The Molecular Basis of Inheritance 1900 - 1940 “ The Golden Age of Genetics”

Searching for Genetic Material Mendel: modes of heredity in pea plants Morgan: genes located on chromosomes Meischer: isolated DNA Came from the nuclei he called it nuclein, later changed to nucleic acid Feulgen: discovered fuchsin dye stained DNA P.A. Levene: analyzed DNA and found out it had: cytosine, thymine, adenine, and guanine a deoxyribose sugar a phosphate group

Frederick Griffith 2 forms (Streptococcus pneumoniae): S strain (pneumonia-causing) R strain (didn’t cause pneumonia) Induced a nonpathogenic strain to become pathogenic. Transforming factor that caused the change. Transformation: change in genotype and phenotype due to assimilation of external substance (DNA) by a cell.

Delbruck and Luria Worked with bacteriophages, (virus that attacks bacteria) Bacteriophages consist of protein coats covering DNA. They infect a cell by injecting DNA into the host cell. Turning the host into a viral “factory”. Host cell bursts, releasing hundreds of new bacteriophage. Phages have DNA and protein, making them ideal to resolve the nature of the hereditary material.

Hershey and Chase Tried to determine if protein or DNA was the hereditary material. Labeled DNA and protein with different isotopes. DNA contains Phosphorous (P) but no Sulfur (S), tagged the DNA with radioactive Phosphorous-32. Protein lacks (P) but does have (S), tagged it with radioactive Sulfur-35. Radioactive S remained outside the cell, and radioactive P was found inside the cell, showing DNA is the physical carrier of heredity. http://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter14/animations.html#

DNA Structure Chargaff ratio of nucleotide bases (A=T; C=G) Wilkins and Franklin took X-ray diffraction photomicrographs of crystalline DNA extract. Watson & Crick came up with the Double Helix model nucleotides: nitrogenous base (thymine, adenine, cytosine, guanine) deoxyribose sugar phosphate group

The final product DNA is a double helix Bases to the center (like rungs on a ladder) Sugar-phosphate backbone (like the sides of a twisted ladder) Strands are complementary, large purine pair with small pyrimidine Purines: ‘A’ & ‘G’ Pyrimidines: ‘C’ & ‘T’ A  T form 2 H+ bonds G  C form 3 H+ bonds http://207.207.4.198/pub/flash/24/24.html

THE CHEMICAL NATURE OF NUCLEIC ACIDS: Phosphate attaches to 5' carbon Base attaches to 1' carbon Hydroxyl (-OH) attaches to 3' carbon Nucleotides strung together in chains Phosphodiester bond connects between Phosphate at 5’ C & hydroxyl at 3’ C Definite direction Sequences conventionally written in 5’ to 3’ direction http://www.johnkyrk.com/DNAanatomy.html

Antiparallel nature Sugar/phosphate backbone runs in opposite directions One strand runs 5’ to 3’, while the other runs 3’ to 5’

DNA Replication Watson & Crick strands are complementary Nucleotides line up on template according to base pair rules (Watson) Semiconservative, each new double helix contains one strand from the parent molecule and one newly synthesized strand. (Meselson, Stahl) Meselson & Stahl replication is semiconservative; Expt: varying densities of radioactive nitrogen http://www.sumanasinc.com/webcontent/animations/content/meselson.html

DNA Replication: a closer look 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 Initiation: Primase: inserts short RNA sequence (Primer) at point of initiation begins the replication process DNA polymerase: catalyzes the elongation of new DNA DNA polymerase only adds nucleotides at the free 3’ end, forming new DNA strands in the 5’ to 3’ direction only Ligase: joins together Okazaki fragments http://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter14/animations.html#

Leading vs. lagging strand 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 Happens multiple times and creates small fragments of DNA called Okazaki fragments. Fragments joined by DNA ligase http://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter14/animations.html#

Visual Representation of DNA Replication http://www.metacafe.com/watch/726018/easily_see_your_dna_at_home/

IMPORTANT CONCEPTS ENZYMES INVOLVED IN DNA REPLICATION WORK SIMULTANEOUSLY TO GET THE JOB DONE. http://www.johnkyrk.com/DNAreplication.html

Quick Review DNA Replication Synthesis of Leading Strand Priming (Primase) Elongation (DNA Polymerase) Replacement of RNA Primer by DNA (DNA Polymerase) Double helix unwinds, providing single-stranded DNA template. (Helicases and Single –Stranded Binding Proteins) Synthesis of Lagging Strand Priming for Okazaki fragments (Primase) Elongation of fragment (DNA Polymerase) Replacement of RNA Primer by DNA (DNA Polymerase) Joining of fragments (Ligase)

DNA Repair Mismatch repair: Excision repair: Telomere ends: DNA polymerase corrects with proper compliment Excision repair: Damaged base removed and replaced with correct base by Nuclease Telomere ends: Found at the ends of Eukaryotic chromosomes Little bit lost each time a cell divides if telomere runs out cell ceases to divide Telomerase replaces what’s lost