DNA

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

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

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

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

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

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

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

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

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

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

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

Helpful Proteins

Proteins in Action

Proofreading Mismatch repair: DNA polymerase Excision repair: nuclease

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