12.2 DNA Structure
Long before the bacteriophage studies were under way, biochemists knew that DNA contains only four types of nucleotides that are the building blocks of nucleic acids. Each consists of a 5 carbon sugar (in DNA it is deoxyribose), a phosphate group and one of the nitrogen bases (A, T, G, and C). All of their parts are joint in the same way. T and C are pyrimidines (single ring structure) and A and G are purines (which are larger, bulkier molecules, double ring structure). Rosalind Franklin had obtained especially good x- ray diffraction images of DNA fibers. By this process, a beam of x-rays is directed at a molecule. The molecule scatters the beam in patterns that can be captured on film. The pattern itself consists only of dots and streaks, in itself; it doesn’t reveal the molecular structure. However, the photographic images of those patterns can be used to calculate the positions of the molecule’s atoms. This molecular constancy and variation among species is the foundation of the unity and diversity of life
Main Points:
The pattern of base pairing between the two strands in DNA in constant for all species (A-T and C- G). However, the DNA molecules of each species show unique differences in the sequence of base pairs along their length.

12.4 DNA Replication and Repair Discovery of DNA structure was a turning point in studies of inheritance. No one could explain DNA replication or how the DNA molecule is duplicated before the cell divides. Watson and Crick assembled a model of it then; Crick understood how it might be done. Enzymes can easily break the hydrogen bonds between the two nucleotide strands of a DNA molecule. These enzymes and other proteins act on the molecule one strand can unwind from the other, thereby exposing stretch of nucleotide bases. Cells have stockpiles of free nucleotides, and these can pair with the exposed bases. Each parent strand remains intact and a companion strand is assembled on each one according to this base pairing rule: A-T and C-G. As soon as a stretch of a new, partner strand forms on a stretch of the parent strand, they