BIOLOGY PROJECT
DNA FORMS
BY: YOMNA HESHAM AND JANA MOSTAFA 12B
WHY DO DIFFERENT FORMS OF DNA EXIST?
 There just isn't enough space for the DNA to extend out into a perfect, linear B-DNA shape. In almost all cells,
from tiny bacteria to large eukaryotes, DNA must be compressed more than a thousand times in order to fit
inside the cell or nucleus.
 Refined resolution of the structure of DNA, based on X-ray crystallography of short synthetic DNA segments,
has revealed that the helical helix of DNA varies significantly depending on the sequence. If a 200-bp portion of
DNA has the correct sequence, it can be run on an acrylamide gel as if it were more than 1000 bp. The double
helix does not follow the same uniform structure.
A-FORM DNA
 A-DNA is a right-handed double helix composed of deoxyribonucleotides. It appears when the relative humidity
in the environment is less than 75%, implying that it is rarely present under normal physiological conditions. The
two strands of A-DNA are antiparallel and not symmetrical. The molecule is asymmetrical because the glycosidic
linkages of a base pair are not diametrically opposed to one another. Each turn contains both major and minor
grooves. One rotation of the helix has 11 base pairs and a length of 2.86nm.
 A-DNA's backbone is made up of sugar phosphates that are connected together continuously via phosphodiester
linkages. The helix's center contains all of its nitrogenous bases. Hydrogen bonding between nitrogenous bases
allow the molecule to form a double helix shape. The helix width of A-DNA is 2.3 nm. Overall, A-DNA is larger
than the more prevalent B-DNA.
B-FORM DNA
 B-form DNA is a right-handed double helix found by Watson and Crick using X-ray diffraction patterns. It is the
most frequent type of DNA found under normal physiological conditions. The two strands of B-DNA run in
opposite directions. The structure is uneven, with major and minor grooves appearing alternately.
 The molecule is asymmetrical because the glycosidic linkages of a base pair are not diametrically opposed to one
another. One turn contains ten base pairs with a length of 3.4nm. The distance between neighboring
deoxyribonucleotides is 0.34 nanometers. The backbone of B-DNA, like that of A-DNA, is made up of sugar
phosphates connected together by phosphodiester linkages, while the core region is made up of nitrogenous
bases. The two strands are kept together by hydrogen bonds formed between nitrogen bases. The helix width of
B-DNA is 2 nm. B-DNA is narrower than A-DNA.
DIFFERENCES
 The primary distinction between A-form and B-form nucleic acids is the confirmation of the deoxyribose sugar
ring. It is in the C2′ endocon formation for the B-form and the C3′ endocon formation for the A-form.
 The positioning of base pairs within the duplex is the second significant variation between A-form and B-form
nucleic acids. In B-form, the base pairs are nearly centered over the helical axis, but in A-form, they are shifted
away from the central axis and closer to the main groove. The end result is a ribbon-like helix with an open
cylindrical core in A-form.
Z-FORM DNA
 Z-form DNA is a left-handed double helix. Its structure differs significantly from that of A-DNA and B-DNA. The
zigzag appearance of the backbone distinguishes it from other kinds of DNA. The helix width is 1.8nm, the
narrowest of the three variants. The structure comprises of both major and minor grooves. One round of Z-DNA
has 12 base pairs and measures 4.56nm in length.
 The distance between two adjacent deoxyribonucleotides is 0.37 nanometers. The two strands are held together
by a hydrogen bond, as is the case with other kinds of DNA. Z-DNA is difficult to monitor due to its instability. It
may play a role in the regulation of gene expression or genetic recombination. It can be found in bacteria,
eukaryotic cells, and viruses. Some viruses require Z-DNA binding proteins for pathogenicity.
CONDITIONS FAVORING A-FORM, B-FORM, AND Z-FORM OF DNA
 A DNA sequence's A-, B-, or Z-DNA conformation is determined by at least three factors.
 The first is the ionic or hydration environment, which can aid in the transition between different helical
configurations.
 Low hydration favors A-DNA, while high salt favors Z-DNA.
 The second prerequisite is DNA sequence: specific purine (or pyrimidine) lengths promote A-DNA, whereas
alternating purine-pyrimidine steps promote Z-DNA formation.
 The third condition is the presence of proteins that can bind to DNA in one helical shape and compel it to adopt
another, such as proteins that attach to B-DNA and drive it to either A- or Z-forms.
 In living cells, the majority of the DNA is in a mix of A and B-DNA conformations, with a few tiny patches capable
of generating Z-DNA