DNA contains sugar and phosphates in their backbone chain which are both water soluble. But the bases are not. a linear DNA would still expose these bases to the water inside the cell. The DNA in cell is actually even more collied than the pictorial representation shown in double stranded DNA in textbook and it does that so that bases are hidden inside away from water molecules. It is at energy lower state.

I want to add to Amar Thaker 's answer that the structure is not completely rigid and keeps changing in bending, twisting and compression. The interaction with proteins in the cell can also alters the coiling of the DNA and specifically the DNA replication will require the unwinding of the DNA helix.

Finally, I would also suggest that you check about alternative conformations of DNA as the helix can have alternative structures. Check this paper for more information on the function: https://www.ncbi.nlm.nih.gov/books/NBK6545/

Yes the DNA contains sugar and phosphates in their backbone chain which are both water soluble .

The DNA structure is not rigid and there is constant bending and meandering, however The degree of supercoiling in a DNA molecule is influenced by environmental conditions, such as ionic strength and temperature; since supercoiling of DNA influences the biological pathways in which it is involved, the level of DNA supercoiling inside cells is tightly regulated.

Coiling delivers significant amount of free energy to DNA molecules and, inside cells, this can be used to drive structural transitions and other metabolic processes that would normally be thermodynamically unfavourable, such as opening of the DNA helix during replication and transcription

1) the curvature is indeed related to the spatial arrangement of the links in the structure.

2) the spiral state is such that bases inside the helix are probably better protected from the environment ; the phosphate skeleton with its negative charge probably ensures a good isolation from the overall proteinaceous equally negatively charged cytoplasm ; the minor groove do not give any indication about the nucleotide sequence, whereas the major groove can offer some specificity of sequence recognition ;

3) the helical structure - but more importantly the double strand structure - is essential to replicate the DNA ;

4) and indeed to help to occupy this long string in a smaller volume, you may have already played with a telephone cord and observed how it is easy to pull it, to condense and may be more difficult to decondense the ball! Specialist of materials will explain if such advantageous properties existed with other polymer forms?

Differences in strength of hydrogen bonds between Adenine (A) and Thymine (T) base pairs and Guanine (G) and Cytosine (C) base pairs seems to provide basis for these base pairs to be set at an angle of 36 degree rather than exactly above each other. This leads to a spiral structure of the double helix DNA.

agree with@ Amar Thaker

regards