DNA: The Genetic Material Coach Fults Chapter 9

Transformation Mendel’s work answered why we resemble our parents But what are those genes made of?

Griffith’s Experiments In 1928, an experiment completely unrelated to the field of genetics led to the discovery of DNA Frederick Griffith, a bacteriologist, was trying to prepare a vaccine against pneumonia Streptococcus pneumoniae is a prokaryote that cause pneumonia Vaccine- weakened or killed disease causing agent injected into the body

Griffith’s Experiments Griffith worked with 2 strands of S. pneumoniae; the 1 st strand enclosed in a capsule composted of polysaccharides. The capsule protects the bacterium from the body’s defense systems Virulent- able to cause disease B/c of this capsule, this strain of S. pneumoniae (S) grows a smooth-edged colonies when grown in a Petri dish and caused disease The 2 nd strain ® lacked the capsule which made rough-edged colonies and didn’t cause disease

Griffith’s Experiments Mice infected with (S) would die, while ® mice weren’t harmed; he wanted to test to see if what caused the death was the capsule Griffith then heat-killed the (S) strain (that still had the capsule) and injected it into a mouse, the mouse lived He mixed the heat-killed (S) and the harmless ® strains; the mice died He looked at the blood of the mice and discovered that the ® strain had now a capsule Thus harmless ® became virulent (S)

Griffith’s Experiments Transformation- is a change in genotype caused when cells take up foreign material

Avery’s Experiments 1944, Oswald Avery showed that DNA is the material responsible for transformation DNA contains the instructions for the making of the capsule in the (S) strain of S. pneumoniae

Viral Genes and DNA: DNA’S Role Revealed In 1952, Alfred Hershey and Martha Chase, scientists at Cold Spring Harbor Laboratory, in New York, performed an experiment that settled the controversy It was known at the time that viruses, which are much simpler than cells, are composed of DNA and RNA surrounded by a protective coat Bacteriophage- is a virus that infects bacteria It was also known that when phages infect bacteria, more phages are produced

Viral Genes and DNA: DNA’S Role Revealed What was not known at the time was how phages reprogram the bacterial cell to make viruses They used a phage named T2; they new the only phosphorus was only contained in the DNA And sulfur was only in the protein coat So in 3 steps they revealed the role of DNA

Viral Genes and DNA: DNA’S Role Revealed: Step 1 They grew T2 with E. coli bacteria in a nutrient medium that contained radioactive sulfur ( 35 S) They grew a second batch of phages with E. coli in a nutrient medium that contained radioactive phosphorus ( 32 P). The radioactive phosphorus would become part of the phages’ DNA

Viral Genes and DNA: DNA’S Role Revealed: Step 2 The 35 S-labeled and 32 P-labeled phages were used to infect two separate batches of E. coli. B/c radioactive elements release particles that can be detected by machines, they can be followed, or traced, in a biological process Scientists could then determine whether it was the protein, DNA, or both that were being transferred into the bacterial cells to reprogram the bacteria

Viral Genes and DNA: DNA’S Role Revealed: Step 3 After a few minutes, they tore the 35 S-labeled phages off the surfaces of the bacteria with a blender The bacteria infected with the 32 P-labeled phage were likewise mixed in a blender; they then used a centrifuge to separate the bacteria and the phages They then examined found that the 35 S-labeled was still part of the phage, meaning the protein wasn’t injected into the bacterial cell They noticed that the 32 P-labeled was with the bacterial cells at the bottom

Viral Genes and DNA: DNA’S Role Revealed: Step 3 The DNA had been injected into the hosts. Moreover, the new generation of phages that was produced by these bacteria also contained radioactive DNA Concluded that DNA of viruses is injected into the bacterial cells, while most of the viral proteins remain outside. So the bacteria produce more more viral DNA Thus DNA is the hereditary material not proteins

DNA: Winding Staircase James Watson and Francis Crick determined that DNA molecule is a double helix- 2 strands twisted around each other Each strand is made of nucleotides Nucleotide- are the subunits that make up DNA; made up of 3 parts: a phosphate group, a 5-carbon sugar molecule, and a nitrogen-containing base The 5-carbon sugar in DNA is called deoxyribose While the sugar and the phosphate group are the same for each nucleotide in a molecule of DNA, the nitrogen base may differ

DNA: Winding Staircase 4 different Nitrogenous Bases: 1. Adenine (A) 2. Thymine (T) 3. Guanine (G) 4. Cytosine (C) (A) and (G) are classified as purines- nitrogen bases made of 2 rings of carbon and nitrogen atoms. (T) and ( C ) are pyrimidines- nitrogen bases made of a single ring of carbon and nitrogen atoms

DNA: Winding Staircase Backbone is made up of sugars and phosphates The nitrogenous bases face each other and are connected by a hydrogen bond

Discovering DNA’S Structure: Chargaff’s Observations In 1949, Edwin Chargaff showed that the amount of adenine always equaled the amount of thymine; and the same with cytosine and guanine The amount did vary between organisms

Discovering DNA’S Structure: Wilkins and Franklin’s Photographs Chargaff’s data became clear when scientists began using X-ray diffraction to study the structure of molecules In X-ray diffraction- X-rays are directed at an object and bounce off hitting a thin film giving a picture 1952, Maurice Wilkins and Rosalind Franklin developed high quality X-ray pics of DNA strands

Discovering DNA’S Structure: Wilkins and Franklin’s Photographs These photos suggested that DNA molecule resembled a tightly coiled helix and was composed of 2 or 3 chains of nucleotides

Discovering DNA’S Structure: Watson and Crick’s Model They used prior knowledge and their knowledge of chemical bonding They produced a 3-D tin and wire model

Paring Between Bases Watson and Crick determined than a purine of one strand would pair up with a pyrimidine of opposite strand (A) forms 2 hydrogen bonds with (T) (C ) forms 3 hydrogen bonds with (G) The hydrogen bonds keep the 2 strands together ATTCGTATCTC TAAGCATAGAG

Roles of Enzymes in DNA Replication Scientist thought the complimentary pairs were used to make copies of DNA for new cells Watson and Crick suggested that one strand was a template, on which the other strand is built DNA replication- making a copy of DNA DNA replication occurs during the S-phase of the cell cycle, before the cell divides

DNA Replication: Step 1 B4 the DNA can be replicated, it unwinds. This is accomplished by DNA Helicases. They open the double helix by breaking the hydrogen bonds that link the bases Once separated, additional proteins attached to each strand, holding them apart and preventing them from reattaching. The areas where the DNA is separated is called the replication fork b/c of their Y-shape

DNA Replication: Step 2 At the replication fork, enzymes known as DNA Polymerases move along each of the DNA strands and add nucleotide to the exposed nitrogenous bases As the DNA polymerases move along, 2 new double helixes are formed

DNA Replication: Step 3 Once DNA polymerases have begun adding nucleotides to a growing double helix, the process continues until all of the DNA has been copied and the polymerases are signaled to detach This process produces 2 DNA molecules, each composed of new and old strands The nucleotide sequences are identical to each other and to the original DNA molecule

Checking for Errors In replication, DNA polymerases have a ”proof- reading role” They can add nucleotide only if the previous nucleotide is matched correctly If a mismatch occurs, it can backtrack and removed the error and replace it with the correct base Proof-reading reduces errors in DNA replication to 1 in 1 billion nucleotides

Rate of Replication Prokaryotes- 2 forks at a single place in the loop; if there was 1 fork it would take 33 days for one replication of DNA Eukaryotes- many forks can take about 8 hours