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biology hw lectures 18-20

biology hw lectures 18-20 - Chapter 25 Lecture 18 The...

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Chapter 25 Lecture 18 The Origin of Multicellularity Some single celled eukaryotes gave rise to multicellular forms, whose descendants include a variety of algae, plants, fungi, and animals The Earliest Multicellular Eukaryotes -based on DNA sequences, common ancestor of multicellular eukaryotes lived 1.5 billion years ago -oldest known fossils of multicellular eukaryotes: relatively small algae that lived about 1.2 billion years ago -larger/diverse multicellular eukaryotes appeared 565 million years ago -a series of severe ice ages occurred from 750-580 million years ago which limited in size and diversity of multicellular eukaryotes -at various times, glaciers covered all of the planet’s landmasses and the seas were iced over (snowball Earth) -snowball earth most life would be confined to areas near deep-sea vents and hot springs or to equatorial regions of the ocean -diversification came to a close about 30 million years ago Cambrian Explosion? : many phyla of living animals appear suddenly in fossils formed early in the Camrian period -Cnidaria, porifera, and mollusca appear in even older rocks dating from the late proterozoic -before Cambrian: all large animals were soft-bodied, herbivores, filter feeders, or scavengers -cambrian explosion predators emerged that had claws and other features for catching prey, new defnsive adaptations -it is possible that many animal phyla originated long before that time
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Chapter 11: Cell Communication Lecture 19 The Cellular Internet -Cell-to-cell communication is essential for multicellular organisms (coordination of activities, grow, survive, reproduce) -also important for many unicellular organisms -same small set of sell-signaling mechanisms…once signal subsides, response is shut down by an enzyme -signals received by cells take various forms, including light and touch…most often communicate with each other by chemical signals 11.1 E XTERNAL S IGNALS ARE C ONVERTED TO R ESPONSES W ITHIN THE C ELL Evolution of Cell Signaling -one topic of cell conversation: sex -ie: certain kind of yeast identify mates by chemical signaling…2 sexes (a and alpha) -cells of mating type a secrete signaling molecule, a factor which can bind to a receptor protein on alpha cells -alpha cells secrete alpha factor which binds to receptors on a cells -2 mating factors cells grow toward each other & bring about other cellular changes fusion - signal transduction pathway : process by which a signal on a cell’s surface converted to a specific cellular response -details of signal transduction in yeast and mammals are similar even though the last common ancestor of these 2 groups of organisms lived over a billion years ago -similarities early versions of cell-signaling mechanisms used today evolved well before the 1 st multicellular creature -signaling mechanisms 1 st evolved in ancient prokaryotes and singe-celled eukaryotes and were adopted for new uses by multicellular descendants -concentration of signaling molecules allows bacteria to sense the local density of bacterial cells:
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