NIS - BIOLOGY

Lecture 50 – Lecture 51DNA: The Genetic Material

Ozgur Unal

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Discovery of the Genetic Material

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Identify the differences in the phenotypes of the individuals below.

Discovery of the Genetic Material

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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:

GriffithAveryHershey and Chase

Discovery of the Genetic Material

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Griffith: Fredrick Griffith in 1928The first major experiment that led to the discovery of DNATwo strains of bacteria Streptococus pneumoniaeGriffith 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 diseaseR strain does not kill the mouse – does not cause disease

Discovery of the Genetic Material

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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?

Discovery of the Genetic Material

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Avery: Oswald Avery in 1944Avery 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 DNABut their results were not widely accepted.

Discovery of the Genetic Material

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

Discovery of the Genetic Material

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Hershey and Chase:Check out Figure 12.3 for the experiment.Radioactive sulfur labels proteinRadioactive phosphorus labels DNA

DNA Structure

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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 baseThere are 2 nucleic acids in cells: DNA and RNADNA contains:

sugar deoxyribose a phosphate one of four nitrogenous bases: Adenine,

Guanine, Cytosine or Thymine

DNA Structure

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Nucleotides:RNA contains:

sugar ribosea phosphateOne of four nitrogenous bases: Adenine, Guanine,

Cytosine or UrasilGuanine (G) and Adenine (A) are double ringed bases Purine baseThymine (T), Cytosine (C) and Uracil (U) are single ringed bases Pyrimidine base

DNA Structure

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Chargaff: Erwin Chargaff in 1950sErwin 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

DNA Structure

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X-ray Diffraction:Maurice Wilkins was working at King’s College on X-ray diffractionRosalind 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.

DNA Structure

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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 phosphate2- C and G bases pair to each other by 3 hydrogen bonds3- T and A bases pair to each other by 2 hydrogen bonds

DNA Structure

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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

DNA Structure

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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.

Chromosome Structure

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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.htmlhttp://bio-alive.com/animations/DNA.htm