The Living World

Transcript The Living World

The Living World

Fourth Edition

GEORGE B. JOHNSON

The First Single Celled Creatures

PowerPoint ® Lectures prepared by Johnny El-Rady

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

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 (NH 3 , CH 4 ) These simple molecules combined to form more complex molecules Lightning provided the energy

Fig. 15.1

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

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

Fig. 15.2

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

Earth is formed 4.5 billion years ago

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

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

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

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

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

Prokaryotes reproduce by binary fission

Fig. 7.1

Some bacteria undergo a process called conjugation The undirectional transfer of plasmid DNA following cell-to cell contact

Fig. 15.5

Pilus connecting the two cells together

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

Prokaryotic Metabolism

Prokaryotes have evolved many more ways than eukaryotes to acquire carbon and energy Acquisition of Carbon Autotrophs = Use CO 2 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

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 CO 2 Cyanobacteria 2. Chemoautotrophs Obtain energy by oxidizing inorganic substances Nitrifiers oxidize ammonia or nitrite

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

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

Prokaryotes are instrumental in genetic engineering Nonpolluting insect control agents Removing environmental pollutants

Fig. 15.7

Exxon Valdez

oil spill

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

15.6 Prokaryotic Lifestyles

Archaeabacteria include Methanogens Use H 2 to CH 4 to reduce CO 2 Live in swamps and marshes Hot spring in Yellowstone National Park Extremophiles Live in unusually harsh environments Thermoacidophiles Hot and acidic environments

Fig. 15.8

15.6 Prokaryotic Lifestyles

Bacteria include

Fig. 15.9

Cyanobacteria Individual cells adhere in filaments The most prominent photosynthetic bacteria Also capable of nitrogen fixation In specialized cells called heterocysts 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

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

Fig. 15.10 The structure of bacterial, plant and animal viruses

Membrane-like layer Not found in all viruses Capsid

15.8 How Bacteriophages Enter Prokaryotic Cells

Bacteriophages are viruses that infect bacteria The most studied are double-stranded DNA bacteriophages, including

Fig. 15.11b

Phage T4 Phage lambda ( l ) Tail fibers Same basic structure as T4 But only possesses one thin tail fiber

15.8 How Bacteriophages Enter Prokaryotic Cells

Bacteriophages undergo two types of reproductive cycles Lytic cycle Example: T4 bacteriophage Lysogenic cycle Example: Bacteriophage l

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

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

Lytic cycle

Bacterial chromosome Virus attaching to cell wall Viral DNA injected into cell

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

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 Prophage exits the bacterial chromosome

Lytic cycle

Bacterial chromosome Virus attaching to cell wall Viral DNA injected into cell Reduction to prophage

Lysogenic cycle

Viral DNA integrated into bacterial chromosome Reproduction of lysogenic bacteria

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

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)

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

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

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

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

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

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

15.10 Disease Viruses

Smallpox Caused by the variola virus 15 million worldwide cases in 1967 A world immunization campaign eradicated the disease in 1977 Ali Maow Maalin of Somalia The virus still exists in Atlanta and Moscow “Weaponized” smallpox remains a bioterrorist threat

The Living World

Transcript The Living World

The Living World

GEORGE B. JOHNSON

The First Single Celled Creatures

15.1 Origin of Life

Forming Life’s Building Blocks

Forming Life’s Building Blocks

Forming Life’s Building Blocks

15.2 How Cells Arose

15.3 The Simplest Organisms

The Structure of a Prokaryote

The Structure of a Prokaryote

15.4 Comparing Prokaryotes to Eukaryotes

Prokaryotic Metabolism

Prokaryotic Metabolism

Prokaryotic Metabolism

15.5 Importance of Prokaryotes

15.6 Prokaryotic Lifestyles

15.6 Prokaryotic Lifestyles

15.6 Prokaryotic Lifestyles

15.6 Prokaryotic Lifestyles

15.7 The Structure of Viruses

15.8 How Bacteriophages Enter Prokaryotic Cells

15.8 How Bacteriophages Enter Prokaryotic Cells

Lytic Cycle

Lysogenic Cycle

Lysogenic Conversion

15.9 How Animal Viruses Enter Cells

HIV

HIV

HIV

HIV

15.10 Disease Viruses

15.10 Disease Viruses

15.10 Disease Viruses

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