Biology Reference
In-Depth Information
nitrogenousbase,forexampleadenine(A),thymine(T),guanine(G),
or cytosine (C). Adenine and guanine are the member of purines
whichconsistoftwoheterocyclicringsofcarbonandnitrogenatoms
while cytosine and thymine have a single such ring. A sugar and a
base are composed of a “nucleoside” and if a phosphate group is
attached on it (carbon atom at 5
or 3
position), then the main unit
ofDNAwhichiscalleda“nucleotide”occurs.Phosphategroupshave
negativecharges [1].
The stable double-stranded DNA structures are held together by
the strong covalent and noncovalent bonds (i.e., hydrogen bonds,
ionic bonds, Van der Waals and hydrophobic forces) which are
theoretically 10 times weaker than covalent bonds. In aqueous
media,thestrengthofthesebondsincreasebecauseofthehydrogen
bonds formed between the partially negative oxygen atom and the
partially positive hydrogen atom of water.
While covalent bonds don't get affected from heat, noncovalent
bondscanbebrokenreversiblybyahightemperature.Formolecular
interactions in living cells, this situation is desired because it
plays an essential role in biological functioning. This reversible
interactions are also used in the development of bisensorsystems.
The stable duplex DNA molecule is also protected via weak
hydrogen bonds, which occurs between A-T and G-C bases, when
a hydrogen atom is sandwiched between two elecron-attracting
atoms, usually oxgen or nitrogen. It should not be forgotten that
hydrogen bonds can also form between bases within a single-
stranded DNA or RNA molecule dependent on the sequence of
molecules and the distance of its complementary region on the
same strand. As a result of this bonding, hairpin DNA structures
or loops occur which are called as “the secondary structure of
DNA” [1]. Some compounds which have planar aromatic ring in
their chemical structure bind DNA between adjacent base pairs (or
between hydrogen bonds) via intercalation such as daunomycin [2]
and bleomycin.
Most of the DNAs have a B-DNA in living cells. DNAs also have
differenthelicalstructuressuchasA-DNAorZ-DNA.AandBformsof
DNAs are both right-handed helices (clockwise direction) and their
one turns contains 11 (A form) and 10 (B form) base pairs. Left-
handed Z-DNAform has 12 basepairs per turn.
