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Unformatted text preview: In-Folding, Endosymbiosis
and the Evolution of the
Eukarya Photosynthetic bacteria (Cyanobacteria) evolved about 3 billion years ago They released oxygen into the atmosphere Bacteria capable of aerobic respiration arose over 2 billion years ago Group Work Working in groups and using your book (Chapter 26) fill in the chart with features of the prokaryotes. Prokaryotes
Nuclear Envelope (membrane around DNA) Absent Membraneenclosed organelles Absent Ribosomes Present, free in cytoplasm, rRNA is structural component of Ribosomes DNA
RNA DNA Present, single circular chromosome, mRNA made on demand as needed Group Work Working in groups and using your book (Chapter 6) fill in the chart with features of the eukaryotes. Eukaryotes
Structure Description Cell membrane Covering of cell Lipid bilayer Chloroplasts Photosynthesis Outer membrane
Circular DNA Mitochondria Function Aerobic respiration Outer membrane
Ribosomes, Circular DNA Eukaryotes, con’t
Endoplasmic reticulum Transport
Regulation of Metabolic activities Lipid bilayer, continuous with cell membrane and nuclear membrane Nucleus
RNA Hereditary material
Controls protein synthesis Linear, double stranded DNA in many chromosomes in a DOUBLE membrane
Lipid bilayer, mRNA is produced on demand, but is always present in cytoplasm Ribosomes &
rRNA Sites of protein synthesis Attached to ER, inside chloroplasts and mitochondria, rRNA is component of ribosomes Theory: How the ER and Nuclear Envelope
Structures evolved in eukaryotes
Endoplasmic reticulum and the nuclear envelope arose from infolding of the cell membrane (Figure 25.9, 7.20). endocytosis brings materials into the cell then pinches off to form a chamber or vesicle. Some eukaryotic cells also extend “arms” of cytoplasm around a food particle and engulf it (phagocytosis in the amoeba). Working in groups, use string as the cell membrane and the paperclip as the nuclear material to demonstrate how the endoplasmic reticulum and the double
layered nuclear envelope may have formed. Share with your peer leader when you think you have figured it out. How Some Organelles Evolved in
Mitochondria and chloroplasts may have evolved as the result of a large prokaryotic cell using a mechanism similar to endocytosis to engulf small bacteria, which were capable of photosynthesis or aerobic respiration. This theory, known as endosymbiosis, was popularized by Lynn Margulis, 1981, Symbiosis in Cell Evolution • suggests that bacteria evolved symbiotic relationships (see Figure 25.9, 28.2) where smaller bacteria photosynthesized and respired for the benefit of larger bacteria, which may have provided protection from predation Working in groups, use the string as the cell membrane and the rubber bands as photosynthetic and aerobic bacteria, demonstrate how endosymbiosis could have led to the evolution of chloroplasts and mitochondria. Questions for Discussion
Why does the infolding of the cell membrane seem a logical explanation for how the endoplasmic reticulum and the nuclear envelope form? Because the membranes of the ER and the nuclear envelope are nearly identical to the cell membrane. ER and the nuclear envelope are connected, but independent of the cell membrane.
1. Why does endosymbiosis seem a logical explanation for the origin of chloroplasts and mitochondria from bacterial cells? Because the outer membrane of these structures is similar to the cell membrane, and the inner membrane is similar to a bacterial membrane Because each structure has its own circular DNA different from the nuclear DNA Because each structure has its own ribosomes
1. Fig. 28.2 3. What point is Fig. 28.2 (go back to see slide) trying to show?
Endosymbiosis is thought to have occurred several different times in the evolution of the Eukarya 4. What is not explained by endosymbiosis?
How the single stranded, circular DNA from the prokaryotes evolved into the double stranded, linear DNA found in eukaryotes
How other organelles and cell structures of eukaryotes evolved
Also…no modern prokaryotes are known to perform endocytosis or phagocytosis! ...
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