2. Origin of the Eukaryotic Cell Accel Bio

3. From where, the first cell? The scene: early earth, 3 billion years ago. Intense lightning, volcanic activity, and UV radiation. Atmosphere of (CH 4 & NH 3 also??). CO, CO 2, N 2, & H2OH2O

4. Prokaryote Fossils from 3.5 BYA _____________ (from 3.5 billion years ago) indicates that the earliest known forms of life were primitive cellular organisms like some of today’s prokaryotes. This evidence is found in fossilized “mats” of prokaryotes called stromatolites (see picture in previous slide!). These prokaryotes are known to have been photosynthetic and were probably preceded by a simpler heterotrophic prokaryotes (from 4 bya ??). Fossil evidence

5. How about Eukaryotic Cells then? The fossil record indicates that eukaryotes evolved from ___________ more than __________ years ago. How did this happen? How were the membrane- bound __________ and __________ first formed? There are two mechanisms, acting in concert, that are thought to be responsible… prokaryotes 1.7 billion organelles the nucleus

6. Membrane Infolding Hypothesis explains the formation of internal membranes by infolding of cell membrane in ancient prokaryote thought to have led to formation of nuclear membrane & ER

7. Endosymbiont Hypothesis explains the formation of mitochondria & chloroplasts by symbiotic relationship between small prokaryotes living inside larger host prokaryote prokaryotic ancestors of mitochondria & chloroplasts probably entered host cell via endocytosis as undigested prey or internal parasites this symbiosis eventually became mutually beneficial & led to greater interdependence --> inseparable single organism --> 1st eukaryotic cell

8. Proof of Endosymbiont Hypothesis This hypothesis was developed extensively by Lynn Margulis (now at Boston University) and is based on following pieces of evidence: mitochondria & chloroplasts both have own DNA which is more similar to prokaryotic DNA than eukaryotic nuclear DNA mitochondria & chloroplasts both have own ribosomes (which also are similar to prokaryotic ribosomes) m & c have ability to self-replicate, indicating autonomy no other cell structures have endosymbiosis explains the double-membranes of m & c, including fact that inner membrane’s enzymes resemble those of prokaryotes