Transcript Origin_of_Life

Origin of Life
Chapter 14
Biogenesis vs. Spontaneous
Generation
• Biogenesis
– States that all living things come from other
living things
• Spontaneous generation
– Early, now disproved, hypothesis that living
organisms develop from nonliving material
Francesco Redi’s
• Middle of 17th century, Italian scientist
Francesco Redi described different
developmental forms of flies
Redi’s Experiment
• Belief: Maggots come from rotting meat
• Redi’s 1668 Hypothesis: Maggots come
from flies
• Redi put meat in 3 separate jars
– Jar 1: Left open
– Jar 2: Covered with netting
– Jar 3: Sealed from outside
Redi’s Experiment Step 1
• Left open
• Maggots developed
• Flies were observed
laying eggs on the
meat in the open jar
Redi’s Experiment Step 2
• Covered with netting
• Maggots appeared
on the netting
• Flies were observed
laying eggs on the
netting
Redi’s Experiment Step 3
• Sealed
• No maggots
developed
Redi’s Conclusions
• Experiment showed convincingly that flies
come only from eggs laid by other flies
• 1st major blow towards idea of
spontaneous generation
Lazzaro Spallanzani
• Italian scientist
• In 1700s, designed experiment to test
hypothesis of spontaneous generation of
microorganisms
• Spallanzani benefited from the invention of
the microscope
Spallanzani’s Experiment
• Belief: Microorganisms came from the air
• Hypothesis: Microorganisms came from other
microorganisms. Boiling will kill the
microorganisms.
• Used meat broth to test his hypothesis
• Spallanzani put broth into four flasks
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–
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Flask 1 was left open
Flask 2 was sealed
Flask 3 was boiled and then left open
Flask 4 was boiled and then sealed
Spallanzani's Experiment Step 1
• Left Open
• Turned cloudy
• Microbes were found
Spallanzani's Experiment Step 2
• Sealed
• Turned cloudy
• Microbes were found
Spallanzani's Experiment Step 3
• Boiled and left open
• Turned cloudy
• Microbes were found
Spallanzani's Experiment Step 4
• Boiled and sealed
• Did not turn cloudy
• Microbes not found
Spallanzani’s Conclusions
• Boiled broth became contaminated only
when microorganisms from the air entered
the flask
• Opponents: Claimed Spallanzani heated
experimental flasks too long, destroying
the “vital force” in air inside them
– Air lacking “vital force” could not generate life
Louis Pasteur
• French scientist
• Developed experiment in mid-1800s that
ended the controversy of Spallanzani’s
experiment
• Used special curved-shaped flasks
Pasteur’s Experiment
• Hypothesis: Microbes come from cells of
organisms on dust particles in the air; not
the air itself
• Each flask was boiled and placed at
various locations
Pasteur's Experiment Step 1
• Filled with broth
• The special shaped
was intended to trap
any dust particles
coming in
Pasteur's Experiment Step 2
• Flasks boiled
• Microbes killed
Pasteur's Experiment Step 3
• Flask left at various
locations
• Did not turn cloudy
• Microbes not found
Pasteur's Experiment Step 4
• Dust collected in the neck
of the flask & prevented
microorganisms from
entering body of the flask
Pasteur’s Conclusions
• Experimental curve-necked flasks remained
clear for up to a year
• Once Pasteur broke off curved necks, broth
became cloudy & contaminated with
microorganisms within a day
• Pasteur reasoned contamination was due to
microorganisms in the air
• Pasteur’s conclusions allowed the principle of
biogenesis to become the cornerstone of biology
Formation of the Earth
• Solar system formed approximately 5
billion years ago
– Began as a swirling mass of gas & dust
– Over time, most of the material collapsed
inward, forming sun
– Left over material circled the young sun
– Collisions between this left over debris formed
the planets
Earth’s Age
• Estimated age of Earth is 4 billion years
old
– Scientists have created a geologic history of
the earth by examining the layers of sediment
• Early estimates were simply based on studying the
sediment layers
• Around mid-twentieth century, radioactive dating
methods were implemented
Radioactive Dating
• Age of a material can be determined by
measuring the amount of a particular
radioactive isotope it contains
– Compare this quantity with the amount of
some other substance in the that remains
constant over time
Background Info for Radioactive
Dating
• Recall, all atoms of an element have the same
atomic number, but their number of neutrons can
vary
– Isotopes are atoms of the same element that differ in
the number of neutrons they contain
• Mass number refers to the total number of
protons & neutrons
• Isotopes are designated by their chemical name
followed by their mass number
– Carbon-12; carbon-14
Background Info for Radioactive
Dating
• Some isotopes, radioactive isotopes, have
unstable nuclei—which undergo
radioactive decay
• Half-life refers to the length of time it takes
for one-half of any size sample of an
isotope to decay
– Length depends on isotope & can vary from
fraction of a second to billions of years
Carbon-14 Dating
• Limited to organic remains less than ~60,000
years old
• Amount of carbon-14 (radioactive isotope) is
compared to the amount of carbon-12 (stable
isotope)
• Living things take carbon into their bodies
constantly, mostly in the form of carbon-12 but
some in the form of carbon-14
• When an organism dies, the intake of carbon
ceases
• Over time, amount of carbon-14 declines with
respect to amount of carbon-14
• Half-life of carbon-14 is 5,730 years
Other Methods of Radioactive
Dating
• Earth’s age has by estimated using the
decay of uranium & thorium in rock
crystals
– Half-life of thorium-230 is 75,000 years
– Half-life of uranium-238 is 4,500,000,000
years
The 1st Organic Compounds
• All elements found in organic compounds
are thought to have existed on Earth &
rest of solar system when Earth formed
• The question then is how & where these
elements assembled into the organic
compounds found in life?
Oparin’s Hypothesis
• In 1923, Alexander I. Oparin, a Soviet
scientist, suggested that the atmosphere
of the primitive Earth was very different
from that of today
– Early atmosphere was thought to contain
ammonia, hydrogen gas, water vapor, &
methane
Oparin’s Hypothesis
• Temperatures above the boiling point of water
could allow these gases to form simple organic
compounds—like amino acids
• As the Earth cooled & water vapor condensed,
lakes & seas formed
• Lakes & seas were a collection ground for the
simple organic compounds
• Over time, these simple compounds could have
entered complex chemical reactions caused by
lightening & UV radiation
• Thus resulting in the macromolecules essential
to life—like proteins
Miller-Urey Experiment
• Oparin never tested his hypothesis
• In 1953, Stanley Miller & Harold Urey set up an
experiment to test Oparin’s hypothesis
• Apparatus included a chamber containing the
gases Oparin assumed were present in the
young Earth’s atmosphere
• Electric sparks substituted the lightening to
supply the energy necessary to drive chemical
reactions
• Experiment produced a variety of organic
compounds—amino acids, ATP, nucleotides in
DNA
New Hypothesis
• Early atmosphere of Earth was composed
largely of carbon dioxide, nitrogen, & water
vapor
– Both carbon dioxide & oxygen interfere with
production or organic compounds
– Believed that areas like undersea hot springs,
favored the production of organic compounds
since they are protected from the atmosphere
Organic Compounds from Beyond
Earth
• A newly fallen meteorite that was
recovered before it was contaminated,
was found to contain a broad mixture of
organic compounds
• Speculation that that life on Earth began in
space and was carried here by space
debris rather than originating here on
Earth
From Molecules to Cell-Like
Structures
• Sidney Fox’s research
– Cell-like structures form spontaneously in
laboratory from solutions of simple organic
chemicals
• Structures include…
– 1). Microspheres
» Spherical in shape, composed of many protein
molecules, & are organized as a membrane
– 2). Coacervates
» Collections of droplets composed of molecules of
different types—amino acids & sugars
Coacervates & Microspheres
• Have many life-like properties
– Ability to take up certain substances from their
surroundings
– Coacervates can grow
– Microsphres can bud to form smaller
microspheres
• Show that some important aspects of
cellular life arose without direction from
genes
The 1st Life Forms
• Oldest known cellular fossils were found in
a desolate corner of Australia
– The unicellular organisms were 3.5 billion
years old
Origin of Heredity
• Recall…
– DNA is transcribed to form mRNA
– tRNA translates the 3-base codons, amino
acids, into mRNA
– RNA message is then translated into a protein
Origin of Heredity
• Question arose—why is RNA necessary to
carry out protein synthesis?
– Understanding function of RNA can be found
in its shape
• Can take on variety of shapes
– Shapes are controlled by hydrogen bonds
• Might behave like proteins & catalyze chemical
reactions
Roles of RNA
• Early 1980s, researcher Thomas Cech
found RNA found in some unicellular
eukaryotes can act as an enzyme
– Called it a ribozyme
• RNA molecule that can act as an enzyme &
promote a specific chemical reaction, but still has
the ability to replicate
Roles of RNA
• Recent studies suggest life started with
self-replicating molecules of RNA
– RNA has hereditary information
– RNA would be able to respond to natural
selection & thus evolve
– Also found that RNA plays a vital role in DNA
replication, protein synthesis, RNA processing
• Still many unanswered questions
– Have not discovered ribozyme that can selfreplicate
1st Prokaryotes
• Characteristics of first cellular life
– Anaerobic—little or no oxygen in existence
– Heterotrophic—took in organic molecules
from environment
• Eventually resources depleted, push for
organisms to evolve
Chemosynthesis
• Archaebacteria
– Unicellular organisms
– Thrive under harsh environmental conditions
– Many are autotrophic, but obtain energy by
chemosynthesis
• In chemosynthesis, CO2, serves as carbon source
for assembly of organic molecules
Photosynthesis & Aerobic
Respiration
• Oxygen is a byproduct of photosynthesis
– Oxygen destroyed coenzymes essential to
cell function of early unicellular organisms
– Some organisms were not damaged--bind the
oxygen—aerobic respiration
• Photosynthetic life formed by 3 billion
years ago
Photosynthesis & Aerobic
Respiration
• Took billion years or more for oxygen
levels to reach today’s levels
– Eventually reached upper atmosphere &
formed ozone
• Layer absorbs UV radiation from sun
– UV radiation damages DNA
1st Eukaryotes
• Differences between Prokaryotes &
Eukaryotes
– Eukaryotic cells are larger
– Eukaryotes DNA is organized into
chromosomes in a nucleus
– Eukaryotes contain membrane bound
organelles
1st Eukaryotes
• How did eukaryotes evolve from
prokaryotes?
– Evidence that between 2 & 1.5 billion years
ago, small aerobic prokaryote entered &
began to live & reproduce inside larger
anaerobic prokaryote
• Endosymbiosis—mutually beneficial relationship
– Believed that aerobic prokaryote gave rise to
modern mitochondria
1st Eukaryotes
• Another later invasion, gave rise to
chloroplasts
– Sites of photosynthesis
• Both chloroplasts & mitochondria…
– Replicate independently from replication cycle
of cell
– Contain some of their own genes