Transcript Slide 1
Dawn of Eukarya Chapter 28 The Steps in the Development of a Eukaryote Cell The evolution of eukaryotic cells included the following components: The origin of a flexible cell surface The origin of a cytoskeleton The origin of a nuclear envelope The appearance of digestive vesicles The endosymbiotic acquisition of certain organelles Revision of the cell – first step The first step toward the eukaryotic condition may have been the loss of the cell wall by an ancestral prokaryotic cell. Steps in the formation of a Eukaryote cell A surface that is flexible enough to allow for infolding lets the cell exchange materials with its environment rapidly enough to sustain a larger volume and more rapid metabolism. A flexible surface also allows endocytosis. An infolded plasma membrane attached to a chromosome within an ancestral prokaryote may have led to the formation of the nuclear envelope. The development of a eukaryote cell The early steps in the evolution of the eukaryotic cell likely included three advances: The formation of ribosome-studded internal membranes, some of which surrounded the DNA The appearance of a cytoskeleton The evolution of digestive vesicles Cytoskeleton and size and shape A cytoskeleton allowed the now much larger cell to manage changes in its shape, distribute daughter chromosomes, and move materials from one part of the cell to another. The origin of the cytoskeleton is a mystery; the genes that encode it are found in neither bacteria nor archaea. A controversial hypothesis suggests that these genes may have originated in a longextinct fourth domain of life that transferred them laterally to an ancestor of the early eukaryotes. Intermediate cell From an intermediate kind of cell, the next advance was likely to have been a motile phagocyte. The first true eukaryotic cell possessed a cytoskeleton and a nuclear envelope; it also may have had an associated endoplasmic reticulum and Golgi apparatus and perhaps one or more flagella The Effects of Oxygen During the early stages of eukaryotic evolution, the O2 levels in the atmosphere were increasing as a result of the photosynthetic activities of the cyanobacteria. Most living things were unable to tolerate this new aerobic, oxidizing environment, but some prokaryotes and ancient phagocytes were able to survive. One hypothesis suggests that the key to the survival of the early phagocytes was the ingestion of a prokaryote that became symbiotic and evolved into the peroxisomes of today. Blending of the two Domains? It is clear that the eukaryotic genome is a mixture of genes with two distinct origins. Recently, it has been suggested that the Eukarya may have arisen from the mutualistic fusion of a Gram-negative bacterium and an archaean. Giardia – A link? Endosymbiosis Endosymbiosis Figure 28.1 Three Protists Polyphyletic Origins Most protists are aquatic, occupying a variety of environments including marine and fresh waters, the body fluids of other organisms, and soil water. Most are unicellular, but some are multicellular, and a few are very large. Some protists are heterotrophs, some are autotrophs, and some switch between these two modes of nutrition. The terms protozoan and algae actually lump together many phylogenetically distant protist groups. Cell Surfaces Cell surfaces Actinopodia Rhizopoda Contractile vacuoles Food vacuoles Euglenoids Apicomplexans - Malaria Advanced ciliates Paramecium Sex – Conjugative exchange Red Tides- Dinoflagellates Reproductive strategies Mitosis/Meiosis Diatoms Phaeophyta Rhodophyta Chlorophyta Volvox Oomycetes Slime molds