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Chapter 13

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Biology 1225
Michael Butler

Chapter 13 Early Life Forms and the Viruses - Life originated on earth more than 3.8 billion years ago - Its origin and subsequent evolution have been linked to the physical and chemical evolution of the universe, the stars and our solar system - In the past century, many studies and experiments have provided indirect evidence that life originated under conditions that presumable existed on the early Earth - In comparison to the age of the Earth, humans (Homo Sapiens) have been around for only a very short period of time - Humans are a recent offshoot of the primate group - Human evolution has been marked by trends that involved changes in our bones, muscles, teeth, sensory systems and the brain Taxonomy – the science of the naming and categorization of organisms according to their evolutionary similarities - Naturalists and biologists have documented more than 1.4 million species that exist on the Earth today, and perhaps millions more wait to be discovered and classifies. - Classification systems are used to organize species into categories based on the evolutionary relatedness. ***We do not need to memorize dates for major events in the history of life. - be sure to know what conditions were like on the early, primitive Earth and how those conditions (such as the development of the gases in the atmosphere) contributed to the appearance of life. - the evidence is that the first cells to evolve were bacteria that only undertook anaerobic metabolism since there was no free atmospheric oxygen at that time to allow aerobic metabolism. 1 13.2 Before There were cells page 237 hydrothermal vent – underwater opening from which mineral rich water heated by geothermal energy streams out iron-sulfur world hypothesis – hypothesis that life began in rocks rich in iron sulphide near deep sea hydrothermal vents protocells – membranous sac that contains interacting organic molecules; hypothesized to have formed prior to the earliest life forms RNA world hypothesis – hypothesis that RNA served as the first material of inheritance 1. Laboratory simulations provide indirect evidence that organic subunits can self-assemble under certain condition. 2. They also show how complex organic compounds and protocells may have formed on the early earth 3. The iron-sulfur world hypothesis holds that these events took place near hydrothermal vents 4. The RNA world hypothesis holds that the first genome was RNA based. Take Home Message What do scientific studies reveal about the origin of life? 1. Small organic subunits could have formed on the early earth, or formed in space and fallen to earth on meteorites 2. Complex organic molecules could have self-assembled from simpler ones. 3. The first genetic material may have been RNA instead of DNA 4. Protocells – chemical filled membranous sacs that grow and divide – may have been the ancestors of the first cells 2 13.3 Origin of the Three domains page 240 ***know all about endosymbiosis for exam*** endosymbiosis – one species lives inside another - Some eukaryotic organelles are though to be descended from bacteria - Mitochondria and chloroplasts resemble bacteria in their structure and genome, so these organelles probably evolved through endosymbiosis. - By this process, one species enters another, then lives and replicates inside it (Endo means within; symbiosis means living together) - Endosymbionts inside a cell can be passed along to the cell’s descendants when the cell divides. Ozone layer – atmospheric layer with a high concentration of ozone that prevents much UV radiation from reaching Earth’s surface Stromatolites – Dome-shaped structures composed of layers of prokaryotic cells and sediments; form in shallow seas 1. Fossil stromatolites are evidence of early bacterial life. 2. An early branching separated the bacteria from the archaea. 3. Production of oxygen by some photosynthetic bacteria altered Earth’s atmosphere and allowed formation of a protective ozone layer 4. Eukaryotes have a composite ancestry with both bacterial and archaeal genes. 5. ***Know for exam*** Mitochondria and chloroplasts are thought to have evolved from bacteria by endosymbiosis. 3 Take Home Message What was early life like and how did it change Earth? 1. Life arose by 3-4 billion years ago; it was initially anaerobic and prokaryotic 2. An early divergence separated ancestors of modern bacteria from the lineage that would lead to archea and eukaryotic cells 3. Evolution of oxygen producing photosynthesis in bacteria led to formation of the ozone layer, and favoured organisms that carried out aerobic respiration 4 13.4 The Viruses page 244 bacteriophage – virus that infects bacteria disease vector – organism that carries a pathogen from one host to the next viral envelope – a layer of cell membrane derived from the host cell in which an enveloped virus was produced virus – a noncellular infectious particle with a protein coat and a genome of RNA or DNA; replicates only in living cells *** Concentrate on being able to identify the general structural features of all viruses – at a minimum know: - a virus consists of nucleic acid (either RNA or DNA but never both) held inside a protein based shell called a protein coat (Capsid) - The viral coat consists of many protein subunits that bond together in a repeating pattern, producing a helical rod or polyhedral shape. - In many viruses that infect animals the protein coat is enclosed within a viral envelope - Viruses do not have cytoplasm, at most will contain only one or two enzymes that have a specialized purpose in their infection process and are absolute obligate parasites of cells. - 1. Viruses consist of RNA or DNA inside a protein coat (capsid) 2. Some also have a viral envelope 3. Viruses replicate only in living cells, as when bacteriophages multiply in bacteria 4. Mutation and viral reassortment produce new types of viruses 5. Insects serve as disease vectors that spread some viral diseases of plants. 6. HIV infects human cells. It began as an epidemic and now is a pandemic. HIV is an enveloped RNA virus. Pg. 246 animated 5 6 Take Home Message What are viruses and how do they affect us? 1. Viruses are noncellular particles that consist of genetic material wrapped in a protein coat (capsid). They replicate only within living cells. 2. Viral structure varies. Each type of virus infects and replicates inside a specific type of host 3. A virus harms and eventually kills a host cell. Viral genes direct the host cell’s metabolic machinery to produce new viral particles 4. Viral genomes can be altered by mutation. Viruses with new combinations of genes also arise as a result of viral reassortment. 7 13.5 Bacteria and Archaea page 248 archaea – prokaryotes most closely related to eukaryotes; many live in extreme environments autotroph – organism that uses carbon dioxide as its carbon source; obtains energy from light or breakdown of minerals heterotroph – organism that obtains both carbon and energy by breaking down organic compounds. bacteria – most diverse and well known lineage of single elled organisms that lack a nucleus (prokaryote) binary fission – method of asexual reproduction in which a prokaryote divides into two identical descendant cells conjugation – one bacterium transfers a plasmid to another decomposer – organism that breaks down organic material into its inorganic subunits plasmid – of many prokaryotes, a small ring of non chromosomal DNA replicated independently of the chromosome extreme halophile – organism that lives where the salt concentration is high extreme thermophile – organism that lives where the temperature i
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