1. Erwin Chargaff (1951): • Rule of Base pairing

2. Rosalind Franklin & Maurice Wilkins (1953):• X-ray diffraction pattern of DNA

3. James Watson & Francis Crick (1953):• Molecular structure of DNA

Beyond the syllabus:The discovery of the double helix

Chargaff’s Results: Similar amount of A-T and C-G

•Franklin’s Results: 2 periodicities for DNA 3.4 Å and 34 Å.

The discovery of the double helix: Watson & Crick Model

DNA is composed of 2 chains of nucleotides that form a double helix shape.

The two strands are antiparallel.

Every complete turn of the helix measured 3.4 nm

The backbone of the DNA molecule is composed of alternating phosphate groups

and sugars.

The complimentary nitrogenous bases form hydrogen bonds between the strands.

A is complimentary to T and G is complimentary to C.

The discovery of the double helix: Watson & Crick Model

1. Unwound and separate the DNA helix:

Helicase

2. New dNTPS are joined to the template by

hydrogen bonds.

3. Complementary Base Pairing ensures

identical copies of DNA. The parent

strands act as a template for the new

(complementary strands)

4. Adenine pairs only with Thymine (A-T)

5. Cytosine pairs only with Guanine (C-G)

6. DNA polymerase enzyme links the

phosphate of the new nucleotide to the

sugar of the nucleotide before it by

covalent bond.

3.4.1 DNA replication

3.4.2 DNA replication: Complementary Base Pairing

Complementary Base Pairing ensures the new DNA molecule is identical to the original

– no mistakes are made – so the base-sequence of nucleotides is conserved

3.4.3 State that DNA replication is…

Meselson & Stahl Experiment:

DNA Replication is semiconservative