DNA Structure Standard 3.1.1 Explain the double-stranded, complimentary nature of DNA as related to its function in the cell.

What we already know about DNA •What class of macromolecule does DNA belong to? • What are the monomers of this macromolecule?

The three parts of a nucleotide: – 1. Phosphate group – 2. Sugar (deoxyribose) – 3. Nitrogen base

Discovery of DNA Frederick Griffith (1928) Tried to figure out how bacteria causes pneumonia Experiment: 1st: injected mice with disease- causing bacteria (all died) and again with harmless bacteria (no sickness) He heated the disease-causing bacteria to kill them and injected it into mice (mice lived) 2nd: mixed heat-killed bacteria with live, harmless bacteria and injected into mice (all died)

Discovery of DNA Frederick Griffith’s experiments showed that: hereditary material can pass from one bacterial cell to another. The transfer of genetic material from one cell to another cell or from one organism to another organism is called transformation. Griffith studied pneumonia bacteria • In 1928, he isolated two strains of bacteria, and injected them into mice – Live R strain was harmless (mice lived) – Live S strain caused pneumonia (mice died) – When he injected the S Strain that was heat-killed, the mice lived • BUT.... When he mixed the live R strain with the heat-killed S strain and injected into mice, the mice died. Griffith's Conclusions When the heat-killed bacteria mixed with the live harmless bacteria, something was exchanged between them, making the live harmless bacteria deadly Transformation = process in which one strain of bacteria changes the gene(s) of another bacteria

Discovery of DNA Oswald Avery (1944): showed that DNA is the hereditary material that transfers information between bacterial cells. Before this scientists thought proteins would code for our traits since they are more diverse. Avery (1944) Repeated Griffith’s work Experiment: Made an extract from heat-killed bacteria and treated it with enzymes (destroyed organic compounds) Transformation still occurred Repeated using enzymes to break down DNA Transformation did not occur Conclusion: DNA stores and transmits genetic info

Discovery of DNA Alfred Hershey and Martha Chase: Did experiments using radioactive viruses to infect bacteria (bacteriophages) Used radioactive markers to determine what actually entered a bacterial cell Conclusion: Discovered DNA was the genetic material of all living things

DNAs Structure Watson and Crick (1953): created a model of DNA by using Franklin’s and Wilkins’s DNA diffraction X-rays. Conclusion: strands in DNA are twisted around each other (helix)

Structure of DNA DNA is made of two nucleotide strands that wrap around each other in the shape of a double helix.

Nucleotide Structure A DNA nucleotide is made of a 5-carbon deoxyribose sugar, a phosphate group, and one of four nitrogenous bases: adenine (A), guanine (G), cytosine (C), or thymine (T).

Nucleotides A Adenine T Thymine G Guanine C Cytosine

Two Kinds of Bases in DNA Pyrimidines are single ring bases. Purines are double ring bases. N C O C C N C C N

Thymine and Cytosine are pyrimidines Thymine and cytosine each have one ring of carbon and nitrogen atoms. C N O cytosine C N O thymine

Adenine and Guanine are purines Adenine and guanine each have two rings of carbon and nitrogen atoms. C N O Guanine C N Adenine

Bonds of DNA Nucleotides along each DNA strand are linked by covalent bonds. Hydrogen bonding between the complementary base pairs, G-C and A-T, holds the two strands of a DNA molecule together.

C with G This is Chargaff’s Rule! Erwin Chargaff Worked with DNA nitrogen bases, discovered (1950): In any sample of DNA – # adenines (A) = # thymines (T) – # cytosines (C) = # guanines (G) Therefore, in DNA, the bases are always paired: A with T and C with G This is Chargaff’s Rule!

What do we know about DNA now? How are the two strands of DNA held together? What does DNA code for? Why do purines pair with pyrimadines?