In a B double helix, the ribose-phosphate backbones run along the outside of the helix, and the base pairs are roughly perpendicular to the axis of the of the helix and lie just offset from the center axis of the helix, This offset causes one of the two B-DNA structure. The helical axis is vertical, and a solid line is drawn from phosphate to phosphate along the backbone.

Fig . Z-DNA structure. Shaded area is the deep groove.

grooves that run along the helix to be larger than the other thus the designation, major groove and minor groove.

Left-handed conformation
It was generally believed that B-DNA is the only form that DNA can adopt. However, in 1979 it was discovered that DNA can also form a left-handed double helix with the same hydrogen bonding between the two bases (Fig. 3). This was discovered by solving the structure of crystals of short DNA fragments;
crystals can yield a more detailed view of the molecule. A solid line drawn between the phosphates in the left-handed conformation follows a zigzag path (Fig. 5), hence it is called Z-DNA. B-DNA has two helical grooves on its outside, one wide and one narrow (Fig. 4). In contrast, Z-DNA has only one deep helical groove extending almost to the axis of the helix.
The base pairs in left-handed Z-DNA are located near the outside of the helix. Both forms of DNA have the two sugar-phosphate backbones oriented in opposite directions (Fig. 2). Z-DNA has been identified in DNA fibers as well as inside living cell.

Conformational changes
In biological systems, DNA is normally subjected to a torsional strain that tends to unwind the double helix. This torsional strain tends to untwist right-handed B-DNA and some times is great enough to stabilize certain regions of DNA in the left­ handed Z-DNA conformation.
The conversion from right handed B-DNA to left-handed Z-DNA is associated with changes in the relationship of the bases to the sugar backbone. Every other base in Z-DNA is rotated about the bond connecting it to the sugar.
This altered form is common in adenine or guanine residues (purines) but less common in thymine or cytosine residues (pyrimidines); thus sections that form Z-DNA often have alternations of purines and pyrimidines.

Right-handed B-DNA is the most common form of DNA, but left-handed Z-DNA is found to occur in a number of places and may play an important role in the regulation of gene expression. In addition, the crossing over or recombination of DNA molecules appears to have Z-DNA as an important component of the process. Although left-handed Z-DNA is less common, it is important in carrying out special functions that are in general associated with special proteins that bind to Z-DNA and not to B-DNA.
Function
For cells to live and grow, the genetic information in DNA must be (1) propagated and maintained from generation to generation, and (2) expressed to synthesize the components of a cell. These two functions are carried out by the processes of DNA replication and transcription, respectively.
Replication
Each of the two strands of a DNA double helix contains all of the information necessary to make a new double-stranded molecule. During replication the two parental strands are separated, and each is used as a template for the synthesis of a new strand of DNA. Base incorporation is directed by the existing DNA strand; nucleotides that base-pair with the template are added to the nascent DNA strand.
The product of replication is two complete double-stranded helices, each of which contains all of the genetic information (has the identical base sequence) of the parental DNA. Each progeny double helix is composed of one parental and one nascent strand. DNA replication is very accurate. In bacteria the mutation rate is about 1 error per 1000 bacteria per generation, or about 1 error in 109 base pairs replicated. This low error rate is due to a combination of the high accuracy of the replication process and cellular pathways which repair misincorporated bases.
Fig. Replication of DNA: A= adenine, C = cytosine, G = guanine, and T = thymine.

Fig. Transcription of DNA. The RNA strand.

Synthesis of the nascent DNA strands is carried out by a family of enzymes called DNA polymerases. These enzymes synthesize a new phosphodiester bond between the 3’ hydroxyl position of the terminal deoxyribose of an existing DNA (or RNA) chain and a deoxynucleotide triphosphate. All DNA polymerases (1) synthesize new DNA in a 5' to 3' direction; (2) require a primer (they must add on to the 3' hydroxyl position of DNA or RNA); and (3) require DNA molecule as a template. DNA polymerases, in conjunction with multiple accessory proteins, can rapidly synthesize new DNA strands. A bacterial replication 'fork moves at a rate of approximately 1000 nucleotides per second, while in eukaryotic cells forks move at a rate of about 100 nucleotides per second.

Transcription

In transcription, DNA acts as a template directing the synthesis of RNA. RNA is single-stranded polymer similar to DNA except that it contains the sugar ribose instead of 2-deoxyribose and the base uracil instead of thymidine . The two strands of DNA separate transiently, and one of the two single­-stranded regions is used as a template to direct the synthesis of an RNA strand. As in DNA replication, base pairing between the incoming ribonucleotide and the template strand determines the sequence of bases incorporated into the nascent RNA. Thus, genetic information in the form of a specific sequence of bases is directly transferred from DNA to RNA in transcription.
After the RNA is synthesized, the DNA reverts to double-stranded form. Transcription is carried out by a family of enzymes called RNA polymerases. In prokaryotes, RNA polymerase interacts directly with the site at which RN A transcription starts (called promoters), while in eukaryotes large complexes of proteins bind to promoters and then direct RNA polymerase to start RNA synthesis. Following transcription, newly synthesized RNA is often processed prior to being used to direct protein synthesis by ribosomes in a process called translation