Evidence from the Hershey–Chase experiment
In 1952, Dr. Hershey and his assistant, Martha Chase, performed one of the most famous experiments in modern biology, which confirmed the concept that DNA is the material basis for heredity. The structure of the DNA was discovered one year later by Watson & Crick.
The T2 phage virus as a basis for research.
For a long time it was not clear whether protein or DNA is the genetic material. It was known that chromosomes are composed of DNA and proteins, but the fact that proteins are made of 20 different amino acids and could have more complexity compared to the 4 bases of DNA, made them the more likely candidate.
Evidence from the Hershey–Chase experiment
In their experiment, Hershey and Chase took advantage of radioactive isotopes of Sulfur (35S) and phosphorus (32P), knowing that DNA and proteins could be distinguished that way. Isotopes are atoms of elements with the same number of protons, but different number of neutrons. While isotopes have the same chemical properties, the stability of them might be different. Unstable isotopes release energy in the form of radiation.
How could the radioactive isotopes 35S and 32P help to identify genetic material?
Proteins: Composed of amino acids – some of which DNA: Contains contain sulfur. None of them contains phosphorus. phosphorus, but no sulfur
Evidence from the Hershey–Chase experiment
How did Hershey and Chase`s experiment provide evidence for DNA being the only genetic material?
Purine-to-pyrimidine bonding ensures DNA stability In the space below, draw a detailed (2D) diagram of the DNA double helix
Purine-to-pyrimidine bonding ensures DNA stability
The nitrogenous bases in DNA are composed of either two rings (Adenine and Guanine) and are then called purines. If they are composed of only one ring (Cytosine, Thymine and Uracil), they are referred to as pyrimidines.
Purine-to-pyrimidine bonding ensures DNA stability
DNA is composed of an equal number of purines (A + G) and pyrimidines (C + T) and X-ray diffraction studies showed that the DNA helix was tightly packed. This tight packing is only possible if a pyrimidine is paired with a purine...
A=T
GC
... and if the bases are upside down in relation to one another.
http://www.uic.edu/classes/bios/bios100/lectures/04_06a_DNA_structure-L.jpg
Only purine – pyrimidine pairs fit inside the double helix without leaving a gap or bulge. This is responsible for complementary base pairing between A – T and G – C and allows tight packing of the DNA helix.
Purine-to-pyrimidine bonding ensures DNA stability
Purine-to-pyrimidine bonding ensures DNA stability
Hydrogen acceptors
Hydrogen donors
Tight bonding between complementary base pairs is enhanced through electrochemical attraction and the formation of hydrogen bonds
http://image.slides harecdn.com/ppa6lecturech16-100309091651-phpapp02/95/ppa6-lecture-ch-16-42-728.jpg?cb=1268126300
The number of partial positive and negative charges between pairs of complementary bases determine the number of hydrogen bonds between them.
Chargaff’s data on the relative amounts of pyrimidine and purine bases across diverse life forms
For a long time scientists thought that DNA contains a repeating sequence of four bases – with an equal number of nucleotides, which would allow little room for variation. This hypothesis was referred to as the tetranucleotide hypothesis.
The Austrian biochemist Erwin Chargaff analysed DNA samples from different species to find their nucleotide composition and to disprove the tetranucleotide hypothesis.