Chapter 24 & 25 Notes PDF

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Notes cover all of Chapter 24 and 25 lecture slides, Prof./Dr. Hall. Topics of Chapter 24 include: conditions of early earth that made origin of life possible, diverse structural and metabolic adaptations in prokaryotes, rapid reproduction, mutation, genetic recombination, genetic diversity, and pro...

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Chapter 24 Early Life and the Diversification of Prokaryotes

The First Cells 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 the dense mats; others were free-floating individual cells Prokaryotes were alone on Earth until the first eukaryote cells appeared 1.8 billion years ago They are the most abundant organisms on Earth They thrive in most environments, including places too acidic, salty, cold, or hot for most other organisms Some prokaryotes colonize the bodies of other organisms

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 four stages: 1. Abiotic synthesis of small organic molecules 2. Joining of these small molecules into macromolecules 3. Packaging of molecule into protocells, membrane-bound droplets that maintain consistent internal chemistry 4. Origin of self-replicating molecules

Synthesis of Organic Compounds on Early Earth Earth’s early atmosphere had little oxygen and likely contained water vapor and compounds released by volcanic eruptions Example: nitrogen, nitrogen oxides, carbon dioxide, methane, ammonia, and hydrogen As Earth cooled, water vapor condensed into oceans, and most hydrogen escaped into space Organic compounds may have originated in reducing conditions by volcanoes or deep-sea vents Amino acids form spontaneously in experimental conditions simulating volcanic eruptions

Protocells Replication and metabolism are key properties of life that likely appeared together in early protocells 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 Abiotically produced vesicles have a semi-permeable membrane and have simple reproduction They absorb molecules from the surroundings and metabolize reagents from external sources

Self-Replicating RNA and Protocells The first genetic material was likely RNA, not DNA Single-stranded RNA molecules have 3-D shapes, making catalytic function possible Ribozymes are RNA molecules that can catalyze reactions; some are also self-replicating Ribozymes with faster, more accurate self-replication would leave more descendent molecules Life today was likely preceded by an “RNA World” Double-stranded DNA is a more stable and accurately replicated genetic material than RNA

Fossil Evidence of Early Life Many of the oldest fossils are stromatolites, layered rocks that formed from the activities of prokaryotes 3.5 bya Stromatolites with distinct morphologies and habitats evolved 3.1 and 2.8 billion years ago Stromatolites living today are formed by cyanobacteria and other photosynthetic bacteria By 2.5 billion years ago, diverse communities of cyanobacteria populated the oceans and were the main photosynthetic organisms for over a billion years Cyanobacteria transformed the atmosphere by releasing oxygen produced via photosynthesis

Diverse Structural and Metabolic Adaptations Have Evolved in Prokaryotes Most prokaryotes are unicellular, although some species form _______ Prokaryotic cells typically have much smaller diameters than eukaryotic cells; 0.5 – 5 um compared to 10 – 100 um for eukaryotes

Prokaryotes have a variety of cell shapes, including spheres (cocci), rods (bacilli), and spirals

Cell – Surface Structures Cell walls protect and maintain prokaryote shape and prevent bursting in hypotonic environments Bacterial cell walls contain peptidoglycan, a network of modified sugars and polypeptides Archaeal cell walls contain polysaccharides and proteins but lack peptidoglycan Eukaryote cell walls are made of cellulose or chitin Biologists categorize bacteria based on cell wall composition determined by response to Gram stain Gram-positive bacteria have simpler cells walls composed of a thick layer of peptidoglycan Gram-negative bacteria have less peptidoglycan and an outer membrane with lipopolysaccharides Lipid portions of lipopolysaccharides can be toxic Many antibiotics target peptidoglycan and damage bacterial cell walls The outer membrane protects gram-negative bacteria, increasing their antibiotic resistance Many prokaryotes also have a sticky, protective outer layer called a capsule that allows them to adhere to the substrate or each other When water or nutrients are lacking, some bacteria develop resistant cells called endospores Endospores are dormant cells that can remain viable for centuries or survive extreme conditions Some prokaryotes stick to the substrate or each other using hair like appendages called fimbriae Pili (or sex pili) are longer, less numerous appendages that are used to pull prokaryotes together during DNA transfer between cells

Motility About half of all bacteria exhibit taxis, the ability to move toward or away from a stimulus Example: some prokaryotes exhibit chemotaxis, movement toward or away from a chemical stimulus Flagella, the most common structures enabling movement in prokaryotes, likely evolved independently in the 3 domains of life Structurally different from eukaryotes flagella

Internal Organization and DNA Prokaryote cells lack the complex compartmentalization found in eukaryote cells

The prokaryotic genome has less DNA than the eukaryotic genome Most of the genome consists of a single circular chromosome The genetic material is not enclosed inside a membrane; it is located in the nucleoid region Some species of bacteria also have smaller rings of DNA called plasmids

Nutritional and Metabolic Prokaryotes can be categorized by how they obtain energy and carbon Phototrophs obtain energy from lights Chemotrophs obtain energy from organic compounds or inorganic chemicals like H2S Autographs use inorganic carbon sources like CO2 Heterotrophs obtain carbon from organic compounds

Metabolic Cooperation Cooperation between different prokaryotic species occurs in surface-coating colonies called biofilms Biofilms are common but can cause a wide range of problems for humans, including chronic infections, tooth decay, and contamination or medical devices

Rapid Reproduction, Mutation, and Genetic Recombination Promote Genetic Diversity in Prokaryotes Prokaryotes reproduce by binary fission; most offspring are genetically identical to the parent cell Prokaryotes have considerable genetic variation Three factors contribute to this genetic diversity 1. Rapid reproduction 2. Mutation 3. Genetic recombination

Genetic Recombination Prokaryotic DNA form different individuals can be brought together by transformation, transduction, and conjugation Movement of genes among individuals from different species is called horizontal gene transfer

Prokaryotes Have Radiated into a Diverse Set of Lineages Prokaryotes have radiated extensively due to diverse structural and metabolic adaptations Prokaryotes inhibit every environment known to support life

Bacteria Proteobacteria is a clade of gram-negative bacteria with diverse metabolic and nutritional modes It has been divided into 5 subgroups (Alpha, Beta, Gamma, Delta, Epsilon) based on molecular relationships Many species in the subgroup alpha proteobacteria are closely associated with eukaryotic hosts Rhizobium forms root nodules in legumes and fixes atmospheric N2 Members of the subgroup beta proteobacteria have diverse nutritional modes Nitrosomonas participates in soil nitrification by oxidizing ammonium (NH4+) and producing nitrite (NO2-) Some heterotrophic gamma proteobacteria are pathogenic Salmonella causes food poisoning, and Vibrio cholerae causes cholera Escherichia coli is a common heterotrophic gamma proteobacteria that not normally pathogenic Most species in the subgroup epsilon proteobacteria are pathogenic Helicobacter pylori causes stomach ulcers Chlamydias are disease-causing parasites that can only live within animal host cells Chlamydia trachomatis causes blindness and the sexually transmitted disease, nongonococcal urethritis Spirochetes are helical gram-negative heterotrophs Many species are free-living, but some are parasitic Treponema pallidum causes syphilis, and Borrelia burgdorferi causes Lyme disease Cyanobacteria are gram-negative photoautotrophs that generate O2 through plantlike photosynthesis Cyanobacteria are common members of the phytoplankton in marine and freshwater communities Gram-positive bacteria: Actinomycetes, many of which are soil decomposers

Streptomyces, which are a source of antibiotics Bacillus anthracis, cause of anthrax Clostridium botulinum, cause of botulism Staphylococcus + Streptococcus, which can be pathogenic Mycoplasmas, which are the smallest known cells and the only bacteria lacking a cell wall

Archaea Archaea have many unique traits, but they also share some traits in common with bacteria and others with eukaryotes Some archaea live in extreme environments and are called extremophiles Extreme halophiles either tolerate or require a highly saline environment Members of the genus Halobacterium cannot survive if salinity drops below 9% Extreme thermophiles thrive in very hot environments Members if the genus Sulfolobus live in hot springs with temperatures up to 90 degrees C. Other species live near deep-sea hydrothermal vents Methanogens are strict anaerobes that produce methane as a waste product. They live in anoxic environments, including swamps marshes, and the guts of cattle, and near deep-sea hydrothermal vents

Prokaryotes Play Crucial Roles in the Biosphere If humans disappeared from Earth, few other species would be driven to extinction The role of prokaryotes in the biosphere is essential to the survival of many other species Prokaryotes play a major role in recycling chemical elements between the living and nonliving components of ecosystems Some chemoheterotrophic prokaryotes are decomposers, organisms that break down dead organic materials and release mineral nutrients

Ecological Interactions Symbiosis is an ecological relationship in which 2 species live in close contact: a larger host and smaller symbiont Prokaryotes often form symbiotic relationships with larger organisms These symbiotic relationships increase the fitness of one or both organisms

Some species produce sugars through photosynthesis and others can fix atmospheric nitrogen (N2) into forms available to plants

Impact on Humans The best-known prokaryotes are pathogens, but many other have positive interactions with humans Human intestines are home to about 500-1,000 species of “good” bacteria Many of these are mutualists and break down food that is undigested by out intestines Bacteroides thetaiotaomicron has genes involved in synthesizing carbohydrates, vitamins, and other nutrients needed by humans

Pathogenic Bacteria All pathogenic prokaryotes are bacteria Prokaryotes cause about half of all human diseases More than 1 million people per year die from lung disease caused by Mycobacterium tuberculosis Some bacterial diseases are transmitted by other species such as fleas or ticks Lyme disease, which infects 300,000 people in the U.S. each year, is caused by a bacterium carried by ticks

Prokaryotes in Research and Technology Some bacteria can be used to make natural, biodegradable plastics Others have been engineered to produce ethanol from plant sources and agricultural and municipal wastes Prokaryotes are also used on bioremediation, the use if organisms to remove pollutants from the environment

Chapter 25 The Origin and Diversification of Eukaryotes

Eukaryotes Arose by Endosymbiosis More Than 1.8 Billion Years Ago Early eukaryotes were unicellular Eukaryotic cells have organelles and are structurally more complex than prokaryotic cells Early eukaryotes had a nucleus, nuclear membrane, and cytoskeleton and varied in size and shape A well-developed cytoskeleton enables eukaryotic cells to have asymmetrical forms and to change shape Chemical evidence for the presence of eukaryotes dates back to 2.7 billion years ago The oldest widely accepted fossils of eukaryotic organisms are 1.8 billion years ago

The Fossil Record of Early Eukaryotes Appearance of Novel Features Red algae from 1.2 billion years ago are the oldest eukaryote fossils to be resolved taxonomically Other fossils from this period include green algae, amoebas, and other unidentified colonial and multicellular protists Many novel biological features evolved during this period Multicellularity Sexual life cycles Eukaryotic photosynthesis

Rise of Large Eukaryotes Large multicellular eukaryotes appear in the fossil record in the Ediacaran period, 635 – 541 million years ago Maximum body size, taxonomic diversity, and morphological variation increased dramatically from 635 – 535 million years ago

The Ediacaran biota declined with the onset of the “Cambrian explosion”

Endosymbiosis in Eukaryotic Evolution Eukaryotes have genes and call characteristics derived from both archaea and bacteria, likely as a consequence of endosymbiosis Endosymbiosis is a symbiotic relationship in which one organism lives inside the body or cell of another organism

Origin of Mitochondria and Plastids – Important to Review This Theory The endosymbiont theory proposes that mitochondria and plastids were formerly small bacteria that began living within larger cells An endosymbiont is a cell that lives withing a host cell Bacterial ancestors of mitochondria and plastids likely entered the host cell as undigested prey or internal parasites The relationship between endosymbiont and host cells could have become mutually beneficial Anaerobic host cells may have benefited from aerobic endosymbionts as oxygen increased in the atmosphere Over time, the host and endosymbionts would have become a single organism Key evidence supporting endosymbiotic theory: Inner membranes of mitochondria and plastids are similar to plasma membranes of living bacteria Replication of mitochondria and plastids is similar to cell division in bacteria Mitochondria and plastids have circular D N A, like bacteria Mitochondria and plastids transcribe and translate their own D N A into proteins Ribosomes of mitochondria and plastids are more similar to bacterial than eukaryotic ribosomes

Multicellularity Has Originated Several Times in Eukaryotes The evolution of eukaryotic cells allowed for a greater range of unicellular forms

A second wave of diversification occurred when multicellularity evolved and gave rise to algae, plants, fungi, and animals The first multicellular forms were colonies, collections of connected cells with little or no differentiation Multicellular colonies consisting of simple filaments, balls, or cell sheets occur often in the fossil record

Independent Origins of Complex Multicellularity Multicellular organisms with differentiated cells evolved multiple times Red, green, and brown algae, plants, fungi, and animals all arose independently from different singlecelled ancestors Volvox is a multicellular green alga that forms a monophyletic group with a single-celled alga (Chlamydomonas) and several colonial species Multicellularity in Volvox may have originated through evolution of increasingly complex colonial forms descended from a single-celled common ancestor

Steps in the Origin of Multicellular Animals Multicellularity in animals requires mechanisms for cells to adhere and signal to each other The transition to multicellularity in animals involved new ways of using proteins encoded by genes found in closely related put primitive choanoflagellates rather than the evolution of many novel genes

Four “Supergroups” of Eukaryotes Have Been Proposed Based on Morphological and Molecular Data Eukaryote diversity is influenced by their hybrid origins and the independent origins of complex multicellularity Eukaryotes are divided into four supergroups

Four Supergroups of Eukaryotes Excavata This group includes 3 clades: parabasalids, diplomonads, and euglenozoans Many are parasites, such as Giardia intestines

SAR This group includes 3 large clades: Stramenopila, Alveolata, and Rhizaria

Stramenopiles include important photosynthetic organisms, such as diatoms Rhizarians include many species of amoebas and forams, such as Globigerina

Archaeplastida This supergroup includes red algae, green algae, and plants Volvox is a multicellular green algae belonging to this group

Unikonta The supergroup Unikonta includes animals, fungi, and some protists This group includes 2 major clades: the amoebozoans (tubulinids and slime molds) and the opisthokonts (animals, fungi, and related protists)...