How is DNA structured as a double helix with two antiparallel strands of nucleotides linked by hydrogen bonds between complementary base pairs?

SouravNovember 8, 2024

Answered step-by-step

The structure of DNA as a double helix is a remarkable feature that underpins its function in storing and transmitting genetic information. Here’s a detailed explanation of how this structure is organized:

Basic Structure of DNA

1. Double Helix Formation

• Helical Shape: DNA is composed of two long strands that twist around each other to form a double helix. This helical structure was famously described by James Watson and Francis Crick in 1953.
• Right-Handed Twist: The most common form of DNA, known as B-DNA, has a right-handed twist, meaning it spirals in a clockwise direction.

2. Antiparallel Strands

• Strand Orientation: The two strands of DNA run in opposite directions, which is referred to as being “antiparallel.” One strand runs in the 5′ to 3′ direction, while the other runs from 3′ to 5′.
• 5′ and 3′ Ends: The numbers refer to the carbon atoms in the sugar backbone (deoxyribose) of the nucleotides. The 5′ end has a phosphate group attached to the fifth carbon, while the 3′ end has a hydroxyl group attached to the third carbon.

3. Nucleotide Composition

• Nucleotides: Each strand of DNA is made up of repeating units called nucleotides. Each nucleotide consists of three components:
  • A phosphate group
  • A deoxyribose sugar
  • A nitrogenous base (adenine, thymine, guanine, or cytosine)

Base Pairing and Hydrogen Bonds

1. Complementary Base Pairing

• Specific Pairing: The nitrogenous bases from each strand pair specifically:
  • Adenine (A) pairs with Thymine (T)
  • Guanine (G) pairs with Cytosine (C)
• This specificity is due to hydrogen bonding patterns:
  • A-T pairs are connected by two hydrogen bonds.
  • G-C pairs are connected by three hydrogen bonds, making G-C pairs slightly stronger than A-T pairs.

2. Hydrogen Bonds

• Stability: The hydrogen bonds between complementary bases hold the two strands together, providing stability to the double helix structure. However, these bonds are weak enough to allow for the separation of strands during processes such as DNA replication and transcription.

Sugar-Phosphate Backbone

• Backbone Structure: Each strand’s backbone is formed by alternating sugar (deoxyribose) and phosphate groups. The phosphate group of one nucleotide links to the sugar of the next nucleotide through phosphodiester bonds, creating a strong covalent bond that provides structural integrity.