Presentation on theme: "Chapter 9 DNA: THE Genetic Material. Transformation Frederick Griffith, a bacteriologist, prepared a vaccine against pneumonia Vaccine – a substance that."— Presentation transcript:
Chapter 9 DNA: THE Genetic Material
Transformation Frederick Griffith, a bacteriologist, prepared a vaccine against pneumonia Vaccine – a substance that is prepared from killed or weakened disease-causing agents, including certain bacteria To protect the body against future infections by the disease-causing agent
Griffith’s Experiments Griffith worked with 2 strains of S. pneumoniae Virulent – (full of poison) able to cause disease 1 st strain had a smooth capsule that protected the bacterium from body’s defense systems (S)1 st strain had a smooth capsule that protected the bacterium from body’s defense systems (S) 2 nd strain lacked capsule and didn’t cause disease (R)2 nd strain lacked capsule and didn’t cause disease (R) Mice injected with (S) strain died; mice injected with (R) strain livedMice injected with (S) strain died; mice injected with (R) strain lived
Griffith’s Experiments Griffith injected mice with dead S bacteria – mice lived Griffith injected mice with heat-killed S bacteria-mice still lived Meaning the capsule was not involved with killing the mice He mixed harmless live R bacteria with the harmless heat-killed S bacteria-mice died
Griffith had discovered what is now called: Transformation- a change in genotype caused when cells take up foreign genetic material
Avery’s Experiments In 1944, a series of experiments proved that transformation is stopped by DNA destroying enzymes. Almost 100 years after Mendel’s experiments, Oswald Avery & co-workers demonstrated that DNA is the material responsible for transformation.
Viral Genes and DNA In 1952, Hershey and Chase used T2 bacteriophage to prove that viruses infect bacteria. A bacteriophage, also referred as a phage, is a virus that infects bacteria. When phages infect bacteria they reproduce more viruses and burst to release more viruses.
DNA’s Role Revealed Alfred Hershey and Martha Chase performed The following experiment: Step 1 – grew E. coli labeled with radioactive sulfur ( 35 S) protein coat. incorporated the sulfur Step 2 Labeled phages used to infect two separate batches of E. coli
Step 3 Used centrifuge to spin the tubes to separate the bacteria (heavy) from the viral parts (lighter). Concluded that the DNA of viruses is injected into the bacterial cells, while viral proteins coat was not. Injected DNA cause bacterial cells to produce more viruses. DNA is the heredity material in viruses.
The Structure of DNA – Section 2 Watson & Crick determined that a DNA molecule is a double helix – two strands twisted around each other Nucleotides – the subunits that make up DNA 3 parts: a phosphate group, a 5-carbon sugar, and a nitrogen- containing base
DNA Base Pairing Rule: Adenine (A) to Thymine (T) & Guanine (G) to Cytosine (C) Adenine (A) and guanine (G) are classified as bulky purines –two rings of carbon & nitrogen atoms Thymine (T) and cytosine (C) are classified as the smaller pyrimidines –single ring C & N atoms
Discovering DNA’s Structure Chargaff’s 1949 observations – the amount of adenine always equaled the amount of thymine; likewise the amount of guanine always equaled the amount of cytosine; however the amount varied between different organisms.
Wilkins & Franklin’s Photographs X-ray diffraction to study the structures of molecules 1952 Wilkins Franklin developed high-quality X-ray diffraction photographs of strands of DNA which suggested that the DNA resembled a tightly coiled helix and was composed of two or three chains of nucleotides.
Watson & Crick’s DNA Model 1953 Watson & Crick to make the “spiral staircase” configuration of DNA. The model takes into account the information from Chargaff, Wilkins, & Franklin along with their knowledge of chemical bonding.
Pairing Between Bases Base-pairing rules – cytosine pairs with guanine and adenine with thymine. These base pairing rules are supported by Chargaff's observations. The strictness of base- pairing results in 2 strands containing complementary base pairs.
The Replication of DNA – section 3 The complementary DNA structure serves as a template, or pattern, on which the other strand is built DNA replication – the process of making a copy of DNA, which occurs during the (S) phase of the cell cycle
Step 1 – The double helix needs to unwind before replication can begin Accomplished by enzymes called DNA helicases which open the double helix by breaking the hydrogen bonds between the two strands. The area where double helix separates is called replication fork.
Additional proteins prevent the strands from assuming their double- helical shape Replication forks – areas where the double helix separates Enzymes known as DNA polymerases add nucleotides to the exposed nitrogen bases, according to the base- pairing rules – forming two double helixes Step 2 –
The process continues until DNA has been copied & polymerases are signaled to detach Nucleotide sequences are identical in the two DNA molecules The enzyme helicase unzips the DNA DNA polymerase adds nucleotides to the exposed strands The end results are two identical strands, each strand with a parent strand and a daughter strand
Checking for errors – errors sometime occur with wrong nucleotide added DNA polymerases “proofreading role” – can backtrack and correct Proofreading reduces errors – DNA replication about one error per 1 billion nucleotides
Section 3 The Replication of DNA Replication doesn’t begin at one end of DNA and end at the other. The circular molecules found in prokaryotes have two replication forks. Replication forks move away from one another until meeting at the other side of DNA circle. A Eukaryotic cell contains one single long strand of DNA. Each human chromosome is replicated in 100 sections, each with its own starting point. Multiple replication forks working at same time. The entire chromosome replicates in 8 hours.