Measurement of viscoelasticity of

DNA

By:-Jasmin

VISCOELASTICITY

• Viscoelasticity is the property of materials that exhibit both  viscoUs and elastic characteristics when undergoing deformation.

• 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.

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

• 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

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

• 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. when is τr extrapolated to zero DNA concentration is called as relaxation time (τro)

• 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)

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

• 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

• This makes the molecules migrate toward the centre of the system

• 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

• 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

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