TLS Online TPP Program

#Id: 3993

#Unit 2. Cellular Organization

TLS Online TPP Program

#Id: 3995

#Unit 2. Cellular Organization

When an ethidium ion intercalates between two base pairs, it causes the DNA to unwind by 260, reducing the normal rotation per base pair from 360 to 100. In other words, ethidium decreases the twist of DNA.

TLS Online TPP Program

#Id: 3997

#Unit 2. Cellular Organization

In a natural closed DNA that is negatively supercoiled, the intercalation of ethidium bromide first removes the negative supercoils and then introduces positive supercoils.

TLS Online TPP Program

#Id: 3998

#Unit 2. Cellular Organization

One approach is to measure the effect of nicking the DNA. Unconstrained supercoils are released by nicking, whereas constrained supercoils are unaffected. Nicking releases -50% of the overall supercoiling.

Another approach uses the crosslinking reagent psoralen, which binds more readily to DNA when it is under torsional tension. The reaction of psoralen with E. coli DNA in vivo corresponds to an average density of 1 negative super helical turn/200 bp (sigma = -0.05).

TLS Online TPP Program

#Id: 3999

#Unit 2. Cellular Organization

Interphase chromatin remain attached to nuclear wall proteins via matrix attachment regions (MARs) or matrix attachment DNA sequences.

TLS Online TPP Program

#Id: 4000

#Unit 2. Cellular Organization

(MARs) also called scaffold attachment regions (SARs), or S/MARs in case of metaphase chromosome as the same sequences appear to attach to the protein substructure in both metaphase and interphase cells.