chapter 12 DNA and RNA Biology Mr. Hines

Transcription

1 chapter 12 DNA and RNA Biology Mr. Hines

2 Transformation What is transformation? Process in which one strain of bacteria is changed by a gene or genes from another strain of bacteria.

3 12.1 DNA Remember from earlier that genes are segments of DNA located on chromosomes. DNA stands for Deoxyribonucleic acid. Mendel pretty much discovered genetics, but he did not know how it worked.

4 This chapter will explore methods for which genetics can be further understood. Questions? How are proteins made? How do chromosomes copy themselves? How does a cell make another cell? Many others???????

5 Griffith and Transformation In many cases in science, a discovery comes by accident or an experiment gives unexpected results. In 1928, a British scientist was trying to figure out how bacteria made people sick. More specifically, he wanted to know how certain bacteria caused the sickness known as pneumonia.

6 Griffith isolated 2 types (strains) of pneumonia bacteria in mice. He was able to grow the bacteria in the laboratory (in agar plates just like we did in class) One of the bacteria caused pneumonia while the other one did not When the bacteria grew in the agar plates, he noticed that the bacteria that caused pneumonia grew as smooth colonies, whereas the harmless colonies grew with rough edges.

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8 When Griffith injected mice with the disease causing strain of bacteria, the mice developed pneumonia and died. When mice were injected with the harmless strain, they didn t get sick at all.

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10 Was the disease causing strain producing poison? To find out, he killed a culture of the disease causing bacteria and with heat. He then injected mice with the dead bacteria. The mice did not develop pneumonia and lived.

11 He concluded that something else was killing the mice, not a poison created by the disease causing bacteria.

12 Transformation Griffith performed a second experiment where he mixed the heat killed disease causing bacteria with the live, harmless bacteria. He then injected the mixture into mice. The mice developed pneumonia and died! How can this be since neither caused disease by themselves.

13 When he examined the lungs of the dead mice and took samples of the bacteria, he grew it in agar plates. To his amazement, the bacteria that grew in the agar were smooth colonies. In other words, the bacteria that was growing in the mice (and dishes) was the disease causing bacteria even though it had been killed.

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15 Griffith reasoned that somehow the disease causing bacteria even though dead passes its disease causing ability to the harmless strain. Griffith called this transformation. Transformation is the process where one strain of bacteria passes its traits to another bacteria permanently.

16 He went on to hypothesize that the harmless strain of bacteria inherited the deadly trait. (transformation) He thought this might be a gene.

17 Avery and DNA About 15 years after Griffith, another scientist by the name of Avery repeated Griffith s work. They wanted to know which of the macromolecules was responsible for passing on the trait. (Carbs, proteins, lipids, nucleic acids)

18 Avery killed the disease causing bacteria with heat and then made a juice out of it (probably by adding water to it) He then added special enzymes to the juice that would destroy all proteins, carbs, and lipids (this leaves only nucleic acids) He then mixed the juice with the harmless bacteria, injected it into mice. The mice developed pneumonia and died. Transformation had occurred.

19 Avery ran the experiment again using an enzyme that destroys nucleic acid, but leaves the proteins, carbs, and lipids in tact. The mice did not develop pneumonia and lived. Transformation DID NOT occur.

20 This meant the molecule responsible for passing on the trait must be a nucleic acid. This was a very important discovery nucleic acid was responsible for storing and transmitting genetic information from one generation to the next.

21 The Hershey-Chase Experiment Later, in 1952, two American scientists Alfred Hershey and Martha Chase made the most important discovery of all. They studied how viruses infect living organisms.

22 Bacteriophages Bacteriophages are viruses that infect bacteria. Yes, bacteria can get sick too from viruses. Viruses are 1000 times smaller than bacteria and are not considered to be living things. Bacteriophages (and most viruses) consist of DNA or RNA coated with a membrane of proteins.

23 Bacteriophages attach to the outer membrane of bacteria and inject its genetic information (DNA) The viral genetic information inside of the bacteria will cause the bacteria to produce more viruses. This bacteria is killed while hundreds or thousands of new viruses are created. The dead bacteria will split open and release the viruses (your throat with a cold virus)

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26 Radioactive markers Bacteriophages are composed of only 2 macromolecules - nucleic acids and proteins. Hershey and Chase wanted to know which of the 2 was injected into bacteria. In other words, they were trying to confirm other experiments performed by earlier scientists.

27 When the viruses injected genetic material into the bacteria, was it protein or nucleic acid? To prove this, they grew viruses in agar plates containing a radioactive isotope called phosphorus-32 and sulfur-35. In other words, as the bacteriophages grew, they absorbed the radioactive phosphorus and sulfur. This was clever since proteins contain almost no phosphorus and nucleic acids contain no sulfur.

28 The isotopes could be used as markers. If the bacteria, after injected by the virus, contained sulfur 35, then the genetic material is protein.

29 If the bacteria, after injected by the virus, contained phosphorus 32, then the genetic material was nucleic acid.

30 Hershey and Chase concluded that the genetic material of the bateriophage was nucleic acid (DNA), not protein.

31 The components and structure of DNA Scientists, when they make discoveries, are never satisfied new knowledge leads to new questions.

32 Scientists now wanted to know how DNA does these 3 things 1. Carry information from one generation to another. 2. Determine the inheritable traits for organisms 3. Make copies of itself

33 DNA structure DNA is a very long macromolecule made of subunits called nucleotides. Each nucleotide is made of 3 basic components. 1. Deoxyribose (5 carbon sugar) 2. Phosphate group 3. Nitrogen base

34 There are 4 kinds of Nitrogen bases in DNA 1. Adenine 2. Guanine 3. Cytosine 4. thymine

35 These 4 nitrogen bases can be placed into 2 groups Purines Adenine and Guanine contain 2 carbon rings Pyrimidines Cytosine and Thymine contain only 1 carbon ring.

36 The nitrogen bases can be arranged in any order and are held together by the deoxyribose and phosphate groups.

37 Section 12-1 Figure 12 5 DNA Nucleotides Purines Pyrimidines Adenine Guanine Cytosine Thymine Phosphate group Deoxyribose

38 Figure 12 7 Structure of DNA Section 12-1 Nucleotide Hydrogen bonds Sugar-phosphate backbone Key Adenine (A) Thymine (T) Cytosine (C) Guanine (G)

39 So, all the information needed to build all living things comes down to 4 molecules imagine if we only had 4 letters in our alphabet.

40 Chargaff s Rule Chargaff noticed that all organisms contained the same amount of 1. Thymines and Adenine 2. Guanine and cytosine He was unable to explain why

41 Franklin and x ray evidence In the 1950s, Franklin photographed DNA and discovered that it was double stranded and coiled up like a spring. This was later named the Double Helix

42 The Double helix Later, scientists Watson and Crick, pieced the 3 dimensional model of DNA together using cardboard and wire. Once the model was complete, they realized how DNA could carry genetic information and how it could be copied.

43 Figure 12 7 Structure of DNA Section 12-1 Nucleotide Hydrogen bonds Sugar-phosphate backbone Key Adenine (A) Thymine (T) Cytosine (C) Guanine (G)

44 The double helix DNA molecule looks similar to a twisted ladder/spiral staircase. The 2 strands are held together by hydrogen bonds between nitrogen bases. After further studies, it was discovered that DNA had a pattern known as base pairing. Base pairing states that each nitrogen base has a corresponding partner.

45 Base pairing states that each nitrogen base has a corresponding partner. Guanine = Cytosine Adenine = Thymine

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