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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