Phage infects bacterialdonor cell with A andB alleles.

Incorporation of phageDNA creates recombinantcell with genotype AB.

Phage with A alleleinfects bacterialrecipient cell.

Recombinantcell

Recipientcell

Crossing over

Donor cell

A B−

A− B−

Phage DNA

Conjugation and Plasmids

Conjugation is the process where genetic material is transferred between prokaryotic cells

In bacteria, the DNA transfer is one way In E. coli, the donor cell attaches to a recipient by a

pilus, pulls it closer, and transfers DNA

Figure 24.16

Sex pilus

The F factor is a piece of DNA required for the production of pili

Cells containing the F plasmid (F+) function as DNA donors during conjugation

Cells without the F factor (F–) function as DNA recipients during conjugation

The F factor is transferable during conjugation

Figure 24.17-1

1 One strand ofF cell plasmidDNA breaks atarrowhead.

Bacterialchromosome

F plasmid

Matingbridge

F cell(donor)

F− cell(recipient)

Figure 24.17-2

Bacterialchromosome

F plasmid

Matingbridge

F cell(donor)

Broken strandpeels off andenters F− cell.

Figure 24.17-3

Bacterialchromosome

F plasmid

Matingbridge

F cell(donor)

Donor andrecipient cellssynthesizecomplementaryDNA strands.

Figure 24.17-4

Bacterialchromosome

F plasmid

Matingbridge

F cell(donor)

Recipient cellis now arecombinantF cell.

Donor andrecipient cellssynthesizecomplementaryDNA strands.

The F factor can also be integrated into the chromosome

A cell with the F factor built into its chromosomes functions as a donor during conjugation

The recipient becomes a recombinant bacterium, with DNA from two different cells

R Plasmids and Antibiotic Resistance Genes for antibiotic resistance are carried in R

plasmids Antibiotics kill sensitive bacteria, but not bacteria with

specific R plasmids Through natural selection, the fraction of bacteria

with genes for resistance increases in a population exposed to antibiotics

Antibiotic-resistant strains of bacteria are becoming more common

Concept 24.4: Prokaryotes have radiated into a diverse set of lineages

Prokaryotes have radiated extensively due to diverse structural and metabolic adaptations

Prokaryotes inhabit every environment known to support life

An Overview of Prokaryotic Diversity

Applying molecular systematics to the investigation of prokaryotic phylogeny has produced dramatic results

Molecular systematics led to the splitting of prokaryotes into bacteria and archaea

Molecular systematists continue to work on the phylogeny of prokaryotes

Figure 24.18

UNIVERSALANCESTOR

ainEukarya

Cyanobacteria

Spirochetes

Chlamydias

Proteobacteria

Nanoarchaeotes

Crenarchaeotes

Euryarchaeotes

Korarchaeotes

Eukaryotes

ain Archaea

ain Bacteria

The use of polymerase chain reaction (PCR) has allowed for more rapid sequencing of prokaryote genomes

A handful of soil may contain 10,000 prokaryotic species

Horizontal gene transfer between prokaryotes obscures the root of the tree of life

Bacteria

Bacteria include the vast majority of prokaryotes familiar to most people

Diverse nutritional types are scattered among the major groups of bacteria

Video: Tubeworms

Figure 24.UN01

Eukarya

BacteriaArchaea

Figure 24.19a

Alpha subgroup Beta subgroup

Gamma subgroup Delta subgroup Epsilon subgroup

Rhizobium (arrows)(TEM)

Nitrosomonas(TEM)

Thiomargaritanamibiensis (LM)

Helicobacter pylori(TEM)

Chondromycescrocatus (SEM)

Proteo-bacteria

AlphaBetaGammaDeltaEpsilon

Proteobacteria are gram-negative bacteria including photoautotrophs, chemoautotrophs, and heterotrophs

Some are anaerobic and others aerobic

Figure 24.19aa

Proteobacteria

AlphaBetaGammaDeltaEpsilon

Members of the subgroup alpha proteobacteria are closely associated with eukaryotic hosts in many cases

Scientists hypothesize that mitochondria evolved from aerobic alpha proteobacteria through endosymbiosis

Example: Rhizobium, which forms root nodules in legumes and fixes atmospheric N2

Example: Agrobacterium, which produces tumors in plants and is used in genetic engineering

Figure 24.19ab

Alpha subgroupRhizobium (arrows)inside a root cell of a legume (TEM)

Members of the subgroup beta proteobacteria are nutritionally diverse

Example: the soil bacterium Nitrosomonas, which converts NH4

+ to NO2–

Figure 24.19ac

Beta subgroupNitrosomonas(colorized TEM)

The subgroup gamma proteobacteria includes sulfur bacteria such as Thiomargarita namibiensis and pathogens such as Legionella, Salmonella, and Vibrio cholerae

Escherichia coli resides in the intestines of many mammals and is not normally pathogenic

Figure 24.19ad

Gamma subgroupThiomargaritanamibiensis containingsulfur wastes (LM)

The subgroup delta proteobacteria includes the slime-secreting myxobacteria and bdellovibrios, a bacteria that attacks other bacteria

Figure 24.19ae

Delta subgroupFruiting bodies ofChondromyces crocatus,a myxobacterium (SEM)

The subgroup epsilon proteobacteria contains many pathogens including Campylobacter, which causes blood poisoning, and Helicobacter pylori, which causes stomach ulcers

Figure 24.19af

Epsilon subgroupHelicobacter pylori(colorized TEM)

Figure 24.19b

Spirochetes Cyanobacteria

Gram-positive bacteria

Chlamydias

Leptospira(TEM)

Oscillatoria

Streptomyces(SEM)

Chlamydia (arrows)(TEM)

Mycoplasmas(SEM)

Chlamydias are parasites that live within animal cells

Chlamydia trachomatis causes blindness and nongonococcal urethritis by sexual transmission

Figure 24.19ba

ChlamydiasChlamydia (arrows)inside an animal cell(colorized TEM)

Spirochetes are helical heterotrophs Some are parasites, including Treponema pallidum,

which causes syphilis, and Borrelia burgdorferi, which causes Lyme disease

Figure 24.19bb

SpirochetesLeptospira,a spirochete(colorized TEM)

Cyanobacteria are photoautotrophs that generate O2

Plant chloroplasts likely evolved from cyanobacteria by the process of endosymbiosis

Figure 24.19bc

CyanobacteriaOscillatoria,a filamentouscyanobacterium

Gram-positive bacteria include Actinomycetes, which decompose soil Streptomyces, which are a source of antibiotics Bacillus anthracis, the cause of anthrax Clostridium botulinum, the cause of botulism Some Staphylococcus and Streptococcus, which can

be pathogenic Mycoplasms, the smallest known cells

Figure 24.19bd

Gram-positive bacteriaStreptomyces,the source ofmany antibiotics(SEM)

Figure 24.19be

Gram-positive bacteriaHundreds of mycoplasmascovering a humanfibroblast cell(colorized SEM)

Archaea

Archaea share certain traits with bacteria and other traits with eukaryotes

Figure 24.UN02

Eukarya

BacteriaArchaea

Table 24.2

Table 24.2a

Table 24.2b

Some archaea live in extreme environments and are called extremophiles

Extreme halophiles live in highly saline environments

Extreme thermophiles thrive in very hot environments

Video: Cyanobacteria (Oscillatoria)

Figure 24.20

Methanogens produce methane as a waste product Methanogens are strict anaerobes and are poisoned

Methanogens live in swamps and marshes, in the guts of cattle, and near deep-sea hydrothermal vents

Figure 24.21

Figure 24.21a

Figure 24.21b

Recent metagenomic studies have revealed many new groups of archaea

Some of these may offer clues to the early evolution of life on Earth

Concept 24.5: Prokaryotes play crucial roles in the biosphere

Prokaryotes are so important that if they were to disappear, the prospects for any other life surviving would be dim

Chemical Recycling

Prokaryotes play a major role in the recycling of chemical elements between the living and nonliving components of ecosystems

Chemoheterotrophic prokaryotes function as decomposers, breaking down dead organisms and waste products

Prokaryotes can sometimes increase the availability of nitrogen, phosphorus, and potassium for plant growth

Prokaryotes can also “immobilize” or decrease the availability of nutrients

Figure 24.22

Seedlings growing in the lab

Soil treatment

Strain 1 Strain 2 Strain 3Nobacteria

Figure 24.22a

Seedlings growing in the lab

Ecological Interactions

Symbiosis is an ecological relationship in which two species live in close contact: a larger host and smaller symbiont

Prokaryotes often form symbiotic relationships with larger organisms

In mutualism, both symbiotic organisms benefit In commensalism, one organism benefits while

neither harming nor helping the other in any significant way

In parasitism, an organism called a parasite harms but does not kill its host

Parasites that cause disease are called pathogens

Figure 24.23

The ecological communities of hydrothermal vents depend on chemoautotrophic bacteria for energy

Impact on Humans

The best-known prokaryotes are pathogens, but many others have positive interactions with humans

Mutualistic Bacteria

Human intestines are home to about 500–1,000 species of bacteria

Many of these are mutualists and break down food that is undigested by our intestines

Pathogenic Bacteria

Prokaryotes cause about half of all human diseases For example, Lyme disease is caused by a bacterium

and carried by ticks

Figure 24.24

Figure 24.24a

Figure 24.24b

Figure 24.24c

Pathogenic prokaryotes typically cause disease by releasing exotoxins or endotoxins

Exotoxins are secreted and cause disease even if the prokaryotes that produce them are not present

Endotoxins are released only when bacteria die and their cell walls break down

Horizontal gene transfer can spread genes associated with virulence

For example, pathogenic strains of the normally harmless E. coli bacteria have emerged through horizontal gene transfer

Prokaryotes in Research and Technology

Experiments using prokaryotes have led to important advances in DNA technology For example, E. coli is used in gene cloning For example, Agrobacterium tumefaciens is used to

produce transgenic plants

Bacteria can now be used to make natural plastics Prokaryotes are the principal agents in

bioremediation, the use of organisms to remove pollutants from the environment

Bacteria can be engineered to produce vitamins, antibiotics, and hormones

Bacteria are also being engineered to produce ethanol from waste biomass

Figure 24.25

Figure 24.25a

Figure 24.25b

Figure 24.26

Figure 24.UN03

Figure 24.UN04

FimbriaeCell wall

Capsule

Flagella

Sex pilus

Internalorganization

Circularchromosome

Figure 24.UN05

Biology in Focus - Chapter 24

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Transcript of Biology in Focus - Chapter 24

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