Top 3 Experiments with DNA

top 3 experiments with dnaOur modern idea of DNA and heredity has led to the development of a large number of trends, such as forensic analysis, paternity testing and genetic screening. Therefore, thanks to such widespread use, today many people have the basic idea of what DNA is.

Proteins and DNA

According to Gregor Mendel signs (such as painting of petals in pea flowers) are not inherited directly, but rather are determined by the genes transmitted from parents to descendants. Subsequent works of scientists, among which Theodore Boveri, Walter Sutton and Thomas Hunt Morgan, proved that the hereditary signs of Mendel are most likely associated with chromosomes.

Initially, scientists believed that the proteins were contained in chromosomes together with DNA are genetic material. After all, it was known that proteins have various amino acid sequences, while DNA was considered a boring, repeating polymer, partly due to the wrong (but common) understanding of its structure and composition.

Today we know that the initial idea of ​​DNA was erroneous. In reality, DNA carries a large amount of information and we will tell about this in more detail in the opening of the DNA structure. But as scientists first realized that the “boring” DNA is actually a genetic material?

Frederick Griffith: bacteria transformation

In 1928, the British bacteriologist Frederick Griffith conducted a series of experiments using Streptococcus Pneumoniae bacteria (pneumococcus) and mice. Griffith did not try to determine the genetic material, and tried to develop a vaccine against pneumonia. In their experiments, Griffith used two kindred strains of bacteria, known as R and S.

R-strain. When growing in a Petri dish, R-bacteria, colonies or clusters of related bacteria were formed, which had clearly defined edges and a grungy appearance (abbreviation “R” from the English “Rough” – coarse, grungy). R-bacteria were invisible, that is, they did not cause diseases when introduced in the mice organism.

S-strain. The S-bacteria formed rounded and smooth colonies (S is the first letter of the English word “smooth”). Their smoothness was due to a shell of polysaccharide created by bacteria. This shell protected the S-bacteria from the immune system of mice, making them virulent (capable of causing diseases). In mice, which was injected into live s-bacteria, pneumonia developed, and they died.

As part of its experiments, Griffith tried to introduce S-bacteria inactivated during the heating (that is, the S-bacteria were heated to a very high temperature, which led to the cell destruction) into a mice organism. It is not surprising that such S-bacteria did not cause diseases in mice.

However, the experiments accepted an unexpected turnover when harmless R-bacteria were introduced with mice together with previously heated S-bacteria. It is not enough that mice developed pneumonia and they died, but when Griffith took a sample of blood in the dead mouse, he found that it contains the living bacteria of the S-strain.

Thus, Griffith came to the conclusion that the R-strain bacteria should have taken over something from inactivated S-bacteria, and this something he called the “transformation factor”. Thanks to this, R-bacteria were able to “transform” in bacteria with a smooth surface and become virulent.

Avery, MacLeod and McCarty: identification of the transformation factor

In 1944, three Canadian and American researchers, Osawald Avery, McLee McCarty and Colin Macleod, decided to deal with the “transformation factor” of Griffith.

To do this, they began with volumetric cultures of S-cells, and after a long sequence of biochemical stages (carefully defined experimental paths), they gradually allocated a transformation factor through laundering, separation or enzymatic destruction of other cell components. Thanks to this method, they were able to get a small amount of highly purified transformation factor and as a result of a number of other analyzes, determine what he represents.

Every and his colleagues offered several direct evidence that DNA could be factor in the transformation factor.

The purified substance in chemical tests for protein detection gave a negative result, but at the same time also gave an explicit positive result in chemical tests for DNA detection.

The composition of the purified factor was very reminder DNA by the ratio of nitrogen and phosphorus.

Enzymes that destroy proteins and RNAs practically did not affect it, but enzymes capable of splitting DNA eliminated transforming activity.

The results indicated that DNA is likely to be a factor in transformation. However, Avery was careful in interpreting their results. He understood that the transformation factor itself could be some additional substance present in small quantities, and not DNA.

And since such an opportunity remained, the disputes on the role of DNA continued until 1952, when Alfred Hershi and Martha Chase, using another approach, could finally establish that it was the DNA that is a carrier of genetic material.