Transcript Life: The Science of Biology, 10e

4
Nucleic Acids and the
Origin of Life
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?
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?
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
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 nitrogencontaining base.
A nucleoside consists only of a
pentose sugar and a nitrogenous
base.
Figure 4.1 Nucleotides Have Three Components
Figure 3.16 Monosaccharides Are Simple Sugars
4.1 What Are the Chemical Structures and Functions of Nucleic
Acids?
RNA contains the sugar ribose.
DNA contains deoxyribose.
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.
Figure 4.2 Linking Nucleotides Together
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 long—up to
millions of monomers.
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.
Table 4.1
4.1 What Are the Chemical Structures and Functions of Nucleic
Acids?
Complementary base pairing: purines pair
with pyrimidines by hydrogen bonds.
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 threedimensional shape of some RNA
molecules.
Complementary base pairing can also
take place between RNA and DNA.
Figure 4.3 RNA
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.
Figure 4.4 DNA
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).
4.1 What Are the Chemical Structures and Functions of Nucleic
Acids?
Transcription plus translation =
expression
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.
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.
Figure 4.5 DNA Replication and Transcription
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.
4.1 What Are the Chemical Structures and Functions of Nucleic
Acids?
Other roles for nucleotides:
• ATP—energy transducer in
biochemical reactions
• GTP—energy source in protein
synthesis
• cAMP—essential to the action of
hormones and transmission of
information in the nervous system
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.
4.2 How and Where Did the Small Molecules of Life Originate?
Francesco Redi first disproved
spontaneous generation in 1668.
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.
Figure 4.6 Disproving the Spontaneous Generation of Life (Part 1)
Figure 4.6 Disproving the Spontaneous Generation of Life (Part 2)
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.
4.2 How and Where Did the Small Molecules of Life Originate?
Two of the theories on the origin of life:
1. 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.
Figure 4.7 The Murchison Meteorite
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.
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 Earth’s 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.
Figure 4.8 Miller and Urey Synthesized Prebiotic Molecules in an Experimental Atmosphere (Part
1)
Figure 4.8 Miller and Urey Synthesized Prebiotic Molecules in an Experimental Atmosphere (Part
2)
Working with Data 4.1: Could Biological Molecules Have Been
Formed from Chemicals Present in Earth’s Early Atmosphere?
In the 1950s Miller and Urey experiments,
the sources of energy impinging on Earth
were:
Working with Data 4.1: Could Biological Molecules Have Been
Formed from Chemicals Present in Earth’s 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?
Working with Data 4.1: Could Biological Molecules Have Been
Formed from Chemicals Present in Earth’s 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?
Working with Data 4.1: Could Biological Molecules Have Been
Formed from Chemicals Present in Earth’s 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?
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 –78°C 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.
4.2 How and Where Did the Small Molecules of Life Originate?
The Miller and Urey experiments
sparked decades of research.
Ideas about Earth’s 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.
4.3 How Did the Large Molecules of Life Originate?
Conditions in which polymers might
have been first synthesized:
• Solid mineral surfaces—silicates within
clay may have been catalysts
• Hydrothermal vents—metals as
catalysts
• Hot pools at ocean edges—
concentrated monomers favored
polymerization (the “primordial soup”)
4.3 How Did the Large Molecules of Life Originate?
In living organisms, the many
biochemical reactions require
catalysts—molecules that speed up
the reactions.
A key to the origin of life is the
appearance of catalysts—proteins
called enzymes.
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?
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.
Figure 4.9 The “RNA World” Hypothesis
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.
Figure 4.10 An Early Catalyst for Life?
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.
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.
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.
Figure 4.11 Protocells
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.
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
(13C:12C), which were found in the
fossils.
Figure 4.12 The Earliest Cells?
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.
Figure 4.13 The Origin of Life
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.