At this time; reading, listening or writing the word DNA doesn’t amaze us, we know that it is deoxyribonucleic acid. But it was not always like this. There was a time when it was a mystery; many of the scientists, researcher and workers spent their whole life in searching out what is DNA.

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University College of Agriculture, Sargodha
MOLECULAR GENETICS
PBG-510
ASSIGNMENT TITLE
Forms of DNA
Course Instructor
Sir Dr. Saeed Rauf
Submitted by
Ishtiaq Shariq
P.B.G. 6th
Roll No. 2
Dated: Mar. 22, 14

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Abstract
At this time; reading, listening or writing the word DNA doesn’t amaze us, we know
that it is deoxyribonucleic acid. But it was not always like this. There was a time when it was
a mystery; many of the scientists, researcher and workers spent their whole life in searching
out what is DNA.
Going back in time, we come to know that O. T. Avery, C. M. Macleod and
McCarty, they worked on proofing that DNA is the genetic material that transfers from one
generation to other. Watson and Crick after their effort of 50 years gave the chemical
structure of DNA. Rosalind Franklin with her work of X-ray crystallography informed
Watson and Crick about the dimension and shape of DNA molecule; helping them to finally
make DNA model.
The science is the name of joint effort that leads to discoveries and inventions. The
DNA studies in 1950s were more based on X-ray diffraction, but the latest studies done using
single-crystal X-ray analysis have given us A-, B-, C-, D-, E-, P- and Z- DNA. All these
based on the high salt or dehydration conditions.

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F O R M S O F D N A
INTRODUCTION AND HISTORICAL EVENTS
Here I am going to give a brief explanation of Forms of dna, before that I feel need of
explaining the word DNA; a polynucleotide, bio-chemical molecule made up of many
nucleotides (monomers) joined together in the form of a chain. DNA is the information
storing molecule and main command center of a living organism that controls the whole
mechanism of an organism (by producing proteins). DNA is found in all living organisms in
exceptional case in bacteria, and some time in meso-biotic organism i.e. virus.
When Watson and Crick were working on DNA, at that time only two forms of DNA
were known, A-DNA and B-DNA. Rosalind Franklin under aqueous, low salt conditions
found out B form of DNA during her X-ray studies, Watson and Crick succeeded her work.
Watson, Crick, shared the Nobel Prize in 1962 but unfortunately she had died in 1958, she
was 37 years old.
In 1950s DNA was studied on the bases of X-ray diffraction, that was not so much
precise, but the investigations made with the new technology single crystal X-ray analysis
have revolutionized the genetics field. The technique mentioned few lines back used to give
the resolution of 5 Ȃ (Angstrom); on the other hand the single crystals diffract X-rays at about
1 Ȃ (Angstrom) interval that’s nearly equal to atomic resolution. That resulted in a fully
detailed atomic level structure of DNA.
A-DNA
A-DNA which is found under high salt or dehydration condition has been examined
properly. In comparison to B-DNA, A-DNA is bit denser, having 9 base pairs in each
complete turn of helix, which is 23 Ȃ (2.3 nm) in diameter. And the helix is right handed. The
bases are moved sideways relative to the axis of the helix that leads to the difference in the
positioning of bases. Resultantly the appearance of the major and minor grooves is improved.
It seems uncertain that A-DNA occurs in vivo (under physiological conditions)
When DNA explored under many laboratory conditions more different forms of right
handed helices were found. And they were named as C-, D-, E- and newly discovered P-
DNA.

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C-DNA
When dehydration condition is higher than that required for A-DNA and B-DNA at
that time C-DNA is observed. It has only 9.3 base pairs per turn and that’s why it’s less
dense. Its helical diameter is 19 Ȃ. Like A-DNA, C-DNA doesn’t have its base pairs lying
flat; they are moved sideways relative to the axis of the helix.
B-DNA
The DNA model described two famous scientists Watson and Crick was the B-DNA.
B-DNA is the conformation that DNA takes under physiological. The majority of the DNA
molecules present in the aqueous protoplasms of living cells exist in the B formation.
Though, DNA is not a motionless, never changing molecule. On the other hand, DNA
molecule displays significant structural elasticity. The constructions of DNA molecules
change as a function of their environment. The exact structure of the DNA molecule depends
upon the nature of molecule with which it is interacting. In fact, intracellular B-DNA appears
to have an average of 10.4 base pairs per turn.
D-, E-DNA
Two other forms of helices, D-DNA, E-DNA occur that doesn’t have guanine in their
base composition, in other words guanine lacking helices. And they have few base pair in
each turn, approximately 8 base pairs in D-DNA and 7 base pairs in E-DNA.
P-DNA
Freshly, Jean Francois Allenmand and colleagues discovered that if we stretch the
DNA artificially another form of DNA will be observed, called P-DNA (named for Linus
Pauling). Comparing with the B form of DNA, P-DNA is remarkable, it’s longer and tighter
and the phosphate groups that are located on outside in B-DNA are found inside the
molecule. Now talking of the nitrogenous bases, the N-bases that are present inside the helix

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in B-DNA, are present to the outer surface in P-DNA, and very less number of hydrogen
bods are formed. There are 2.62 bases per turn, in compare to the 10.4 bases per turn in B-
DNA.
Z-DNA
Andrew Wang, Alexander Rich and their fellows in 1979 discovered the Z-DNA.
This all happened when they examined a tiny synthetic DNA oligonucleotide that had only
the G C. The Z-DNA takes the form of a left handed double helix. Like A- and B-DNA the
Z-DNA is also comprises of two antiparallel chains like structure joined together by Watson-
Crick base pair. The Z-DNA is unique in its nature. The left handed helix is 18 Ȃ (1.8 nm) in
diameter, having 12 base pairs in each turn. Having zigzag formation hence named Z. The
major groove of B-DNA is nearly nominal in Z-DNA. The unique helical symmetry provides
a recognition point for interface with DNA binding molecules. The point to which this form
happens in vivo is still debatable.
The scientists have studied the alternate form of DNA, Z and P, because scientists
believe (to some extent) that B-DNA has to take other forms to facilitate many genetic
functions. As in the processes of transcription, DNA replication; during the unwinding,
providing a site of action to enzymes of large size. However this field requires much more
study.
Tabular description of all mentioned forms of DNA:
HELIX FORM
HELIX
DIRECTION
BASE PAIRS PER
TURN
HELIX
DIAMETER
A Right handed 9 23 Ȃ (2.3 nm)
C 9.3 19 Ȃ.
D Right handed 8
E 7
B Right-handed 10.4 1.9 nm
P 2.62
Z Left handed 12 18 Ȃ (1.8 nm)

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REFRENCES
Concepts of Genetics 10th Edition 2011, William S. Klug, Michael R. Cummings,
Charlotte A. Spencer, Michael A. Palladino
Topic: Alternative Forms of DNA Exist, page number: 256-257