Biologist first had to discover the chemical nature of the genes.

In 1928, Frederick Griffith was trying to figure out how bacteria makes people sick. He wanted to learn more on pneumonia.

Transformation One strain of bacteria had apparently been changed permanently into another.

Avery and DNA What molecule in the heat- killed bacteria was most important for transformation. Transformation requires one particular molecule? Will it be the gene??

Enzymes that destroyed proteins, lipids, carbohydrates, and other molecules including RNA + Transformation still occurred

Enzymes that destroyed DNA + No transformation

Nucleic Acid DNA stores and transmits the genetic information from one generation of an organisms to the next.

Hershey - Chase They collaborated in studding viruses, nonliving particles smaller than cells that can infect living organisms.

Bacteriophages One kind of virus that infects bacteria. “Bacteria eater” They are composed of DNA or RNA and protein coat

Radioactive Markers They could determine which part of the virus (protein coat or the DNA core) entered the infected cell. Learn whether genes were made of protein or DNA.

Hershey and Chase concluded that the genetic material of the bacteriophage was DNA, not protein

The components and Structures of DNA Genes had to carry information from one generation to the next. They had to put information to work by determining the heritable characteristics of organisms Genes had to be easily copied because all of a cell’s genetic information is replicated every time a cell divides.

DNA is a long molecule made up of units called nucletides. Each one is made up of three basic components: - 5- carbon sugar – desoxyribose - a phosphate group - a nitrogenous base

4 kinds of nitrogenous bases They differ on the number of rings 2 for purines 1 for pyramidines

The backbone of a DNA chain is formed by sugar and phosphate groups of each nucleotide. The nitrogenous bases stick out sideways from the chain. The nucleotides can be joined together in any order.

Chargaff’s Rules There is a curios relationship between nucleotides. He discovered there is an equal percentage of: [A] = [T] [G]=[C]

X- Ray Evidence In 1950 Rosalind Franklin used a technique called X- Ray diffraction to get information about the structure of the DNA molecule. She aimed a powerful X-ray beam at concentrated DNA samples and record them on film.

It does not reveal the structure of DNA, but it carry some very important clues. The X- shaped pattern shows that the strands in DNA are twisted around each other like coils of a spring.

The angle X suggests that there are two strands in the structure. The nitrogenous bases are near the center of the molecule.

The Double Helix Francis Crick and James Watson were trying to understand the structure of DNA by building a model of the molecule.

They twisted and stretch the models in various ways but they couldn’t explain the DNA properties.

Watson was shown a copy of Franklin’s X- ray pattern and he instantly get the structural model.

Watson and Crick’s model of DNA was a double helix, in which two strands were wound around each other.

Hydrogen bonds could form between certain nitrogenous bases and provide enough force to hold together Base pairing – hydrogen bonds can form only between certain bases. And that makes sense on Chargaff’s rule.