1 / 17

DNA

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

marlow
Download Presentation

DNA

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. DNA

  2. 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)

  3. 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

  4. 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

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

  6. 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

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

  8. 3 Replication Models • Meselson and Stahl concluded that DNA follows the semiconservative model

  9. 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

  10. 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

  11. 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

  12. 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

  13. Helpful Proteins

  14. Proteins in Action

  15. Proofreading • Mismatch repair: DNA polymerase • Excision repair: nuclease

  16. 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

More Related