STRUCTURE OF DNA

Watson and Crick Model of DNA
James Watson and Francis Crick proposed a structure for the DNA molecule that suggested the basic mechanism of DNA replication.

COMPONENTS OF DNA:

DNA is composed of two chains of repeating nucleotides. Each nucleotide consists of three components. These components are:

1.    Phosphate Group (H3PO4)
2. Deoxyribose sugar (C5H10O4)
3. A nitrogen containing base which may be:

• cytosine

• adenine

• guanine

• thymine

STRUCTURE OF A DNA STRAND:

The following are the features of the DNA molecule as described by Watson and Crick in 1953.

The model proposes that DNA is composed of two complimentary strands of DNA running anti-parallel to each other.The two strands wrap around each other to form a double helix structure.

5'---------------3'

3'---------------5'

The hydrophilic sugar(ribose) and phosphate groups of the nucleotides face the outside of the molecule (where water would be) and the relatively hydrophobic nitrogenous bases are on the inside of the molecule, hidden from water.

The purine(adenine & guanine) bases are opposite to the pyrimidine(cytosine & thymine) bases.

THE BONDS WITHIN THE DNA:

• The nucleotides within each strand are held together by the phosphodiester bonds between the 5' carbon of one nucleotide and the 3 ' carbon of the adjacent nucleotide. These strong covalent bonds hold the sugar/phosphate backbone together.

• The two strands of DNA are held together by weak hydrogen bonds between the nitrogenous bases.

• Hydrogen bonds are base specific. That is, A (adenine) can only form 2 hydrogen bonds with T (thymine) and C (cytosine) can only form 3 bonds with G (guanine).

ADVANTAGES OF THE STRUCTURE:

• If one knows the sequence of bases of one strand, one can deduce the sequence in the complementary one. This property explains how a cell can replicate a DNA molecule to produce two identical molecule.

• 'Chargraff's Rule' of DNA states that the amount of A in any DNA sample always equaled the amount of T and that the amount of G always equaled the amount of C. This structure proves it.