DNA has been copying itself over and over for over 3 billion years.  It started as a simple system and eventually evolved all life forms to make sure it gets copied with great efficiency.  Have we let it down?

1. Discovery of DNA Function
1. Early and Puzzling Clues
0. In 1928, Fred Griffith was working with S (pathogenic) and R (nonpathogenic) strains of streptococcus pnumoniae
1. He performed four experiments summarized here:
1. Inject mice with R cells; mice lived.



2. Inject mice with S cells; mice died; blood samples contained many live S cells.



3. S cells were heat-killed then injected into mice; mice lived.



4. Live R cells plus heat-killed S cells were injected into mice; mice died; live S cells were found in the blood.



2. Some substance from the S cells had transformed the R cells.
1. Both proteins and nucleic acids were candidates.
2. In 1944, Oswald Avery showed that the substance was DNA.
2. DNA Structure
1. Components of DNA
0. DNA is composed of four kinds of nucleotides, each of which consists of:
1. a five-carbon sugar–deoxyribose;
2. a phosphate group;
3. a nitrogenous base–adenine (A), guanine (G), thymine (T), cytosine (C).
2. Patterns of Base Pairing
0. Watson and Crick built a model of DNA.



3. DNA Replication and Repair
1. How a DNA Molecule Gets Duplicated
0. First, the two strands of DNA unwind and expose their bases.
1. Then unattached nucleotides pair with exposed bases.
2. Thus, replication results in DNA molecules that consist of one "old" strand and one "new" strand; this is designated "semi-conservative replication."



2. Monitoring and Fixing the DNA
0. Enzymes "proofread" the new bases for mismatched pairs, which are replaced with correct bases.
3. Regarding the Chromosomal Proteins
0. The DNA of humans and other eukaryotes is highly organized to prevent tangling.
1. Some histones (a type of protein) act as spools to wind the DNA into units forming looped regions and "domains."