dna double helix structure

what is chargaff’s rule 1?

% of adenine = % of thymine

% of guanine = % of cytosine

in ALL organisms due to complementary base paring in double helix (A+T, G+C)

what is chargaff’s rule 2?

concentration of pyrimidines = concentration of purines

direct consequence of rule 1

what is the watson and crick model?

idea that DNA consists of two associated polynucleotide strands that wind together

phosphate-sugar backbone, bases project into interior

strands are antiparallel

strands are held together by hydrogen bonds between bases

how many hydrogen bonds are there between A and T bases

2 H bonds

how many hydrogen bonds are there between G and C bases

3 H bonds

what happens when there are thousands of hydrogen bonds

stability

is the A+T bond or G+C bond weaker?

A+T bond is weaker because there are only two hydrogen bonds

start of a transcription or replication site is usually rich in A’s and T’s because it’s easier to separate strands (e.g. TATA box)