Introduction to B-DNA

No, no, it’s B DNA, not Be DNA. But you can be DNA if you want; no one is stopping you.
B- DNA is the canonical right-hand structure of the DNA helix.
B- DNA is the most common structure of DNA.
It is a double helix made of two antiparallel strands with hydrogen bonds between adenine and thymine and guanine and cytosine.
The hydrogen bonds between adenine, thymine, guanine, and cytosine ( nucleobases) organize the DNA structure into a right-handed helix shape.
Genetic information is generally stored in long double-stranded DNA molecules. Hydrogen bonding between nucleobases keeps the complementary DNA strands organized into a right-handed helical structure called B-DNA. Structural transitions into other DNA forms can occur within certain sequence elements of DNA and these can be functionally important.
Several non-B-DNA structures (oftentimes called unusual or alternative DNA structures) can be important for interactions with proteins involved in replication, gene expression and recombination.
They may also play different roles in the formation of nucleosomes and other supramolecular structures involving DNA. DNA sequences characterized as “random” or “mixed sequence” typically only form A-DNA or B-DNA.
Special sequence characteristics or defined symmetry elements are required to form alternative structures such as left-handed Z-DNA, cruciforms, intramolecular triplexes, quadruplex DNA, slipped-strand DNA, parallel-stranded DNA, and unpaired DNA structures.
1 Together with variations in DNA supercoiling, local alternative structures provide enormous potential for autoregulation of DNA functions. This chapter will briefly review major alternative DNA structures and their potential involvement in biology.

Structure

B-DNA contains 10.5 base pairs for each helical turn.
These base pairs are buried deep in the helix and are almost perpendicular to the helical axis.
The distance between the base pairs in B-DNA is 3.4Å.
The pitch or length of each helix is 34Å.
The base pair tilt is found to be -6ᐤ.
The diameter of the helix is 20Å.

type of DNA structures

Apart from B-DNA, there are two other forms of DNA with different structures: A-DNA and Z-DNA.
Now the question arises: Why do we need different forms of DNA; these alternate forms of DNA are seen to be protected from damage.
The arrangement of A-DNA has been seen to be less affected and more stabilized from UV radiation damage and stabilization from Bacillus subtilis.
Z-DNA is found to be thinner than B-DNA, and they have one major groove as compared to B-DNA.
The zig-zag configuration in Z-DNA gives higher electrostatic repulsion, which helps the Z-DNA to tolerate more increased concentration of salt. It is stabilized in that situation.

What is the different between B-DNA and A-DNA

A-DNA

A-DNA is a right-handed double helix made up of deoxyribonucleotides. It appears when the relative humidity of the environment is less than 75%, which means that it is rarely present in normal physiological condition.
The two strands of A-DNA are anti-parallel with each other and not symmetrical. The molecule is asymmetrical is because the glycosidic bonds of a base pair are not diametrically opposite to each other.
Therefore, major grooves and minor grooves can be observed in each turn. One turn of the helix consists of 11 base pairs with a length of 2.86nm. The backbone of A-DNA is formed by sugar phosphates that are linked continuous using phosphodiester bonds.
All the nitrogenous bases are at the core centre of the helix. Hydrogen bonds between nitrogenous bases allow the molecule to exhibit the double helix structure. The helix width of A-DNA is 2.3nm. Overall, A-DNA is wider than the more commonly found B-DNA.

B-DNA

B-form DNA is a right-handed double helix, which was discovered by Watson and Crick based on the X-ray diffraction patterns. It is the common form of DNA exists under normal physiological condition. The double strands of B-DNA run in opposite directions.
The structure is asymmetrical with major grooves and minor grooves present alternatively. The molecule is asymmetrical is because the glycosidic bonds of a base pair are not diametrically opposite to each other. In one turn, there are 10 base pairs with a length of 3.4nm.
The distance between adjacent deoxyribonucleotides is 0.34nm. Same as A-DNA, the backbone of B-DNA is formed by sugar phosphates that are linked continuous using phosphodiester bonds and the core region consists of nitrogenous bases.
The two strands are held together by hydrogen bonds between nitrogenous bases. The helix width of B-DNA is 2nm. B-DNA is narrower than A-DNA.

Conclusion

The general structure of B-DNA is found to be helpful.
But the alternative DNA structures are also helpful in other extreme environmental situations.
We can also find these alternate structures because of environmental situations, protein binding, and superhelical tension.
These types of DNA are found in prokaryotes and some eukaryotic cells, and information about their replication and expression continues to be studied.

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Recombinant DNA technology

By :- ANUSHKA ASH