the discovery that dna is the genetic code involved many

The discovery that DNA is the genetic code involved many experiments.

Section 1: DNA: The Genetic Material

KWhat I Know

WWhat I Want to Find Out

LWhat I Learned

Essential Questions• Which experiments led to the discovery of DNA as the genetic material?• What is the basic structure of DNA?• What is the basic structure of eukaryotic chromosomes?

DNA: The Genetic MaterialCopyright © McGraw-Hill Education

Review• nucleic acid

New• double helix• nucleosome


Vocabulary


Discovery of the Genetic Material• After the rediscovery of Mendel’s work, scientist began to look for the

molecule involved in inheritance.

• For many years, scientists struggled to determine if DNA or protein was the source of genetic information.


Discovery of the Genetic MaterialGriffith• First major experiment searching for the genetic material

• Involved transformation between two forms of S. pneumoniae

• Set the stage for the search to identify the transforming substance


Discovery of the Genetic MaterialAvery• Identified the molecule that transformed the R strain of S. pneumoniae into

the S strain

• Concluded that when the S cells were killed, DNA was released

• R bacteria incorporated this DNA into their cells and changed into S cells


Discovery of the Genetic Material Hershey and Chase• Used radioactive labeling to trace bacteriophage

DNA and protein.

• Concluded that the bacteriophage DNA was injected into the cell and provided the genetic information needed to produce new viruses.

Summary of Hershey-Chase ResultsInteractive Table

FPOAdd link to interactive table from page 328 (table 1) here.



DNA StructureNucleotides• Nucleotides are the subunits of nucleic acids, and consist of

• 5-carbon sugar

• Phosphate group

• Nitrogenous base


DNA StructureChargaff• Analyzed the amount of A, G, T,

and C in the DNA of various species

• Chargaff’s rule: C = G and T = A


DNA StructureThe structure question• Search for the structure of DNA was lead by four scientists:

• Rosalind Franklin, British chemist

• Maurice Wilkins, British physicist

• Francis Crick, British physicist

• James Watson, American biologist


DNA StructureX-ray diffraction• X-ray diffraction techniques indicated that DNA was a double helix, or

twisted ladder shape.

• The double helix shape was formed by two strands of nucleotides twisted around each other.


DNA StructureWatson and Crick• Using Franklin and Chargaff’s data, Watson and Crick measured the

width of the helix and the spacing of the bases.

• They built a model that conformed the Franklin and Chargaff’s data.


DNA StructureDNA structure• DNA often is compared to a twisted ladder.

• Rails of the ladder are represented by the alternating deoxyribose and phosphate.

• The pairs of bases (cytosine-guanine or thymine-adenine) form the steps.

• Purine base always binds to a pyrimidine base


DNA StructureOrientation• DNA molecules have specific orientations of the two strands

• On the top rail, the strand is said to be oriented 5’ to 3’.• The strand on the bottom runs in the opposite direction and is

oriented 3’ to 5’.

Structure of DNA Animation

FPOAdd link to animation from page 331(Figure 8) here.



Chromosome Structure• In prokaryotes, DNA molecules are contained in the cytoplasm, and

consist mainly of a ring of DNA and associated proteins.

• In eukaryotes, DNA is organized into individual chromosomes.


Chromosome Structure• To fit into a cell, DNA coils around a group of beadlike proteins called

histones.

• DNA + histones form a nucleosome, which group together into chromatin fibers, which supercoil to form a chromosome.


ReviewEssential Questions• Which experiments led to the discovery of DNA as the genetic material?• What is the basic structure of DNA?• What is the basic structure of eukaryotic chromosomes?

Vocabulary• double helix• nucleosome

the discovery that dna is the genetic code involved many

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