DNA the basics Double helix structure of DNA

DNA: the basics

Double helix structure of DNA “It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material. ” Watson & Crick

Directionality of DNA n You need to number the carbons! ¨ it matters! This will be IMPORTANT!! nucleotide PO 4 N base 5 CH 2 4 O 1 ribose 3 OH 2

The DNA backbone n Putting the DNA backbone together ¨ refer to the 3 and 5 ends of the DNA n the last trailing carbon Sounds trivial, but… this will be IMPORTANT!! 5 PO 4 5 CH 2 4 base O 1 C 3 O –O P O O 5 CH 2 2 base O 4 1 2 3 OH 3

Anti-parallel strands n Nucleotides in DNA backbone are bonded from phosphate to sugar between 3 & 5 carbons 5 3 3 5 ¨ DNA molecule has “direction” ¨ complementary strand runs in opposite direction

Bonding in DNA 5 hydrogen bonds 3 covalent phosphodiester bonds 3 5 …. strong or weak bonds? How do the bonds fit the mechanism for copying DNA?

Base pairing in DNA n Purines ¨ adenine (A) ¨ guanine (G) n Pyrimidines ¨ thymine (T) ¨ cytosine (C) n Pairing ¨A n ¨C n : T 2 bonds : G 3 bonds

Copying DNA n Replication of DNA ¨ base pairing allows each strand to serve as a template for a new strand ¨ new strand is 1/2 parent template & 1/2 new DNA