1.5.7 The Watson Crick Model

Key Features of the Watson-Crick Model

1. Double Helix Structure:

• DNA consists of two polynucleotide chains wound around each other to form a helical structure.
• The two strands run antiparallel (one strand runs 5' to 3' while the other runs 3' to 5').

2. Components of DNA:

• Nucleotides: Building blocks of DNA, each made of:
• A phosphate group.
• A pentose sugar (deoxyribose).
• A nitrogenous base (adenine, thymine, cytosine, or guanine).
• Phosphodiester Bonds: Nucleotides in a single strand are joined by these bonds, forming the sugar-phosphate backbone.

3. Complementary Base Pairing:

• The bases of one strand pair with the bases of the other strand via hydrogen bonds:
• Adenine (A) pairs with Thymine (T) via 2 hydrogen bonds.
• Cytosine (C) pairs with Guanine (G) via 3 hydrogen bonds.
• This base pairing ensures that the sequence of bases in one strand determines the sequence in the other.

4. Hydrogen Bonding and Stability:

• Hydrogen bonds hold the two strands together.
• The large number of hydrogen bonds in a DNA molecule provides stability while allowing the strands to separate during replication.

5. Helical Turn:

• Each turn of the helix contains 10 base pairs, with a rise of 3.4 nm per turn.

6. Replication:

• Watson and Crick suggested a semi-conservative model of replication:
• Each strand serves as a template for a new complementary strand.
• Results in two DNA molecules, each with one original strand and one new strand.