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NIS - BIOLOGY

NIS - BIOLOGY. Lecture 50 – Lecture 51 DNA: The Genetic Material Ozgur Unal. Discovery of the Genetic Material. Identify the differences in the phenotypes of the individuals below. Discovery of the Genetic Material.

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NIS - BIOLOGY

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  1. NIS - BIOLOGY Lecture 50 – Lecture 51 DNA: The Genetic Material OzgurUnal

  2. Discovery of the Genetic Material • Identify the differences in the phenotypes of the individuals below.

  3. Discovery of the Genetic Material • After Mendel’s work was rediscovered in the 1900s, scientists began to search for the molecule involved in inheritance. • They knew that the genetic information was carried on the chromosomes in eukaryotic cells  two main components of chromosomes are DNA and protein. • Several experiments were performed to identify the source of genetic information: • Griffith • Avery • Hershey and Chase

  4. Discovery of the Genetic Material • Griffith: Fredrick Griffith in 1928 • The first major experiment that led to the discovery of DNA • Two strains of bacteria Streptococus pneumoniae • Griffith found that one strain could be transformed into the other form. • One strain, called S (smooth) strain had a sugar coat. • The other strain, called R (rough) strain did not have a sugar coat. • S strain kills the mouse – causes disease • R strain does not kill the mouse – does not • cause disease

  5. Discovery of the Genetic Material • Griffith: • Check out Figure 12.2 for the experiment. • The experiment showed that R cells were transformed into S cells  What is the transforming substance?

  6. Discovery of the Genetic Material • Avery: Oswald Avery in 1944 • Avery and his colleagues isolated different macromolecules, such as DNA, protein and lipids from the killed S cells. • Then, the exposed the R cells to these macromolecules. • Only the R cells exposed to DNA were transformed into S strain  transforming substance is DNA • But their results were not widely accepted.

  7. Discovery of the Genetic Material • Hershey and Chase: Alfred Hershey & Martha Chase in 1952 • Definitive evidence that DNA is the transforming factor. • The experiment involved a bacteriophage  a type of virus that attacks bacteria • Bacteriophage in the experiment was made of DNA and protein. • Viruses cannot replicate themselves  they inject themselves into a living cell to reproduce • Hershey and Chase labeled both parts of virus (DNA and protein) by radioactive elements  they could • keep track of them

  8. Discovery of the Genetic Material • Hershey and Chase: • Check out Figure 12.3 for the experiment. • Radioactive sulfur labels protein • Radioactive phosphorus labels • DNA

  9. DNA Structure • Nucleotides: • Nucleic acids are complex macromolecules • that store and transmit genetic information  made of smaller subunits called nucleotides. • 5-carbon sugar, a phosphate group and a nitrogenous base • There are 2 nucleic acids in cells: DNA • and RNA • DNA contains: • sugar deoxyribose • a phosphate • one of four nitrogenous bases: Adenine, • Guanine, Cytosine or Thymine

  10. DNA Structure • Nucleotides: • RNA contains: • sugar ribose • a phosphate • One of four nitrogenous bases: Adenine, Guanine, Cytosine or Urasil • Guanine (G) and Adenine (A) are • double ringed bases  Purine base • Thymine (T), Cytosine (C) and • Uracil (U) are single ringed • bases  Pyrimidine base

  11. DNA Structure • Chargaff: Erwin Chargaff in 1950s • Erwin Chargaff analyzed the amount of A, G, T and C in the DNA of various species. • Check out Figure 12.5!! • He found that G = C and T = A

  12. DNA Structure • X-ray Diffraction: • Maurice Wilkins was working at King’s College on X-ray diffraction • Rosalind Franklin, also at King’s College, took “Photo 51” using X-ray diffraction. • This photo indicated that DNA was a double helix. • The specific structure of the DNA double helix was determined later by James Watson and Francis • Crick.

  13. DNA Structure • Watson and Crick: • Watson and Crick (at Cambridge Unv.) used Franklin’s picture and Chargaff’s data. • They built a model of the double helix: • 1- Two outside strands consist • of alternating deoxyribose and • phosphate • 2- C and G bases pair to each • other by 3 hydrogen bonds • 3- T and A bases pair to each • other by 2 hydrogen bonds

  14. DNA Structure • DNA is often compared to a twisted ladder. • The pairs of bases (C-G and A-T) form the rungs of the ladder. • A purine always binds to a pyrimidine • base. • C + T = A + G

  15. DNA Structure • Another unique feature of the DNA molecule is the direction of the two strands. • C atoms can be numbered in organic molecules. • Check out Figure 12.8!! • On the top rail, the orientation of the sugar has the 5’ C on the left and 3’ C on the right • The strand on the bottom runs • in opposite direction  3’ to 5’. • This orientation of the two • strands is called antiparallel.

  16. Chromosome Structure • In eukaryotes DNA is organized into chromosomes. • In order to fit into the nucleus of a eukaryotic cell, the DNA tightly coils around a group of beadlike proteins called histones. • Histones and DNA form nucleosome. • The nucleosomes then group together into chromatin fibers, which supercoil to make • up the DNA structure recognized • as a chromosome. • http://www.johnkyrk.com/DNAanatomy.html • http://bio-alive.com/animations/DNA.htm

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