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?
- Discovery of DNA Function
- Early and Puzzling Clues
- In 1928, Fred Griffith was working with S
(pathogenic) and R (nonpathogenic) strains of streptococcus pnumoniae
- He performed four experiments summarized here:
- Inject mice with R cells; mice lived.
- Inject mice with S cells; mice died; blood samples contained many live S cells.
- S cells were heat-killed then injected into mice; mice lived.
- Live R cells plus heat-killed S cells were injected into mice; mice died; live S cells
were found in the blood.
- Some substance from the S cells had transformed the R cells.
- Both proteins and nucleic acids were candidates.
- In 1944, Oswald Avery showed that the substance was
- DNA Structure
- Components of DNA
- DNA is composed of four kinds of nucleotides, each of which
- a five-carbon sugardeoxyribose;
- a phosphate group;
- a nitrogenous baseadenine (A), guanine (G),
thymine (T), cytosine (C).
- Patterns of Base Pairing
- Watson and Crick built a model of DNA.
- DNA Replication and Repair
- How a DNA Molecule Gets Duplicated
- First, the two strands of DNA unwind and expose their bases.
- Then unattached nucleotides pair with exposed bases.
- Thus, replication results in DNA molecules that consist of one "old" strand
and one "new" strand; this is designated "semi-conservative
- Monitoring and Fixing the DNA
- Enzymes "proofread" the new bases
for mismatched pairs, which are replaced with correct bases.
- Regarding the Chromosomal Proteins
- The DNA of humans and other eukaryotes is highly organized to prevent tangling.
- Some histones (a type of protein) act as spools to
wind the DNA into units forming looped regions and "domains."