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DNA The Molecular Basis of Inheritance. Identifying the Genetic Material 1928 Fredrick Griffith (English Bacteriologist) Trying to find a vaccine for.

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Presentation on theme: "DNA The Molecular Basis of Inheritance. Identifying the Genetic Material 1928 Fredrick Griffith (English Bacteriologist) Trying to find a vaccine for."— Presentation transcript:

1 DNA The Molecular Basis of Inheritance

2 Identifying the Genetic Material 1928 Fredrick Griffith (English Bacteriologist) Trying to find a vaccine for pneumonia Vaccine: prepared from killed/weakened microorganisms introduced into the body to produce immunity Griffith worked with 2 strains of Streptococcus pneumoniae bacteria S strain  Polysaccharide Capsule  “Smooth” edged colonies  Virulent – able to cause disease R strain  No Capsule  “Rough” edged colonies  Nonvirulent - does not cause disease

3 Griffith’s Experiment Griffith’s Conclusion: Something had passed from heat killed bacteria to the nonvirulent R strain making them virulent… he called this the “transforming principal” Griffith did not know what it was, but many scientists thought it was proteins

4 Today we know… Transformation – cells take up foreign genetic material, changing their own genes (used for genetic engineering) Heat killed S bacteria – enzymes were denatured therefore the DNA could not be copied Proteins are denatured at 60 0 C and DNA is denatured at 90 0 C DNA of heat killed S bacteria survived and transformed DNA of R bacteria

5 The Search for what caused the Transformation… 1944 – Oswald Avery, MacLeod, & McCarty (American Bacteriologists) Experiment: 1. Added protease to “R and heat-killed S” mixture Result  Mice died 2. Added DNAase to “R and heat-killed S” mixture Result  Mice Lived Conclusion: DNA, not protein, is the transforming factor in Griffith’s experiment

6 More Evidence that DNA is the Genetic Material… 1952 – Alfred Hershey & Martha Chase (NY) Used T2 bacteriophages (phage) – virus that infects bacteria Composed of nucleic acid surrounded by a protein coat Viruses infect specific host Viruses are not living Not composed of cells Cannot reproduce on their own Do not grow and develop

7 Background Info on Viruses

8 Hershey & Chase Experiment Experiment: 1. Grew T2 w/radioactive Sulfur 35 S (protein coat takes in 35 S) 2. Grew another group of T2 w/ radioactive Phosphorus 32 P (DNA takes in the 32 P) S-labeled and 32 P–labeled phages were used to infect E.Coli bacteria 4. Separated phages from bacteria using a blender and a centrifuge… the bacterial cells at bottom and viral parts at the top Results: 35 S-labels still in viral parts 32 P-labels mostly in the bacterial cells, and new phages also contained 32 P DNA Conclusion: Viral DNA (not protein) enters bacteria and carries instructions on how to make more phages Without a doubt, DNA is the hereditary material!

9 Hershey & Chase Experiment

10 Structure of DNA By 1950’s most scientists were convinced that Chromosomes carry genetic material Genes are on chromosomes Genes are made of DNA Basic Structure of DNA Composed of nucleotides Nucleotides made of 3 parts deoxyribose, phosphate, N base 2 types nitrogen bases: Purines – double ring of C and N  Adenine  Guanine Pyrimidines – single ring of C and N  Cytosine  Thymine

11 Discovering DNA’s Structure Erwin Chargaff (NYC) 1947 – DNA composition varies among different species Chargaff’s Rules- Discovered regularity of ratios: # Adenines = # Thymines  (ie. Humans A =30%, T=30%) # Guanines = # Cytosines  (ie. Humans G = 20%, C = 20%) 1952 Rosalind Franklin & Maurice Wilkins (England) Developed X-ray crystallography photographs of DNA Suggested “helix” shape of 2-3 chains of nucleotides

12 April 25 th, 1953 James Watson & Francis Crick (England) Built the 1 st accurate 3D (tin and wire) model of DNA “Double Helix” – spiral staircase Purine is always linked by h- bond to a pyrimidine 2 strands of DNA are complimentary to each other 2 strands are anti-parallel 5’(phosphate end) 3’(deoxyribose end) 1962 Awarded the Nobel Prize

13 More on DNA Ex. If the sequence of bases on one strand is AATGCGCAT, than the complimentary strand will be: ________________ Human DNA has 3 billion base pairs.. Less than 1% of our DNA makes us different from one another!

14 Models of DNA Synthesis Semiconservative ea/ daughter molecule will have 1 new strand and 1old strand Conservative Parent molecule reforms Dispersive All 4 strands have a combination of old and new strands

15 1950’s Meselson & Stahl Cultured Ecoli on medium labeled w/ 15 N nt Transferred EColi to medium labeled w/ 14 N nt Centrifuge after each replication and analyze

16 Origin of Replication Prokaryotic Cell – single origin of replication where proteins separate the 2 strands and create a replication bubble, replication proceeds in both directions from the replication fork Eukaryotic Cells – hundreds or thousands of replication bubbles form to speed up the copying process, replication proceeds in both directions from the replication fork

17 DNA Replication Watson and Crick proposed that the complimentary strand of DNA serves as a template for which the other strand is built…experiments confirmed this 5 years later DNA Replication: Process of Synthesizing new molecules of DNA 1. Helicase breaks H-bonds and opens up the double helix forming replication forks (point at which DNA separates) 2. Topoisomerase relieves strain ahead of replication fork 3. Single-strand binding protein – binds to unpaired DNA strands until they serve as templates for new complimentary strand

18 Elongation DNA pol adds 50 nt/sec in Euk cells Each nt is a nucleoside triphosphate 1. At the replication fork, DNA Polymerase III continuously adds complimentary nucleotides to exposed bases on 3’ end of new strand, this is called the leading strand 2. DNA polymerase III must work away from the replication fork on the other strand, the lagging strand, to follow the 5’-3’ direction creating short segments of DNA called Okazaki fragments. DNA Ligase joins the Okazaki fragments together. 3. Process continues until all DNA has been copied, end result is 2 new molecules of DNA each identical to the original and composed of one new and one old strand

19 Priming DNA Synthesis DNA pol can not initiate – only add nt to 3’ end of existing chain Primer – short chain (5-10nt) of RNA Primase – enzyme starts RNA chain from scratch Leading strand – 1 primer needed Lagging strand – 1 primer needed for ea/Okazaki fragment DNA pol I replaces RNA nt of primers w/DNA versions

20 DNA Synthesis

21 Proofreading DNA polymerase only moves to the next nucleotide if the previous nucleotide was a correct match If mismatched, DNA Polymerase backs up, removes the mismatched nucleotide(s) and replaces it with the correct one(s). Only 1 error per 1 billion nucleotides! 6I&mode=related&search=

22 DNA Replication & Aging Every time DNA is copied, DNA polymerase cannot complete replication on the ends Eukaryotic DNA has a non- coding, repeating nucleotide sequence on the ends called telomeres that protects genes from being eroded over successive replications It is believed that telomeres are directly related to the aging process


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