Thomas Hunt Morgan

September 25, 1866 – December 4, 1945

The importance of Morgan's earlier work with Drosophila (fruit flies) was that it demonstrated that the associations known as coupling and repulsion demonstrated using the Sweet Pea, are in reality the obverse and reverse of the same phenomenon, which was later called linkage. Morgan's first papers dealt with the demonstration of sex linkage of the gene for white eyes in the fly, the male fly being heterogametic. His work also showed that very large progenies of Drosophila could be bred. The flies were, in fact, bred by the million, and all the material thus obtained was carefully analysed. His work also demonstrated the important fact that spontaneous mutations frequently appeared in the cultures of the flies. On the basis of the analysis of the large body of facts thus obtained, Morgan put forward a theory of the linear arrangement of the genes in the chromosomes, expanding this theory in his book, Mechanism of Mendelian Heredity (1915).

Morgan was able to demonstrate that genes are carried on chromosomes and are the mechanical basis of heredity. These discoveries formed the basis of the modern science of genetics. He was the first person to be awarded the Nobel Prize in Physiology or Medicine for his work in genetics.

Hershey & Chase

Hershey and Chase concluded that DNA must be the genetic code material, not protein as many poeple believed. When their experiment was published and people finally acknowledged that DNA was the genetic material

1952- Hershey-Chase Experiment

Hershey and Chase sought an answer to the question, “Is it the viral DNA or viral protein coat (capsid) that is the viral genetic code material which gets injected into a host bacterium cell? They were using a bacterium named Escherichia coli, or E. coli and a virus called T2 (bacteriophage that infects E. Coli)

Frederick Griffith

In 1928, Frederick Griffith performed an experiment using pneumonia bacteria and mice. This was one of the first experiments that hinted that DNA was the genetic code material. Click on the “mouse button” to study his experiment. He used two strains of Streptococcus pneumoniae: a “smooth” strain which has a polysaccharide coating around it that makes it look smooth when viewed with a microscope, and a “rough” strain which doesn’t have the coating, thus looks rough under the microscope.

Griffith concluded that the live R strain bacteria must have absorbed genetic material from the dead S strain bacteria, and since heat denatures protein, the protein in the bacterial chromosomes was not the genetic material. This evidence pointed to DNA as being the genetic material. Transformation is the process whereby one strain of a bacterium absorbs genetic material from another strain of bacteria and “turns into” the type of bacterium whose genetic material it absorbed.

Erwin Chargaff

He took samples of DNA of different cells and found that the amount of adenine was almost equal to the amount of thymine, and that the amount of guanine was almost equal to the amount of cytosine. Thus you could say: A=T, and G=C. This discovery later became Chargaff’s Rule.

The adenine and thymine are connected together through hydrogen bonds and also connected to the sugar phosphate spine. Erwin found this to be true by using the percent of adenine in the DNA molecule commpared to that of the thymine. With these results he found that %A and %T were the same. Therefore %A=%T. This also concluded that the remaining nucleiotides were paired together as well. %C=%G.

Wilking & Franklin

By using a technique called X-ray diffraction, Franklin obtained results which led to the realization that a DNA molecule consists of an intertwined double helix of atoms. An X-ray diffraction experiment directs a beam of X-rays through a sample of a substance onto a screen. A pattern of spots is formed. This is recorded, and used to calculate the arrangement of atoms in the sample





Watson & Crick

In 1953, James Watson and Francis Crick determined the structure of DNA, in what is one of the most significant biological discoveries ever made. An examination of x-ray crystallography work by another researcher allowed the duo to determine that DNA is a double-helical molecule – a helical structure with two DNA strands, each with a carbon-phosphate backbone, and pairs of nucleotides arranged like rungs on a ladder.

This discovery was important not only for its own sake but also because it suggested two important facts about genetic inheritance:

1.That genetic information was carried by the sequence of nucleotides on the DNA strands
2.That DNA replication could be achieved if the strands were unwound, with each single strand used as the template for a new strand