DNA and the Birth of Molecular Genetics

DNA - the molecule of heredity

DNA is a Polymer - composed of nucleotides

Pentose sugar - phosphate group - one of 4 nitrogenous bases

Watson & Crick - Nature 1953

cracked the genetic code with two major clues

Chargaff's rule - in any DNA molecule, the [A] = [T] and [C] = [G]
X-ray diffraction patters - Rosalind Franklin did all the work

determined that the DNA is composed of a double helix
(much like a circular staircase)

DNA Replication

- is DNA replication conservative or semiconservative?

Conservative - old strand acts as a template

One daughter strand is the original template while the other strand is composed entirely out of new nucleotides

Semiconservative - old strand splits apart and acts as a template

Both daughter strands are composed of one of the old strands and one comprised out of new nucleotides

Isotopes were used to prove that DNA replication was semiconservative

How was this done?

DNA replication - the complete story

DNA replication

DNA unwinds

DNA polymerase pulls in new nucleotides and pairs them up with the old strand acting as a template

Nucleotides are paired up as complements - A always pairs with T and C always pairs with G. This is why Chargaff's rule exists: the base complementation

Numerous "DNA proofreaders" cruise up and down the helix looking for miss-matched pairs. When found, they are excised (removed) and the proper nucleotides are paired up.

Example: UV radiation causes adjacent TT nucleotides to form T=T dimers, leadint to, among other things, basal cell carcinoma

The Central Dogma

If you have any questions or comments, please feel free to e-mail me.

The image of DNA, DNA replication, and the Central Dogma were modified from graphics found at http://www.gene.com/ae/AB/GG/

All other images by Mike Muller