Chapter 14

• Biogenesis
– States that all living things come from other living things
• Spontaneous generation
– Early, now disproved, hypothesis that living organisms develop from nonliving material

• Middle of 17 th century, Italian scientist
Francesco Redi described different developmental forms of flies

• 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

• Left open
• Maggots developed
• Flies were observed laying eggs on the meat in the open jar

• Covered with netting
• Maggots appeared on the netting
• Flies were observed laying eggs on the netting

• Sealed
• No maggots developed

• Experiment showed convincingly that flies come only from eggs laid by other flies
• 1 st major blow towards idea of spontaneous generation

• Italian scientist
• In 1700s, designed experiment to test hypothesis of spontaneous generation of microorganisms
• Spallanzani benefited from the invention of the microscope

• 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
– Flask 1 was left open
– Flask 2 was sealed
– Flask 3 was boiled and then left open
– Flask 4 was boiled and then sealed

• Left Open
• Turned cloudy
• Microbes were found

• Sealed
• Turned cloudy
• Microbes were found

• Boiled and left open
• Turned cloudy
• Microbes were found

• Boiled and sealed
• Did not turn cloudy
• Microbes not found

• 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

• French scientist
• Developed experiment in mid-1800s that ended the controversy of Spallanzani’s experiment
• Used special curved-shaped flasks

• 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

• Filled with broth
• The special shaped was intended to trap any dust particles coming in

• Flasks boiled
• Microbes killed

• Flask left at various locations
• Did not turn cloudy
• Microbes not found

• Dust collected in the neck of the flask & prevented microorganisms from entering body of the flask

• 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

• 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

• 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

• 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

• 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

• 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

• 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

• 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

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• 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?

• 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

• 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

• 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

• 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

• 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

• 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

• 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

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• Oldest known cellular fossils were found in a desolate corner of Australia
– The unicellular organisms were 3.5 billion years old

• 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

• 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

• 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

• 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

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• 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

• Archaebacteria
– Unicellular organisms
– Thrive under harsh environmental conditions
– Many are autotrophic, but obtain energy by chemosynthesis
• In chemosynthesis, CO
2
, serves as carbon source for assembly of organic molecules

• 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

• 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

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• Differences between Prokaryotes &
Eukaryotes
– Eukaryotic cells are larger
– Eukaryotes DNA is organized into chromosomes in a nucleus
– Eukaryotes contain membrane bound organelles

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• 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

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• 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