Griffith’s Experiments 1928: Frederick Griffith – bacteriologist Goal: To prepare a vaccine for pneumonia Vaccine: injectable substance created with a weakened disease to help produce memory cells to protect against future infections

Griffith’s Experiments Cont. Worked with two strains of S. pneumoniae Disease-causing strain (S) contains a capsule composed of polysaccharides allows it to be virulent Virulent: ability to cause disease Second strain (R) does not contain the capsule – which in turn doesn’t cause disease Mice injected with S = death Mice injected with R = no harm

Griffith’s Experiments Cont. What specifically causes the death of the mice? Capsule? Griffith treated the S bacteria with heat (no reproduction) and injected the vaccine No – the mice still lived What happens if we mix heat-killed S (harmless) and R strain (harmless)? R bacteria changed to a virulent bacteria – why? Transformation: A change in genotype caused by cells taking up foreign genetic material No known cause of transformation

Avery’s Experiments 1944: Oswald Avery – Rockefeller Institute What substance is responsible for the process of transformation? Discovered DNA is the material responsible for transformation DNA contains the genetic instructions for producing the capsule

Genetic Material & Replication Chapter 9 DNA Genetic Material & Replication

DNA’s Role Revealed The scientific community was skeptical of DNA and its purpose Alfred Hershey & Martha Chase (1952) Proved that DNA indeed is the source of transformation using phages Bacteriophage: virus that infects bacteria Used bacteriophage T2 and E. coli to perform their experiment

DNA’s Role Revealed Cont. The Experiment: Step 1: Cultures Grew one batch of T2 and E. coli in a nutrient medium containing radioactive sulfur (35S) Another batch in radioactive phosphorous (32P) Step 2: Infection Each labeled batch of phages were used to infect separate batches of E. coli Step 3: Blending After some time for infection they used a blender to pull off the phages Ask Why use radioactive sulfur? Tagging Draw the picture for visual Step 2: radioactive tags can track the type of material – protein or DNA

DNA’s Role Revealed Cont. Examination: In 35S phage – most of the phage (upper layer) was still labeled This means that protein was not injected into the bacteria In 32P phage – most of the 32P was found in the inner bacterial layers The DNA was affected and will be passed on to the offspring Ask Why use radioactive sulfur? Tagging Draw the picture for visual Step 2: radioactive tags can track the type of material – protein or DNA

Double Helix James Watson & Francis Crick (1950s) DNA was modeled as a double helix Meaning it was two strand of DNA twisted with nucleotides linking them together Nucleotides: subunits containing a phosphate group, a five carbon sugar & a nitrogen-containing base Deoxyribose: The five carbon sugar that is contained within DNA (the D in DNA)

Double Helix Cont. Purines Pyrimidines Purines: two rings of carbon and nitrogen (shorter names) Pyrimidines: one ring (longer names)

Chargaff’s Observations Erwin Chargaff – Biochemist (1949) Data from different organism DNA analysis concluded that: Adenine = the amount of Thymine Guanine = the amount of Cytosine However the amount of each pair varied from organism to organism

Base Pairing Rules and Chargaff’s observations allow for only two complementary pairs: Adenine and Thymine Guanine and Cytosine Complementary base pair concluded that the sequence of one strand determines the sequence of its complementary strand Let the student work one out

Roles of Enzymes DNA Replication: Step 1: Unwinding Process of copying DNA using enzymes Step 1: Unwinding DNA Helicase: an enzyme that unwinds the DNA strand by breaking hydrogen bonds between the pairs Other enzymes are laid down to prevents the strands from reattaching Replication Forks: The areas were the strands are split forming a Y Draw on the board

Roles of Enzymes Step 2: Nucleotide Addition Step 3: Finishing DNA polymerase: An enzyme that adds complementary nucleotides to each exposed strand follow the base pairing rules Two new double helices are formed Step 3: Finishing Polymerases are signaled to end with a special nucleotide sequence The new DNA strand is identical to the template

Checking for Errors DNA polymerases have a “proofreading” ability The ability allows it to only continue if the previous pair is correctly paired If a mismatch is found – the polymerase can go back and fix it Proofreading reduces the odds of an error to one error in every one billion nucleotides

Rate of Replication Prokaryote – Circle DNA Eukaryote – Helix Two replication forks to increase rate Eukaryote – Helix If there were only one replication fork it would take 33 days to copy Each section has a replication fork allowing a replication to take 100 seconds

Video https://www.youtube.com/watch?v=itsb2SqR-R0