Nucleic Acids and the Origin of Life

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Nucleic Acids and the Origin of Life
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4 Nucleic Acids and the Origin of Life

• 4.1 What Are the Chemical Structures and
  Functions of Nucleic Acids?
• 4.2 How and Where Did the Small Molecules of Life
  Originate?
• 4.3 How Did the Large Molecules of Life
  Originate?
• 4.4 How Did the First Cells Originate?

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4 Nucleic Acids and the Origin of Life
About 7,000 cheetahs survive in the world today.
The genomes (DNA) of all cheetahs are extremely
similar, suggesting that they all derive from a
few individuals that survived an event that
almost wiped out their species.
Opening Question Can DNA analysis be used in the
conservation and expansion of the cheetah
population?
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4.1 What Are the Chemical Structures and
Functions of Nucleic Acids?

• Nucleic acids are polymers specialized for the
  storage, transmission, and use of genetic
  information.
• DNA deoxyribonucleic acid
• RNA ribonucleic acid

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4.1 What Are the Chemical Structures and
Functions of Nucleic Acids?

• Nucleotides are the monomers that make up nucleic
  acids.
• Nucleotides consist of a pentose sugar, a
  phosphate group, and a nitrogen-containing base.
• A nucleoside consists only of a pentose sugar and
  a nitrogenous base.

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Figure 4.1 Nucleotides Have Three Components
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Figure 3.16 Monosaccharides Are Simple Sugars
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4.1 What Are the Chemical Structures and
Functions of Nucleic Acids?

• RNA contains the sugar ribose.
• DNA contains deoxyribose.

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4.1 What Are the Chemical Structures and
Functions of Nucleic Acids?

• Nucleotides are linked together in condensation
  reactions to form phosphodiester linkages.
• The phosphate groups link carbon 3' in one sugar
  to carbon 5' in another sugar.
• Nucleic acids are said to grow in the 5'-to-3'
  direction.

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Figure 4.2 Linking Nucleotides Together
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4.1 What Are the Chemical Structures and
Functions of Nucleic Acids?

• Oligonucleotides (about 20 monomers) RNA
  primers to start DNA duplication, RNA that
  regulates gene expression, etc.
• Polynucleotides, or nucleic acids (DNA and RNA)
  can be very longup to millions of monomers.

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4.1 What Are the Chemical Structures and
Functions of Nucleic Acids?

• DNA bases
• Adenine (A)
• Cytosine (C)
• Guanine (G)
• Thymine (T)
• RNA has uracil (U) instead of thymine.

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Table 4.1
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4.1 What Are the Chemical Structures and
Functions of Nucleic Acids?

• Complementary base pairing purines pair with
  pyrimidines by hydrogen bonds.

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4.1 What Are the Chemical Structures and
Functions of Nucleic Acids?

• RNA is single-stranded, but base pairing occurs
  between different regions of the molecule.
• Base pairing determines the three-dimensional
  shape of some RNA molecules.
• Complementary base pairing can also take place
  between RNA and DNA.

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Figure 4.3 RNA
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4.1 What Are the Chemical Structures and
Functions of Nucleic Acids?

• The two strands of a DNA molecule form a double
  helix.
• All DNA molecules have the same structure
  diversity lies in the sequence of base pairs.
• DNA is an informational molecule information is
  encoded in the sequences of bases.

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Figure 4.4 DNA
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4.1 What Are the Chemical Structures and
Functions of Nucleic Acids?

• DNA transmits information in two ways
• DNA can reproduce itself (replication).
• DNA sequences can be copied into RNA
  (transcription). The RNA can specify a sequence
  of amino acids in a polypeptide (translation).

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4.1 What Are the Chemical Structures and
Functions of Nucleic Acids?

• Transcription plus translation expression

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4.1 What Are the Chemical Structures and
Functions of Nucleic Acids?

• DNA replication and transcription depend on base
  pairing.
• DNA replication involves the entire molecule,
  but only relatively small sections of the DNA are
  transcribed into RNA.

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4.1 What Are the Chemical Structures and
Functions of Nucleic Acids?

• The complete set of DNA in a living organism is
  called its genome.
• Not all the information is needed at all times
  sequences of DNA that encode specific proteins
  are called genes.

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Figure 4.5 DNA Replication and Transcription
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4.1 What Are the Chemical Structures and
Functions of Nucleic Acids?

• DNA carries hereditary information between
  generations.
• Determining the sequence of bases helps reveal
  evolutionary relationships.
• The closest living relative of humans is the
  chimpanzee.

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4.1 What Are the Chemical Structures and
Functions of Nucleic Acids?

• Other roles for nucleotides
• ATPenergy transducer in biochemical reactions
• GTPenergy source in protein synthesis
• cAMPessential to the action of hormones and
  transmission of information in the nervous system

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4.2 How and Where Did the Small Molecules of Life
Originate?

• During the European Renaissance (14th to 17th
  centuries), most people thought that at least
  some forms of life arose repeatedly from
  inanimate or decaying matter by spontaneous
  generation.

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4.2 How and Where Did the Small Molecules of Life
Originate?

• Francesco Redi first disproved spontaneous
  generation in 1668.

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4.2 How and Where Did the Small Molecules of Life
Originate?

• Experiments by Louis Pasteur showed that
  microorganisms can arise only from other
  microorganisms.

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Figure 4.6 Disproving the Spontaneous Generation
of Life (Part 1)
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Figure 4.6 Disproving the Spontaneous Generation
of Life (Part 2)
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4.2 How and Where Did the Small Molecules of Life
Originate?

• But these experiments did not prove that
  spontaneous generation had never occurred.
• Eons ago, conditions on Earth and in the
  atmosphere were vastly different.
• About 4 billion years ago, chemical conditions,
  including the presence of water, became just
  right for life.

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4.2 How and Where Did the Small Molecules of Life
Originate?

• Two of the theories on the origin of life
• Life came from outside Earth.
• In 1969, fragments of a meteorite were found to
  contain molecules unique to life, including
  purines, pyrimidines, sugars, and ten amino
  acids.
• Evidence from other meteorites suggest that
  living organisms could possibly have reached
  Earth within a meteorite.

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Figure 4.7 The Murchison Meteorite
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4.2 How and Where Did the Small Molecules of Life
Originate?

• 2. Life arose on Earth through chemical
  evolution.
• Chemical evolution conditions on primitive Earth
  led to formation of simple molecules (prebiotic
  synthesis) these molecules led to formation of
  life forms.
• Scientists have experimented with reconstructing
  those primitive conditions.

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4.2 How and Where Did the Small Molecules of Life
Originate?

• Miller and Urey (1950s) set up an experiment with
  gases thought to have been present in Earths
  early atmosphere.
• An electric spark simulated lightning as a source
  of energy to drive chemical reactions.
• After several days, organic molecules had formed,
  including amino acids.

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Figure 4.8 Miller and Urey Synthesized Prebiotic
Molecules in an Experimental Atmosphere (Part 1)
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Figure 4.8 Miller and Urey Synthesized Prebiotic
Molecules in an Experimental Atmosphere (Part 2)
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Working with Data 4.1 Could Biological Molecules
Have Been Formed from Chemicals Present in
Earths Early Atmosphere?

• In the 1950s Miller and Urey experiments, the
  sources of energy impinging on Earth were

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Working with Data 4.1 Could Biological Molecules
Have Been Formed from Chemicals Present in
Earths Early Atmosphere?

• Question 1
• Of the total energy from the sun, only a small
  fraction is in the ultraviolet range, less than
  250 nm.
• What proportion of total solar energy is the
  energy with wavelengths below 250 nm?

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Working with Data 4.1 Could Biological Molecules
Have Been Formed from Chemicals Present in
Earths Early Atmosphere?

• Question 2
• The molecules CH4, H2O, NH3, and CO2 absorb
  light at wavelengths of less than 200 nm.
• What fraction of total solar radiation is in
  this range?

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Working with Data 4.1 Could Biological Molecules
Have Been Formed from Chemicals Present in
Earths Early Atmosphere?

• Question 3
• Miller and Urey used electric discharges as
  their energy source.
• What other sources of energy could be used in
  similar experiments?

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4.2 How and Where Did the Small Molecules of Life
Originate?

• In another experiment, Miller filled tubes with
  NH3, HCN, and water and kept them sealed at 78C
  for 27 years.
• When opened, they contained amino acids and
  nucleotide bases.
• Cold water within ice on ancient Earth or other
  planets may have allowed prebiotic synthesis of
  organic molecules.

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4.2 How and Where Did the Small Molecules of Life
Originate?

• The Miller and Urey experiments sparked decades
  of research.
• Ideas about Earths original atmosphere have
  changed volcanoes may have added CO2, N2, H2S,
  and SO2 to the atmosphere.
• Adding these gases to the experimental atmosphere
  results in formation of more small organic
  molecules.

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4.3 How Did the Large Molecules of Life Originate?

• Conditions in which polymers might have been
  first synthesized
• Solid mineral surfacessilicates within clay may
  have been catalysts
• Hydrothermal ventsmetals as catalysts
• Hot pools at ocean edgesconcentrated monomers
  favored polymerization (the primordial soup)

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4.3 How Did the Large Molecules of Life Originate?

• In living organisms, the many biochemical
  reactions require catalystsmolecules that speed
  up the reactions.
• A key to the origin of life is the appearance of
  catalystsproteins called enzymes.

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4.3 How Did the Large Molecules of Life Originate?

• Proteins are synthesized from information
  contained in nucleic acids.
• So which came first, nucleic acids or protein
  catalysts?

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4.3 How Did the Large Molecules of Life Originate?

• RNA may have been the first catalyst.
• The 3-D shape and other properties of some RNA
  molecules (ribozymes) are similar to enzymes.
• RNA could have acted as a catalyst for its own
  replication and for synthesis of proteins. DNA
  could eventually have evolved from RNA.

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Figure 4.9 The RNA World Hypothesis
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4.3 How Did the Large Molecules of Life Originate?

• Several lines of evidence support this RNA
  world hypothesis
• Peptide linkages are catalyzed by ribozymes
  today.
• In retroviruses, an enzyme called reverse
  transcriptase catalyzes the synthesis of DNA from
  RNA.

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Figure 4.10 An Early Catalyst for Life?
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4.3 How Did the Large Molecules of Life Originate?

• Short, naturally occurring RNA molecules catalyze
  polymerization of nucleotides in experimental
  settings.
• An artificial ribozyme has been developed that
  can catalyze assembly of short RNAs into a longer
  molecule that is an exact copy of itself.

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4.4 How Did the First Cells Originate?

• The chemical reactions of metabolism and
  replication could not occur in a dilute aqueous
  environment.
• The compounds involved must have been
  concentrated in a compartment.
• Today, living cells are separated from their
  environment by a membrane.

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4.4 How Did the First Cells Originate?

• In water, fatty acids will form a lipid bilayer
  around a compartment.
• These protocells allow small molecules such as
  sugars and nucleotides to pass through.
• If short nucleic acid strands capable of
  self-replication are placed inside protocells,
  nucleotides can enter and be incorporated into
  polynucleotide chains.

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Figure 4.11 Protocells
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4.4 How Did the First Cells Originate?

• Protocells may be a reasonable model for the
  evolution of cells
• They are organized systems of parts with
  substances interacting, in some cases
  catalytically.
• They have an interior that is distinct from the
  exterior environment.
• They can self-replicate.

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4.4 How Did the First Cells Originate?

• In the 1990s, evidence of cells in rocks 3.5
  billion years old was found in Australia.
• The cells were probably cyanobacteria (blue-green
  bacteria) that could perform photosynthesis.
• Photosynthesis uses CO2, and leaves a specific
  ratio of carbon isotopes (13C12C), which were
  found in the fossils.

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Figure 4.12 The Earliest Cells?
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4.4 How Did the First Cells Originate?

• It is plausible that it took about 500 million to
  a billion years from the formation of the Earth
  until the appearance of the first cells.

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Figure 4.13 The Origin of Life
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4 Answer to Opening Question

• DNA sequencing allows conservation biologists to
  mate pairs of cheetahs with the greatest
  differences in DNA.
• The offspring will thus have the greatest
  possible diversity of DNA.
• Genetic homogeneity causes male cheetahs to have
  low sperm counts. Artificial insemination is used
  to overcome this problem.