2. VISCOELASTICITY
• Viscoelasticity is the property of
materials that exhibit
both viscoUs and elastic characteristics
when undergoing deformation.
3. • viscosity is the result of the diffusion of
atoms or molecules inside
an amorphous material.
• elasticity is usually the result of bond
stretching along crystallographic planes in
an ordered solid.
• Viscoelastic materials have elements of
both of these properties and their
relationship between stress and strain
depends on time.
4. DNA
• Deoxyribonucleic acid (DNA) is an
important biopolymer that contains the
genetic information and is present in the
nucleus of cells.
• DNA are long polymers made of repeating
units called deoxyribo nucleotides, which
are made up of a phosphate group, a sugar
moiety and four different nitrogenous
bases
5. • If the solutions of DNA are subjected to
shear stress , the DNA molecules are
extended
• After stress has been removed. The
molecules returns to a relaxed
configuration in which molecule , if it is long
approximates a random coil
6. Couette viscometer
• A solution of an extendable molecule shows
viscoelasticity when sheared in couette viscometer
Zimm-crothers Cartesian diver
viscometer
• The application of shear stress cause the inner
cylinder to rotate.
• When the stress is removed,if viscometer contains
a pure solvent,the angular velocity approaches to
zero
7.
8.
9. • However if the rotor is suspended in
solution of high molecular weight DNA, the
rotor stops and the direction of rotor
reverses owing to relaxation of previously
stretched DNA
• Ultimately the rotor stops owing to
friction
• The exponential decay rate at which rotor
comes to rest can be characterized by
time constant τr called retardation time.
10. • The molecular weight of a linear DNA
molecule is determined by specific
viscosity by extrapolating value of
retardation time to zero shear rate and
zero DNA concentration
• 0.665 logM = 2.0863 + log([n]+5)
11. RADIAL MIGRATION
METHOD
• Developed by Ken Dill and Bruno Zimm
• Novel method for separation of large DNA
molecules according to molecular weight
• Two concentric cones are arranged
12.
13. • The space between the cones is filled by
the DNA solution and the inner cone is
rotating slowly (approx I rotation per 5
seconds)
• The elastic molecule are aligned and
stretched
• The molecule is stretched in curve
therefore, there is small force directed
toward the axis of rotation
14. • The rate of migration is proportional to 5/2
power of molecular weight so this
separation occurs during migration
• After several hours of migration , the
rotation stopped and the stop cock at the
base of the outer cone is opened
• The solution is then fractioned by drop
collection
• The largest molecule emerge first
15. • This method is being used to
concentrate and separate intact DNA
molecules of chromosomes from
fragments that are produced during
the isolation of DNA from the tissue
