Why DNA is twisted?

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In Biology, "double helix" is a term used to describe the structure of DNA. A DNA double helix consists of two spiral chains of deoxyribonucleic acid. The shape is similar to that of a spiral staircase. DNA is nucleic acid composed of nitrogenous bases (adenine, cytosine, guanine and thymine), a five carbon sugar (deoxyribose), and phosphate molecules. The nucleotide bases of DNA represent the stair steps of tyhe staircase, and the deoxyribose and phosphate molecules from the sides of the staircase.

Why is DNA twisted?

DNA is coiled into chromosomes and tightly packed in the nucleus of our cells. The twisting aspect of DNA is a result of interactions between the molecules that make up DNA and water (H2O). The nitrogenous base that comprise the steps of twisted staircase are held together by hydrogen bonds. Adenine is bonded with thymine (A-T) and guanine pairs with cytosine (G-C). These nitrogenous bases are hydrophobic, meaning that they lack an affinity for water. Since the cell cytoplasm and cytosol contain water based liquids, the nitrogenous bases want to avoid contact with cell fluids.
Types of DNA-
B DNA (surrounded by 91-93% of water) i.e, 36 degree turn per base pair
A DNA (surrounded by 71-73% of water) i.e,  32.72 degree turn per base pair
 The sugar and phosphate molecules that form the sugar phosphate backbone of the molecule are hydrophilic, which means they are water loving and have an affinity for water. DNA is arrange such that the phosphate and the sugar backbone are on the outside and in contact with fluid, while the nitrogenous bases are in the inner portion of the molecule. In order to further prevent the nitrogenous bases from coming into contact with cell fluid, the molecule twist to reduce space between the nitrogenous bases and the phosphate and sugar strands.The fact that the two DNA strands that form the double helix are anti parallel helps to twist the molecule as well. Anti- parallel means that the DNA strands run in opposite directions, ensuring that the strands fit tightly together. This reduces the potential for fluid to seep between the bases. 


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