The Molecular Basis of Inheritance AP Biology Ch. 16

Life’s Operating Instructions Nucleic Acids are unique in their ability Direct their own replication Hereditary info is encoded in the DNA DNA program directs biochemical, anatomical, physiological and some behavioral traits.

The Search for the Genetic Material T.H. Morgan demonstrated that genes are located on the chromosomes Chromosomes contain two chemical components Proteins and DNA Scientists debated over Proteins or DNA as the genetic material

DNA can Transform Bacteria Frederick Griffith Studying 2 strains of Streptococcus pneumoniae Heat killed pathogenic strain was a mixed with living nonpathogenic strain, injected into mouse caused pneumonia Some chemical changed the genotype and phenotype of the nonpathogenic bacteria

Avery 14 year study to id transformation agent Isolated purified molecules (Carbohydrates, Lipids, proteins, nucleic acids), from pathogenic bacteria Transformation occurred only with DNA.

Hershey and Chase Bacteriophages are viruses that infect bacteria Composed of DNA and protein Viruses are capable of reprogram cells to produce viruses Used Phosphorus-32 and Sulfur-32 (radioactive isotopes) to label proteins and DNA Infected bacteria and then traced the markers to identify which molecule was incorporated into the bacterial cells. Results showed proteins remained outside the cells, but the DNA entered the cells Concluded that DNA was the genetic material.

Edwin Chargaff DNA is a polymer of nucleotides Nucleotides contain nitrogenous base, phosphate group and a pentose sugar called deoxyribose Nitrogen base can be adenine(A), thymine(T), guanine(G) or cytosine(C) Chargaff analyzed the base composition of DNA from several organisms. He found that the percentage of A was equal to the percentage of T and % of G = % of C Chargaff’s Rules A=T and G=C

The Structural Model of DNA Challenge to determine the structure of the molecule. Its structure could account for its role in inheritance Scientists working on this challenged included Linus Pauling in California, Maurice Wilkins and Rosalind Franklin at Kings College in London The winners were James Watson and Francis Crick at Cambridge University

The Double Helix Wilkins shows x-ray diffraction image of molecule, made by Franklin, to Watson and Crick. Image revealed that molecule is helical in shape Two strands with the nitrogenous spacing between the them.

The structure Two strands, sugar and phosphate backbones Nitrogen bases stick out sideways Hydrogen bonds form between the purines and pyrimdines A forms 2 hydrogen bonds T and G forms 3 hydrogen bonds with C

DNA Replication Two strands are complementary, each strand serves as a template for a new strand. Semiconservative model of replication

Meselson and Stahl Labeled nucleotides with isotopes of nitrogen. Bacteria cultured in two different mediums Concluded that DNA replicates using semi-conservative replication.

Replication Begins at origin of replication at each end is a replication fork Elongation is catalyzed by enzymes called DNA Polymerases.

Antiparallel Elongation Two strands are oriented in opposite directions

DNA polymerases add nucleotides only to the free 3’ end of a growing DNA strand, never to the 5’ end. The DNA strand can only elongate in the 5’ 3’ direction. Leading strand is synthesized in this direction Lagging strand is synthesized in a series of segments. These segments are called Okazaki Fragments.

DNA ligase is an enzyme that joins the sugar-phosphate backbones of the Okazaki Fragments. DNA polymerase can not start synthesis of a polynucleotide it can only add to an existing chain. Primase is an enzyme that starts an RNA chain which will become the primer.