DNA

Searching for Genetic Material n Mendel: modes of heredity in pea plants (1850’s) n Morgan: genes located on chromosomes (early 1900’s) n Griffith: bacterial work (1920’s) –transformation- change in genotype and phenotype due to assimilation of external substance (DNA) by a cell n Avery: transformation agent was DNA (1944)

Hershey-Chase Experiment n 1952 Experiment with bacteriophages –viruses that infect bacteria n Tested if DNA or protein was the hereditary material –in T2 (a bacteriophage that infects E. coli) n Sulfur (S) is in protein, phosphorus (P) is in DNA n Only P was found in host cell

DNA Structure n Chargaff –ratio of nucleotide bases (A=T; C=G) n Structure of DNA researched by Pauling and Wilkins/Franklin n Watson & Crick (1953) –Proposed the structure of DNA after viewing an X-ray diffraction photo by Rosalind Franklin

The Double Helix n Nucleotides: nitrogenous base (thymine, adenine, cytosine, guanine); sugar (deoxyribose); phosphate group

Base Pairing Rules n Purines: A & G n Pyrimidines: C & T (Chargaff rules) n A’s H+ bonds (2x) with T and C’s H+ bonds (3x) with G n Van der Waals attractions between the stacked pairs

DNA Replication n Watson & Crick strands are complementary; nucleotides line up on template according to base pair rules n Meselson & Stahl replication is semiconservative; Expt: varying densities of radioactive nitrogen

3 Replication Models n Meselson and Stahl concluded that DNA follows the semiconservative model

DNA Replication in Action n Origin of replication (“bubbles”)= beginning of replication n Replication fork: Y-shaped region where new strands of DNA are elongating n Helicase: catalyzes the untwisting of the DNA at the replication fork n DNA polymerase: catalyzes the elongation of new DNA

Antiparallel Nature n Sugar/phosphate backbone runs in opposite directions (Crick) n One strand runs 5’ to 3’, while the other runs 3’ to 5’ n DNA polymerase only adds nucleotides at the free 3’ end, forming new DNA strands in the 5’ to 3’ direction only

Leading and Lagging n Leading strand: –synthesis toward the replication fork (only in a 5’ to 3’ direction from the 3’ to 5’ master strand) n Lagging strand: –synthesis away from the replication fork in small pieces (Okazaki fragments) –joined by DNA ligase

The Lagging Strand n Primase enzyme attaches a primer –a short sequence of RNA (sometimes DNA) n Small segments replicated 5’  3’ n After strand replicates, primer falls off

Helpful Proteins

Proteins in Action

Proofreading n Mismatch repair: DNA polymerase n Excision repair: nuclease

Shortening Ends n Because lagging strands need 3’ end, replication cannot extend to the very end of a DNA strand n Chromosomes contain telomeres –Repeating sequences of DNA –Don’t contain genes n Telomerase lengthens telomeres so that they don’t continually get shorter –Chromosomes can divide without losing genes