The reason for double helical structure of DNA is operation of: (a) Van der waals forces (b) Hydrogen bonding (c) Dipole - dipole moment (d) Electrostatic attractions

DNA, or deoxyribonucleic acid, is the hereditary material in humans and almost all other organisms. Its structure is critical to its function in carrying genetic information. A fundamental characteristic of DNA’s structure is hydrogen bonding, which is key to the formation of the double helix.

Hydrogen bonds are weak attractions between a hydrogen atom in one molecule and a more electronegative atom, such as nitrogen or oxygen, in another. In the context of DNA, these bonds occur between nitrogenous bases of nucleotides on opposite strands of DNA. Specifically, adenine (A) pairs with thymine (T) through two hydrogen bonds, and guanine (G) pairs with cytosine (C) with three hydrogen bonds. This bonding pattern is highly specific and often referred to as complementary base pairing.

In order to fully appreciate the significance of hydrogen bonding in DNA, it's important to understand that these bonds are strong enough to hold the two strands together but weak enough to allow for the strands to separate during processes such as DNA replication and transcription. This is crucial for the transfer of genetic information to new cells and to messenger RNA.

Each DNA strand is a linear polymer composed of small units called nucleotides. Understanding the structure of these nucleotides is essential to grasp the overall architecture of DNA.

Each nucleotide subunit includes three components: a phosphate group, a five-carbon sugar (deoxyribose in DNA), and a nitrogenous base. There are four different types of nitrogenous bases in DNA which are adenine (A), thymine (T), cytosine (C), and guanine (G). These bases are categorized into two groups - purines (A and G) and pyrimidines (C and T).

The order of these nucleotides along a DNA strand is what constitutes the genetic code. It's essentially a biological language that defines the synthesis of proteins, which are crucial for the structure and function of cells. The backbone of the DNA strand is formed by alternating phosphate and sugar groups, which are connected via strong covalent bonds. This backbone is identical in all DNA molecules, whereas the sequence of bases varies and contains the genetic instructions unique to each organism.

The double helix is the iconic shape recognized for DNA, and it's more than just an aesthetic feature – it's essential for DNA's function in storing genetic information.

The double helix structure consists of two DNA strands twisted around each other, with each turn of the helix having approximately 10 nucleotide pairs. The strands run in opposite directions, an arrangement that is described as antiparallel. This orientation ensures that the nitrogenous bases are aligned properly for hydrogen bonding.

Structural Stability

This arrangement provides structural stability and puts the hydrophobic base pairs in the interior, protected from water, while exposing the negatively charged phosphate backbone to the aqueous environment. The double helix is further stabilized by base-stacking interactions; the flat bases stack on top of one another in an arrangement that is energy-efficient. In exploring the double helix, we recognize a structure perfectly tuned for the repair, replication, and reading of vital genetic information, representing a masterpiece of biological architecture.