Molecular Basis of Inheritance DNA Structure DNA Replication

Evidence that DNA is the Hereditary Material of Life? Griffith-Avery Experiment—DNA can transform bacteria Hershey-Chase Experiment—Viral DNA can program cells Chargaff—Analysis of DNA composition

Griffith-Avery Experiment: Transformation of Bacteria 2 strains of Streptococcus pneumonia --S strain—pathogenic (disease-causing) --R strain—non-pathogenic -Griffith found that if he mixed heat-killed S-strain bacteria with the harmless strain, some of the living cells were converted to the pathogenic form (TRANSFORMATION—assimilation of external DNA) -Avery purified various chemicals from the heat-killed pathogenic bacteria, only DNA worked to transform the cells -Discovery met with criticism—most still believe protein is the hereditary material Controls

Hershey-Chase Experiment: DNA, the Hereditary Material in Viruses Bacteriophages—viruses that infect bacteria --attach to host cell and inject their DNA into the cell --experiment demonstrated that it was DNA, not protein, that functioned as the phage’s genetic material Radioactively-labeled (sulfur) viral proteins remained outside the host cell during infection Radioactively-labeled (phosphorus) viral DNA traveled inside the host cell -the injected DNA molecules cause the cells to to produce new viral DNA and proteins

Chargaff:The Composition of DNA Base composition of DNA varies between species Regularity in ratios of nucleotide bases Chargaff’s Rules: A = T and G = C Adenine = 30.9% Thymine = 29.4% Guanine = 19.9% Cytosine = 19.8%

Scientists in the Race for the Double Helix Linus Pauling James Watson Francis Crick Rosalind Franklin

Structure of DNA Purines Pyrimidines Adenine Thymine Guanine Cytosine

Structure of DNA DNA is a double helix with a uniform width Purine and pyrimidine bases stacked

Purine + Pyrimidine: width consistent with X-ray data, base ratios consistent with Chargaff’s rules: A = T and G  C

Structure of DNA is related to 2 primary functions: 1. Copy itself exactly for new cells that are created 2. Store and use information to direct cell activities

Complementary (Anti-Parallel) Strands of DNA If one strand is known, the other strand can be determined 3’ = T 5’ A C G T  G  C  C = A = T = A  G  G 3’ 5’

DNA Replication: Semi-Conservative Model 2 strands of the parental DNA separate, and each functions as a template for the synthesis of a new complementary strand

DNA Replication This replication process assures that daughter cells will carry the same genetic information as each other and as the parent cell. Each daughter DNA has one old strand of DNA and one new strand of DNA

DNA Replication Replication occurs simultaneously at many sites (replication bubbles) on a double helix Allows DNA replication to occur in a shorter period of time

Elongating a New Strand DNA polymerases can only attach nucleotides to the 3’ end of a growing daughter strand Thus, replication always proceeds in the 5’ to 3’ direction

Priming DNA Synthesis Primase adds RNA primer to strand DNA polymerase adds DNA nucleotides to strand Another DNA polymerase replaces RNA with DNA

DNA Replication Within the replication bubbles, one daughter strand is made continuously (leading strand) while the other daughter strand must be made in short pieces (lagging strand) which are then joined together by DNA ligase These short pieces of DNA are called Okazaki fragments Overall Direction of Replication-5’ to 3’

DNA Replication

Checking for Errors 1/1,000,000,000 chance of an error in DNA replication Can lead to mutations DNA polymerases have a “proofreading” role Can only add nucleotide to a growing strand if the previous nucleotide is correctly paired to its complementary base If mistake happens, DNA polymerase backtracks, removes the incorrect nucleotide, and replaces it with the correct base

Mismatch Repair DNA polymerase proofreads each nucleotide and catches mistakes The polymerase removes mistake and replaces with correct nucleotide

Excision Repair Nuclease Used by skin cells when repairing genetic damage caused by UV rays of sunlight

End-Replication Problems

End-Replication Problems Telomerase—enzyme lengthens telomeres (ends of nucleotide sequences)