Ch. 15 Evolution Section 1: Origin of Life Continued

RNA’s Role in Early Life: RNA World Hypothesis For DNA to replicate it needs enzymes like helicase and polymerase, but RNA does not need these enzymes (this property makes it an ideal candidate for early replication) RNA can: Carry its own genetic material act as its own enzyme called a ribozyme

Protobionts: A Step towards Cell Membranes? Not technically cells, but may have features that helped early cells evolve Form spontaneously Protect polymers from outside environment by forming primitive “cell membranes”

Prokaryotes make Oxygen Early (~ 4bya) life forms (bacteria) were anaerobic and heterotrophic Eventually over-production led to competition and some of these organisms had an advantage b/c they made their own food (autotrophs) instead of consuming other molecules (heterotrophs) ~3.5 bya some bacteria evolved the ability to photosynthesize

The photosynthesizing bacteria take in CO2 but releases O2 which caused a massive oxygen build up in the atmosphere called the Oxygen Catastrophe which occurred ~2 bya (this caused anaerobic bacteria to be killed off in large numbers..the survivors were bacteria that were protected from the atmosphere…in mud, underground, protected inside cells)

Other Results No Need to Write! Caused the oxidation of sediments in the ocean (visible in iron-rich rocks that date back to that time) Ozone layer (O3) formed in atmosphere and blocked out UV radiation; blocked production of future organic chemicals

Endosymbiotic Theory Bacteria were the only organisms from ~3.8 - 2 bya Primitive eukaryotes evolved ~ 2 bya Theory states that organelles (like chloroplasts and mitochondria) that are found inside eukaryotic cells were once independent prokaryotes These organelles were not digested, but instead were kept alive and a symbiotic relationship evolved Since natural selection favored eukaryotes that could make their own food or eukaryotes that could metabolize food more efficiently, these organisms survived better than other organisms

SUPPORT of THEORY PROBLEMS with THEORY Both chloroplast and mitochondria have two membranes One cell engulfing another does not guarantee that the parent cell will pass on to its offspring the ability to synthesize the new organelle Both have their own circular loop of DNA which is a circular loop like prokaryotes When you remove chloroplasts and mitochondria from a cell, they cannot survive on their own Both can perform protein synthesis using their small ribosomes Both can replicate themselves

Brought into cell via endocytosis which leads to being double-membraned