DNA and RNA. Gene Composition. Gene Composition Introduction to DNA

Transcription

1 DNA and RNA 12.1 Introduction to DNA Gene Composition We know now that genes dictate characteristics of organisms. But what is it about the genes that produce this control? Not until the late 1920s did scientist begin to gain a grasp on DNA. Gene Composition To understand how genes work, we first have to know what it s made of. Three Major s were critical: Griffith s Transformation (1928) Avery s DNA isolation (1944) Hershey-Chase Radiation (1952) 1

2 The Griffith Transformation British Scientist Frederick Griffith was trying to identify how bacteria was making people sick. He worked with the Streptococcus pneumoniae bacterium which had two different strains: R Strain: Rough (did not cause disease) S Strain: Smooth (did cause disease) The Griffith Transformation The two strains were easily distinguishable visually. Griffith observed the following phenomena Mice injected with R strain bacteria lived Mice injected with S strain bacteria died Mice injected with heat killed S strain bacteria lived. The Griffith Transformation Heat killed S strain bacteria mixed with live R strain bacteria, once injected into the mice, killed them. 2

3 The Griffith Transformation Griffith hypothesized that the agent in bacteria that made us sick was a poison produced by the bacteria. Does this experiment prove or disprove Griffith s theory? Avery s DNA Isolation Canadian Biologist Oswald Avery repeated Griffith s experiment. However, they isolated lipids, proteins, carbohydrates and other molecules individually from the heat killed S strained bacteria. Individually, they added each to a suspension of R strain bacteria and infected mice. Avery s DNA Isolation The only factor to be able to transform the R strain bacteria into S strain bacteria is DNA. 3

4 Hershey-Chase Radiation American scientists Alfred Hershey and Martha Chase studied a type of virus that infect bacteria, a bacteriophage. The bacteriaphage is DNA or RNA enclosed in a Protein shell. Hershey-Chase Radiation Using radioactive isotopes they were able to determined what is was in the bacteriophage that transformed bacteria. Phosphorus 32 ( 32 P) Phosphorus is found in DNA, not in protein. Sulfur 35 ( 35 S) Sulfur is found only in protein, not in DNA. How can we use these radioactive isotopes? Hershey-Chase Radiation 4

5 Hershey-Chase Radiation When Sulfur 35 ( 35 S) was introduced to the buffer, it was found outside the cell. Protein is not the transforming Factor. When Phosphorus 32 ( 32 P) was introduced into the buffer, it was found inside the cell DNA is the transforming Factor. DNA s Components and Structure DNA is a long molecule made p of units called Nucleotides. These nucleotides consists of 3 components. A 5-carbon sugar called deoxyribose A phosphate group A Nitrogenous base Nucleotide Types There are 4 different nucleotides types that fall under two categories. Purines (adenine and guanine) have two nitrogenous ring bases Pyrimidines (cytosine and thymine) have only one nitrogenous ring base 5

6 Chargraff s Rules Early research found that the concentrations of: Adenine and Thymine are very similar Guanine and Cytosine are very similar Later, though x-ray evidence, we find that Adenine base pairs always pair with a Thymine, and a Guanine base pair always bind with a Cytosine. (Principle of Base Pairing) The Double Helix Watson and Crick s model of DNA was a double helix, in which two strands were wound around each other. Conformed to Chargraff s Rule Section 12.1 Assessment 1. List the conclusions Griffith, Avery, Hershey, and chase drew from their experiments. 2. Describe Watson and Crick s model of the DNA Molecule 3. What are the 4 kinds of bases found in DNA? 4. Did Watson and Crick s Model account for the equal amounts of Thymine and Adenine in DNA? Explain 6