Chapter 12 Section 1: DNA

Objective Describe the experiments and research that lead to the discovery of DNA as the genetic material and the structure of DNA. Describe the experiments and research that lead to the discovery of DNA as the genetic material and the structure of DNA. Describe the components of a DNA molecule. Describe the components of a DNA molecule.

Discovery of molecular nature of the gene  Frederick Griffith (1928) Discovery of molecular nature of the gene  Frederick Griffith (1928) Working w/2 types of bacteria isolated from ice to see how they produced disease Working w/2 types of bacteria isolated from ice to see how they produced disease Strain 1: Grew in smooth colonies when plated Strain 1: Grew in smooth colonies when plated Strain 1: Grew in rough colonies when plated Strain 1: Grew in rough colonies when plated

Griffith’s Experiment 1. Injected mice w/disease-causing bacteria (smooth colonies) RESULT: mice died

2. Injected mice w/harmless bacteria (rough colonies) RESULT: mice lived

3. Heat-killed the disease-causing bacteria and mixed it w/the harmless bacteria RESULT: mice died

Transformation Heat-killed bacteria passed their disease- causing ability to the harmless strain. (one strain of bacteria had been changed permanently into another.) Heat-killed bacteria passed their disease- causing ability to the harmless strain. (one strain of bacteria had been changed permanently into another.) “Transformed” bacteria then passed the ability to cause disease onto their offspring. “Transformed” bacteria then passed the ability to cause disease onto their offspring. Hypothesis: disease-causing ability is transferred by GENES. Hypothesis: disease-causing ability is transferred by GENES.

Avery and DNA Scientists did not know which macromolecule was responsible for transferring genetic info. Scientists did not know which macromolecule was responsible for transferring genetic info. Avery’s experiment: Destroyed lipids, proteins, carbs and RNA and repeated Griffith’s experiment  Transformation still occurred. Avery’s experiment: Destroyed lipids, proteins, carbs and RNA and repeated Griffith’s experiment  Transformation still occurred.

When DNA was destroyed, no genes were transferred. When DNA was destroyed, no genes were transferred. KEY: DNA stores and transmits the genetic information from one generation to the next. KEY: DNA stores and transmits the genetic information from one generation to the next.

The Hershey-Chase Experiment Studied viruses Studied viruses Bacteriophage: type of virus that infects bacteria. Bacteriophage: type of virus that infects bacteria. Skeptical about whether or not genetic info was transferred by DNA or PROTEIN. Skeptical about whether or not genetic info was transferred by DNA or PROTEIN.

The Components and Structure of DNA DNA: composed of units called “nucleotides.” DNA: composed of units called “nucleotides.” Nucleotides have 3 parts Nucleotides have 3 parts 5-carbon sugar (deoxyribose) 5-carbon sugar (deoxyribose) Phosphate group Phosphate group Nitrogen base (four types) Nitrogen base (four types) Adenine (3-ringed structure) Adenine (3-ringed structure) Guanine (3-ringed structure) Guanine (3-ringed structure) Cytosine (2-ringed structure) Cytosine (2-ringed structure) Thymine (2-ringed structure) Thymine (2-ringed structure)

DNA is a double helix (twisted ladder) DNA is a double helix (twisted ladder) Sugar- phosphate backbone “Rungs” of the ladder = nitrogen bases

Chargaff’s Rules Erwin Chargaff discovered there was an equal % of guanines and cytosines AND adenines and thymines in almost every sample of DNA that he tested (from bacteria – humans ) Erwin Chargaff discovered there was an equal % of guanines and cytosines AND adenines and thymines in almost every sample of DNA that he tested (from bacteria – humans )

X-Ray Evidence Rosalind Franklin’s x-ray diffraction photos provided info about the structure of DNA. Rosalind Franklin’s x-ray diffraction photos provided info about the structure of DNA.

The Double Helix (1953) James Watson and Francis Crick used all of the research from other scientists to develop a model of DNA that accurately depicted its structure and how it could be copied. James Watson and Francis Crick used all of the research from other scientists to develop a model of DNA that accurately depicted its structure and how it could be copied. Double Helix: DNA looks like a twisted ladder, 2 strands are held together by hydrogen bonds linking A-T and G-C. (BASE PAIRING) Double Helix: DNA looks like a twisted ladder, 2 strands are held together by hydrogen bonds linking A-T and G-C. (BASE PAIRING)