Yang Yang, Xianfeng Song Advisor: Sima Setayeshgar
Journal Club April 11th, 2008
Liquid Crystal Phases of DNA and Implications for the origin
of life
Outline
o Part I: Introduction to liquid crystals
o Part II: Introduction to DNA
o Part III: Background on theories of origin of life
o Part IV: Liquid crystal condensation of 6-to-20-base pair DNA duplexes
Part I: Introduction to liquid crystals
Introduction to Liquid Crystal
o Phases between liquid and solido Can be divided into two types:
o Thermotropic: exhibit phase transition into the LC phase as temperature is changed
o Lyotropic: exhibit phase transition into the LC phase as a function of concentration of the mesogen
o Mesogen is the fundamental unit of a liquid crystal that induces structural order in the crystals.
Birefringence (Double Refraction)
o A typical behavior due to anisotropyo Two different refraction indexo no is the refractive indices for o-ray (polarization direction
is perpendicular to the optical axis, called director)o ne is the refractive indices for e-ray (polarization direction
is parallel to the optical axis)o Utilized to view the texture of different phases of LC.
Optical Devices: Crossed polarizers
When the polarizers are arranged so that their planes of polarization are perpendicular to each other, the light is blocked. When the second filter (called the analyzer) is parallel to the first, all of the light passed by the first filter is also transmitted by the second.
When putting LC in between two polarizers, the polarization state is modified by LC. Now there will be light come through depends on the director’s direction, LC’s thickness, ray’s frequency.
Liquid Crystal Phases: Nematic phase
o Nematic phaseoThe mesogens have no
positional order, but exhibits long-range orientational order.
oMost nematics are uniaxial, but some liquid crystals are biaxial nematics.
The Schlieren texture, is characteristic of the nematic phase. The dark regions that represent alignment parallel or perpendicular to the director are called brushes.
Liquid Crystal Phases: Chiral Nematic Phase
o The chiral nematic (cholesteric) liquid crystal phase is typically composed of nematic mesogenic molecules containing a chiral center which produces intermolecular forces that favor alignment between molecules at a slight angle to one another.
o This leads to the formation of a structure which can be visualized as a stack of very thin 2-D nematic-like layers with the director in each layer twisted with respect to those above and below.
A typical texture of chiral nematic liquid crystal with long pitch helix. Network-like defect
lines are oily-streak lines.
The structure of chiral nematic liquid crystals
Liquid Crystal Phases: Smectic Phase
o Form well-defined layers that can slide over one anothero Smectic A phase: the mesogen are oriented along the layer
normalo Smectic C phase: the mesogen are tilted away from the layer
normal
Picture of the smectic A phase
Picture of the smectic C phase
Texture of the smectic A phase
Liquid Crystal Phases: Columnar Phases
100× of texture exhibited by the
hexagonal columnar mesophase
Columnar phase formed
by discotic molecules
Columnar phase formed
by rod-like molecules
A class of liquid crystal phases in which molecules assemble into cylindrical structures
Part II: Introduction to DNA
DNAIn 1952, Rosalind Franklin took the first X-ray diffraction image of DNA , photo 51, which was critical evidence
in identifying the structure of DNA.
In 1953, James D. Watson and Francis Crick described the double helix structure of DNA .oDNA is a double helix, with the sugar and phosphate parts of nucleotides forming the two strands of the helix, and the nucleotide bases pointing into the helix and stacking on top of each other.oThe nucleotide bases use hydrogen bonds to pair specifically, with an A always opposing a T, and a C always opposing a G.oThe two strands of the double helix run in opposite directions.
DNA and RNA Structure
A, B , Z form of DNAA B Z
Helix sense Right handed
Right-handed
Left handed
Repeating unit 1 bp 1bp 2 bp
Rotation/bp 33.6° 35.9° 60°/2
Mean bp/turn 10.7 10.0 12
Inclination of bp to axis
+19° -1.2° -9°
Rise/bp along axis
2.3Å 3.32Å 3.8Å
Pitch/turn of helix
24.6Å 33.2Å 45.6Å
Mean propeller twist
+18° +16° 0°
Glycosyl angle anti anti C: anti, G: syn
Sugar pucker C3'-endo C2'-endo C: C2'-endo, G: C2'-exo
Diameter 26Å 20Å 18Å
Genetic Information Flowo A gene is a sequence of DNA that contains genetic information
and can influence the phenotype of an organism. The genetic code consists of three-letter 'words' called codons formed from a sequence of three nucleotides (e.g. ACT, CAG, TTT).
o In transcription, the codons of a gene are copied into messenger RNA by RNA polymerase.
o In translation, messenger RNA (mRNA) is decoded to produce a specific polypeptide according to the rules specified by the genetic code. This uses an mRNA sequence as a template to guide the synthesis of a chain of amino acids that form a protein.
Part III: Theories of origin of life
Origin of Life
o Before the mid-17th century, most people believed that God had created humankind and other higher organisms and that insects, frogs and other small creatures could arise spontaneously in mud or decaying matter.
o Scientific theoryoOrigin of organic moleculesoFrom organic molecules to protocells
Origin of organic
molecules
o Miller's experiments (The Primordial Soup Theory): using a highly reduced mixture of gases – methane, ammonia and hydrogen – to form basic organic monomers, such as amino acids.
o The Deep Sea Vent Theory: hydrogen-sulfide and hydrogen coming out from the vent in contact with a suitable oxidant, such as carbon dioxide can release chemical energy.
o Wächtershäuser's hypothesis: In 1997, Wächtershäuser and Claudia Huber mixed carbon monoxide, hydrogen sulfide, nickel sulfide, and iron sulfide particles at 100°C and demonstrated that amino acids could form.
o Radioactive beach theory: stronger tidal processes from a much closer moon may have concentrated radioactive grains of uranium and other radioactive elements at the high water mark on primordial beaches where they may have been responsible for generating life's building blocks.
Origin of Organic Molecules: Miller's experiments
In 1953 a graduate student, Stanley Miller, and his professor, Harold Urey, performed an experiment that proved organic molecules could have spontaneously formed on early Earth from inorganic precursors. The now-famous “Miller-Urey experiment” used a highly reduced mixture of gases – methane, ammonia and hydrogen – to form basic organic monomers, such as amino acids.The Miller-Urey experiment attempted to recreate the chemical conditions of the primitive Earth in the laboratory, and synthesized some of the building blocks of life.
How the relatively simple organic building blocks polymerase and form more complex structures?
From Organic Molecules to Protocells
o "Genes first" models-the RNA world: the nucleic acids comes first.
o "Metabolism first" models-iron-sulfur world : the evolution of biochemical reactions and pathways comes first
oBubble Theory: Bubble on the shore is a hypothetical precursor to the modern cell membrane. If protein appears in the bubble, it will spread when bubble burst. When the material accumulate enough, protocell may be occur.
From
Organic
Molecules to Protocells:
RNA
world hypothesis
In the late 1960s Carl R. Woese present this independent RNA idea. The phrase "RNA World" was first used by Nobel laureate Walter Gilbert in 1986, in a commentary on recent observations of the catalytic properties of various forms of RNA. This is a world in which RNA catalyzed all the reactions necessary for a precursor of life's last common ancestor to survive and replicate.
Difficulties
o The formation of molecular chains as uniform as DNA by random chemistry is essentially impossible.
The paper gives us an idea how the small molecule s tend to self-organize themselves to larger molecule
Part IV: Liquid crystal condensation of 6-to-20-base pair DNA duplexes
Background
o Duplex lDNA can form liquid crystal phases when hydrated:o Four phases: isotropic phase (I), chiral nematic (N), uniaxial
columnar (CU)o Ranging from mega base pair (bp) semi-flexible polymers
down to approximately 100 bp rigid rod-like segments (B-DNA has bend persistence length ~50nm)
o Onsager-Bolhuis-Frenkel(OBF) criterionoModel: Monodisperse repulsive hard rods (length L,
diameter D)o Conclusions: If the rods are sufficiently anisotropic in shape,
the appearance of nematic phase require: L/D>4.7 (N>28bp). If L/D<4.7, there should be no LC phases at any volume fraction.
Experiments
o Subject: The solutions which contains a series of self-complementary sDNA duplex-forming “palindromic” oligomers, along with a variety of noncomplementary and partially complementary oligomers
o Result: Short complementary B-form DNA oligomers, 6 to 20 base pairs in length, are found to exhibit nematic and columnar liquid crystal phases, even though such duplexes lack the shape anisotropy required for liquid crystal ordering
Phase Diagramo The phase diagram includes the phase boundaries
measured for sDNA with those obtained from the literature for lDNA, along with the predictions from the Onsager and other models of interacting semi-flexible rod-shaped particle and aggregate solutes.
For N < 20, phase transitions from our data are marked by red open symbols (I-N, triangles; N-CU, circles; CU-C2, squares), and the range of each phase is indicated by colored columns (I, magenta; N, cyan, CU, yellow), at T = 20°C for 20 > N > 8 and T = 10°C for N = 6.
LC Ordering from Mixed Solutions of Complementary and Non-complementary
Oligomers
o The addition of unpaired bases at the sDNA duplex ends, eliminates LC ordering by weakening end-to-end adhesion. This interplay of sequence and LC ordering leads to a remarkable means of condensation of complementary sDNA duplexes from mixed solutions of complementary and noncomplementary oligomers.
o Experiment show if there is a
large excess of noncomplementary oligomers, the LC phase appears as isolated drops.
Discussion & Conclusion
o The observation of nematic and columnar LC phase provides clear evidence for end-to-end stacking of sDNA into rod-shaped aggregates.
o The end-to-end stacking makes the DNA concentration is much higher than in the surrounding isotropic, which has potential implications for the prebiotic chemical generation of complementarily H-bonded molecular assemblies, which will promote ligation in the LC phase.
o Additionally, every ligation in the LC phase produces an
extended complementary oligomer. Thus, the formation of the LC phase by the complementary duplexes has the autocatalytic effect of establishing conditions that would strongly promote their own growth into longer complementary chains relative to the non-LC-forming oligomers.
Thanks!
