Download 1.5 The Origin of Cells

1.5 The Origin of Cells
1.5 The origin of cells
Understanding
• 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
Applications
* Evidence fro Pasteur’s experiments
that spontaneous generation of cells &
organisms does not now occur on Earth
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 preexisting 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:
1.
2.
3.
4.
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