1.5 The Origin of Cells

1.5 The origin of cells Understanding Applications * Evidence fro Pasteur’s experiments that spontaneous generation of cells & organisms does not now occur on Earth Cells can only be formed by division of pre-existing cells First cells must have arisen from non-living material Origin of eukaryotic cells can be explained by endosymbiotic theory Nature of Science * Testing the general principles that underlie the natural world: the principle that cells only come from pre-existing cells needs to be verified

Understanding: Cells can only be formed by division of pre-existing cells Our trillions of cells originated from one single zygote There’s a continuity of life from its origins on Earth to our bodies today. Have not been able to create artificial cell from non-cell materials

Nature of Science: Verifying general principles that underlie the natural world: the principle that “cells only come from pre-existing cells” needs to be verified. Spontaneous generation = living from non-living * Theophrastus – Silphium from soil * Aristotle – insects from dew, hair, flesh, feces * Paracelsus – mice, frogs, eels from water, air, decaying matter Cells only come from pre-existing cells * Redi – maggots & rotting meat experiment * Spallanzani – boiled soup experiment * Pasteur – swan-necked flask experiment * Today’s scientists know: 1) Cells are highly complex! 2) The only thing that produces more cells is cell division. 3) Viruses are not cells & can only be produced inside host cells

Francesco Redi (1600’s)

Lazzaro Spallanzani (1700’s)

Application: Evidence from Pasteur’s experiments that spontaneous generation of cells & organisms does not now occur on Earth (1800’s)

Understanding: First cells must have arisen from non-living material How did cells gets here in the first place? Theory 1: “panspermia” (seeds everywhere!) – first cells on earth came from outer space Directed Panspermia – proposed by Francis Crick (co-discoverer of double helix) in 1973 = seeds purposely spread by an advanced extraterrestrial civilization, or can be spread from Earth to other planets by humans Crick found it impossible that the complexity of DNA could have evolved naturally

… or Theory 2: cells arose from non-living material on Earth Complex structures can’t arise by evolution… it would have to happen in tiny steps over millions of years: Production of C compounds (sugars & amino acids) Assembly of C compounds into polymers Formation of phospholipid membranes Development of mechanism for inheritance

1.Production of C compounds (sugars & amino acids) Miller & Urey experiment

2. Assembly of C compounds into polymers (in deep sea vents where chemicals supplied energy needed)

3.Formation of phospholipid membranes

4. Development of mechanism for inheritance Which came first? Creation of DNA or creation of proteins (enzymes)? For DNA replication & DNA transcription/translation, you need enzymes. But to get enzymes, you need DNA to code for them. Solution: RNA was first genetic material – it can act as a catalyst and can self-replicate!

Understanding: Origin of eukaryotic cells can be explained by endosymbiotic theory Endo = inside Symbiosis = mutualistic = relationship where both organisms benefit Small aerobic or photosynthetic prokaryote taken into larger anaerobic prokaryote by endocytosis * Small one is supplied with food * Large one is supplied with energy Endosymbiotic theory = small prokaryotes living inside big ones developed into mitochondria and chloroplasts.

Endosymbiotic theory

Endocytosis vs Exocytosis

Mitochondria & chloroplast similar to prokaryotes: Circular DNA 70s ribosomes Use DNA to make their own proteins Can only be produced by division of pre-existing mitochondria & chloroplasts