INTRODUCTION In 1865 Mendel showed that genes transmitted genetic information and in the first part of the 20 th century their patterns of transmission were studied extensively Although these classical genetic studies provided little insight into the molecular nature of genes they did demon ID: 1040763
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1. GENETIC MATERIAL (DNA/RNA)
2. INTRODUCTIONIn 1865, Mendel showed that genes transmitted genetic information and in the first part of the 20th century, their patterns of transmission were studied extensively.Although these classical genetic studies provided little insight into the molecular nature of genes, they did demonstrate that the genetic material must perform three essential functions:
3. 1.The genotypic function, REPLICATION. The genetic material must store genetic information and accurately transmit that information from parents to offspring, generation after generation.
4. 2.The phenotypic function, GENE EXPRESSION. The genetic material must control the development of the phenotype of the organism i.e. the genetic material must dictate the growth of the organism from single-celled zygote to the mature adult.
5. 3.The evolutionary function, MUTATION. The genetic material must undergo changes to produce variations that allow organisms to adapt to modifications in the environment so that evolution can occur.
6. MOLECULES IN CHROMOSOMESChromosomes are composed of two types of large organic molecules (macromolecules):PROTEINSNUCLEIC ACIDS
7. TYPES OF NUCLEIC ACIDSThe nucleic acids are of two types:Deoxyribonucleic acid (DNA)Ribonucleic acid (RNA)
8. Nucleic acids were discovered in 1868 by Friedrich Miescher, who called the material 'nuclein' since it was found in the nucleus. It was later discovered that prokaryotic cells, which do not have a nucleus, also contain nucleic acids.
9. Storage of Genetic InformationEvidences for DNAEvidences for RNAWe’ll study few experiments in these regards…
10. DISCOVERY OF NUCLEINIn 1868, Friedrich Miescher, a young Swiss medical student, became fascinated with an acidic substance that he isolated from the pus cells obtained from bandages used to dress human wounds.
11. He first separated the pus cells from the bandages and associated debris and then treated the cells with pepsin (a proteolytic enzyme isolated from the stomach of pigs).
12. After the pepsin treatment, he rediscovered an acidic substance which he named as “nuclein”.Meischer’s nuclein was unusual in that it contained large amounts of both nitrogen and phosphorous, which at that time known to occur in only few fats.
13. Meischer wrote a paper describing his discovery of nuclein in human puss cells and submitted it for publication in the year 1869.But it was published in 1871 because the editor of the journal…………………………
14. At that time, the importance of the substance (nuclein) could not have been anticipated.The existence of polynucleotide chains, the key component of the acidic material in nuclein was not documented until 1940s.The role of nucleic acids as genetic material was not published till 1944.
15. DNA Genetic Material Griffith Effect…The fact that DNA is genetic material came from the experiments using bacteria and viruses.The first series of experiments were performed by a British bacteriologist F. Griffith in 1928, using the bacterium Diplococcus pneumoniae which causes pneumonia in animals.Griffith noticed that this bacterium had two types of strains.S-type, which was capsulated and produced a smooth colony on a synthetic medium.R-type, which was non-capsulated and produced rough colony on a synthetic medium.When S-type of bacteria was injected into healthy mice, the mice developed pneumonia and died. So S-type was named as virulent or pathogenic.However, R-type of bacteria was non-pathogenic.
16. If heat killed S-type of bacteria were injected into healthy mice, they did not cause disease and the mice remained healthy.When heat killed S-type of bacteria were mixed with R-type living bacteria and the mixture injected into healthy mice, the mice developed pneumonia and died.When bacteria were isolated from the dead mice, they were of living S-type and R-type.Griffith announced that it was because of a phenomenon other than mutation, which he called transformation.
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18. ConclusionOn the basis of experiment, Griffith concluded that there were transformation of Avirulent R type to Virulent S type by picking up the genetic material encoding the lipopolysaccharide capsules from the heat killed S type.This bacterial transformation clearly shows the role of DNA as genetic material was further confirmed by another experiment.
19. 1944: Avery, Macleod, McCartyThe Transforming Factor...Oswald Avery, Colin MacLeod, and Maclyn McCarty.These scientists did almost exactly what Griffith did in his experiments but with the following changes.First, after heat-killing the S strain of bacteria, the mixture was separated into six test tubes.Thus, each of the test tubes would contain the unknown "transforming agent.”A different enzyme was then added to each tube except one – the control – which received nothing.To the other five tubes, one of the following enzymes was added: RNase, an enzyme that destroys RNA; protease, an enzyme that destroys protein; DNase, an enzyme that destroys DNA; lipase, an enzyme that destroys lipids; or a combination of enzymes that break down carbohydrates.
20. The theory behind this experiment was that if the "transforming agent" was, for example, protein – the transforming agent would be destroyed in the test tube containing protease, but not the others.Thus, whatever the transforming agents was, the liquid in one of the tubes would no longer be able to transform the S. pneumonia strains.When they did this, the result was both dramatic and clear.The liquid from the tubes that received RNase, protease, lipase, and the carbohydrate-digesting enzymes was still able to transform the R strain of pneumonia into the S strain.However, the liquid that was treated with DNase completely lost the ability to transform the bacteria.
21. Thus, it was apparent that the "transforming agent" in the liquid was DNA.To further demonstrate this, the scientists took liquid extracted from heat-killed S. pneumoniae (S strain) and subjected it to extensive preparation and purification, isolating only the pure DNA from the mixture.This pure DNA was also able to transform the R strain into the S strain and generate pathogenic S. pneumoniae.These results provided powerful evidence that DNA, and not protein, was actually the genetic material inside of living cells.
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23. ConclusionAvery et al. (1944) revealed the chemical nature of the transforming substance to be DNA. The experiment showed that DNA isolated from S type could confer the pathogenic properties to R type.Two conclusions were made:Active factor is DNA that cause transformationS type contains the active factor
24. Hershey and Chase - DNA vs. protein‘DNA is the genetic material’ is demonstrated by Alfred Hershey and Martha Chase (1952).This experiment known as the blender experiment because a kitchen warring blender was used as a major piece of apparatus.They demonstrated that the DNA injected by a phage particle into a bacterium contains all the information required to synthesize progeny phage particles.A single particle of phage T2 consists of DNA encased in a protein shell .The DNA is the only phosphorus containing substance in the phage particle; the proteins of the shell, which contain the amino acids methionine and cysteine, have the only sulphur atoms.
25. In these experiments, phage DNA was made radioactive by growing infected bacteria on a medium containing radioactive phosphate(32P).Since phage proteins do not contain phosphorus, only DNA would be labeled. Similarly, phage proteins were labeled with the help of 35S.Since DNA does not contain sulphur, only protein would be labeled with 35S. Such differential labeling would enable one to distinguish between DNA and proteins of the phage without performing any chemical tests.
26. Hershey and Chase then allowed both kinds of labeled phage particles to infect E. coli bacteria .The infected bacteria were immediately agitated in a warring blender.After shaking, only radioactive32P was found associated with bacterial cells and 35S was found only in surrounding medium and not in bacterial cells.When phage progeny was studied for radioactivity in this experiment, it was found that the phage progeny carried labeled only with 32P. The progeny was not labeled with 35S.This clearly indicates that only DNA and not protein is injected into bacterial cells.The empty protein coat (ghost) left outside.Thus, Hershey and Chase experiment proved that DNA entering the host cell carries all the genetic information or synthesis of new phage particles, hence, is the sole genetic material in DNA bacteriophages.
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28. ConclusionAs coat of Bacteriophage were not labeled with 32P and only with 35S, the results of experiment clearly indicate that only DNA and not proteins enter bacterial cell. Protein coat is left outside. The DNA entering the host carries all the genetic information for synthesis of new phage particle. This certainly proves that DNA is the genetic material in bacteriophage and not proteins.