THE STRUCTURE OF DNA Section 4.2 Page 210

DNA Deoxyribonucleic acid A polymer of many nucleotide subunits

Nucleotides Three main components: Deoxyribose sugar Phosphate group (negatively charged) Nitrogenous base The identity of a nucleotide is determined by its nitrogenous base

Purines Adenine (A) Guanine (G) Differences in the sequence of nucleotides are the source of variation between individuals and organisms. Pyrimidines Thymine (T) Cytosine (C)

Nucleotide structure Ribose – five carbons Carbon naming: Clockwise Start at C to the right of O 5’ 4’ 1’ 3’ 2’

DNA COMPOSITION: Chargaff Experiment Erwin Chargaff, 1949 Chemical analysis: Proportion of adenine in DNA is equal to thymine Proportion of guanine is equal to cytosine # of purines = # of pyrimidines

DNA STRUCTURE Rosalind Franklin and Maurice Wilkins X-ray diffraction analysis (imaging technique) James Watson and Francis Crick Used X-ray diffraction pattern obtained by Franklin Combined with information from Chargaff experiment Developed the double-helical model of DNA

...injustice? Nobel Prize in Physiology or Medicine, 1962: Watson, Crick and Wilkins   ...but what about Rosalind Franklin? Not many women in science at that time Hostile working relationship with Maurice Wilkins Franklin died in 1958 of cancer Nobel prizes are not award posthumously Would she have also been awarded the Nobel prize?

The Double Helix Two antiparallel strands of nucleotides Parallel, but running in opposite directions 3`end (OH group) 5`end (phosphate)

The two strands wind around each other  helical structure Clockwise direction One complete turn of the helix : 10 nucleotides 3.4 nm Helix is 2 nm in diameter 3.4 nm 0.34 nm 2 mm

Sugar-phosphate backbone Nitrogenous bases facing in Bases are paired with one another: complementary base pairing

Complementary base pairing hydrogen bonds purine + pyrimidine A-T (2 H-bonds) G-C (3 H-bonds) this pairing is crucial for the storage and transfer of genetic information

Convention: Write only the 5`to 3`strand... why? Can easily deduce the sequence of the complementary strand Homework: Pg. 216 Q. 1-3, 5-7