#structureofdna #dna #biology #textbook #igcse

Pg:- 67,68,69,71

Ch:- 4

The structure of DNA (deoxyribonucleic acid) is famously described as a double helix, resembling a twisted ladder.

1. Double Helix Shape

• DNA is composed of two long strands that spiral around each other, forming a helical shape. This double-helix structure was discovered by James Watson and Francis Crick in 1953, with key insights from Rosalind Franklin’s X-ray diffraction images.

2. Nucleotides: The Building Blocks

• DNA strands are made of smaller repeating units called nucleotides. Each nucleotide consists of:

  • A phosphate group

  • A deoxyribose sugar (a five-carbon sugar)

  • A nitrogenous base

3. Nitrogenous Bases and Base Pairing

• There are four types of nitrogenous bases in DNA:

  • Adenine (A)

  • Thymine (T)

  • Cytosine (C)

  • Guanine (G)

• The bases pair specifically with each other through hydrogen bonds:

  • Adenine pairs with Thymine (A-T) with two hydrogen bonds

  • Cytosine pairs with Guanine (C-G) with three hydrogen bonds

4. Backbone of DNA

• The two strands of DNA are held together by a backbone made of alternating phosphate and deoxyribose sugar groups. These form covalent bonds along each strand, giving structural stability.

5. Antiparallel Orientation

• The two DNA strands run in opposite directions, or "antiparallel." One strand runs from the 5' to the 3' end, while the other runs from the 3' to the 5' end, which is essential for replication and transcription.

6. Major and Minor Grooves

• The twisting of the double helix creates grooves along the structure. The major groove and minor groove are areas where proteins and other molecules can bind to interact with the DNA.

This structure allows DNA to store genetic information efficiently, replicate accurately, and support the vast diversity of life through the variations in its sequence.