Chapter 12 DNA and RNA 12-1: DNA

Structure of DNA nucleotides: monomers of nucleotides made up of a 5-carbon sugar (deoxyribose), a phosphate group, and a nitrogen-containing base 4 different bases two purines (double-ringed): adenine and guanine two pyrimidines (single-ringed): cytosine and thymine

Purines Pyrimidines Phosphate group Deoxyribose Adenine Guanine Cytosine Thymine Phosphate group Deoxyribose

What do you notice? Source of DNA A T G C Pneumonia virus 29.8 31.6 20.5 18.0 Yeast 31.3 32.9 18.7 17.1 Herring 27.8 27.5 22.2 22.6 Human 30.9 29.4 19.9 19.8 What do you notice?

Chargaff’s Rules X-ray Evidence The amount of thymine always equaled adenine ( [A] = [T] ) and the amount of guanine always equaled cytosine ( [C] = [G] ). X-ray Evidence Rosalind Franklin (1950, Britain): used x-ray diffraction to make an x-ray pattern of DNA which suggested there were two strands.

The Double Helix Watson / Crick (1953): Made a 3-dimensional model to show the structure of DNA (with Franklin’s X-ray photos). Hydrogen bonds form between the two strands. Base pairing can ONLY occur between adenine (A) and thymine (T), and between guanine (G) and cytosine (C). A-T G-C

Double Helix The phosphates and sugars are on the outside The bases are on the inside

Nitrogen bases sugar phosphate

Sugar-phosphate backbone Nucleotide Hydrogen bonds Sugar-phosphate backbone Key Adenine (A) Thymine (T) Cytosine (C) Guanine (G)