Test: Double Stranded DNA - MCAT MCQ

Type 1 topoisomerase break only one strand of DNA while gyrase works on both the strands, it breaks and reseal both DNA strands and introduce negative supercoils. Ethidium bromide and SYBR gold are used to stain DNA molecules.

When DNA duplex is laid open to the specific conditions like temperature, pH, or ionic strength it will interrupt the hydrogen bond between strands and they are no longer held together. After denaturation, the strand separates as individual coils and the double helix is denatured.

When nucleic acid sequences are rich in guanine and can form four-stranded structure, it will term as G-quadruplex. It is a square arrangement of guanine, stabilized by hoogsteen hydrogen bonding.

Anti glycosidic bond conformation is incorrect as in Z-DNA the sugar and glycosidic bond conformations alternate, anti for C and syn for G while in B form there is only anti glycosidic bond conformation.

There are many forms of DNA which are biologically important, out of which Watson-Crick double helix model describes the B form of DNA. The confirmation of DNA would depend on the hydration level, base modification etc.

Linking number defines the no. of times one strand crosses other in closed circular DNA. It is the sum of twist number (total number of helical turn) and writhe number (supercoiling in the helix).
Lk = Tw + Wr.

A monovalent cation is responsible for the stability of G-quadruplex as it is present in the center of the tetrad. It can be formed in either DNA or RNA.

Triple helix formed by three strands polypurine, polypyrimidine and the third strand which lay in the major groove of DNA and makes a hydrogen bond to duplex. The third strand is paired in hoogsteen base pairing scheme where the central strand is purine rich.

Watson-Crick double helix model consists of 10.4 base pairs per helical turn. Since one helical turn formed by 360° and there would be 34.3° twist angle per residue among adjacent base pair.

DNA double helix was first described in 1953 by Watson and Crick using X-ray diffraction. DNA fibers were obtained by Franklin and Wilkins. Watson, Crick, and Wilkins were awarded a noble prize in 1962.