Question

1.Briefly describe both the Griffith, and Hershey-Chase experiments and what information they provided concerning DNA as the genetic material of the cell.(please type)

2. Discuss Chargaff, Franklin, Watson and Crick, and what they contributed to our current understand of DNA.(please type)

Answer 1

Griffith's experiment, reported in 1928 by Frederick Griffith, was the first experiment suggesting that bacteria are capable of transferring genetic information through a process known as Transformation

Griffith used two strains of pneumococcus (Streptococcus pneumoniae) bacteria which infect mice – a type III-S (smooth) which was virulent, and a type II-R (rough) strain which was nonvirulent.

In this experiment, bacteria from the III-S strain were killed by heat, and their remains were added to II-R strain bacteria.

While neither alone harmed the mice, the combination was able to kill its host.

Griffith was also able to isolate both live II-R and live III-S strains of pneumococcus from the blood of these dead mice. Griffith concluded that the type II-R had been "transformed" into the lethal III-S strain by a "transforming principle" that was somehow part of the dead III-S strain bacteria.

Griffith's experiment

Conclusion = "transforming principle" Griffith observed was the DNA of the III-s strain bacteria. While the bacteria had been killed, the DNA had survived the heating process and was taken up by the II-R strain bacteria.

The III-S strain DNA contains the genes that form the smooth protective polysaccharide capsule. Equipped with this gene, the former II-R strain bacteria were now protected from the host's immune system and could kill the host.

The exact nature of the transforming principle (DNA) was verified in the experiments done by Hershey and Chase.

Hershey and Chase experiment

Hershey and Chase showed that when bacteriophages, which are composed of DNA and protein, infect bacteria, their DNA enters the host bacterial cell, but most of their protein does not.

Hershey and Chase and subsequent discoveries all served to prove that DNA is the hereditary material.

Overview of experiment

Conclusion =

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

They determined that a protective protein coat was formed around the bacteriophage, but that the internal DNA is what conferred its ability to produce progeny inside a bacterium. They showed that, in growth, protein has no function, while DNA has some function.

2) Chargaff's rules state that DNA from any cell of any organism should have a 1:1 stoichiometric ratio (base pair rule) of pyrimidine and purine bases and, more specifically, that the amount of guanine should be equal to cytosine and the amount of adenine should be equal to thymine.

Franklin is best known for her work on the X-ray diffraction images of DNA.

Rosalind Franklin's first important contributions to the model popularised by Crick and Watson . Where she presented , among the Watson model , the two forms of the molecule, type A and type B, position being that the phosphate units are located in the external part of the molecule.

Watson and Crick model

• DNA is a double-stranded helix, with the two strands connected by hydrogen bonds. A bases are always paired with T, and C are always paired with G, which is consistent with and accounts for Chargaff's rule.
• Most DNA double helices are right-handed; that is, if you were to hold your right hand out, with your thumb pointed up and your fingers curled around your thumb, your thumb would represent the axis of the helix and your fingers would represent the sugar-phosphate backbone. Only one type of DNA, called Z-DNA, is left-handed.
• The DNA double helix is anti-parallel, which means that the 5' end of one strand is paired with the 3' end of its complementary strand (and vice versa). As shown in Figure 4, nucleotides are linked to each other by their phosphate groups, which bind the 3' end of one sugar to the 5' end of the next sugar.
• Not only are the DNA base pairs connected via hydrogen bonding, but the outer edges of the nitrogen-containing bases are exposed and available for potential hydrogen bonding as well. These hydrogen bonds provide easy access to the DNA for other molecules, including the proteins that play vital roles in the replication and expression of DNA .

Summary

Watson and Crick were not the discoverers of DNA, but rather the first scientists to formulate an accurate description of this molecule's complex, double-helical structure.