Question: How did we find out about DNA?

Claire Bryer answered on 19 Jun 2015:

The first idea was that there could be something that controls how living things are different from one another. We now know this ‘something’ is DNA but they did not know this back then. A guy called Mendel did some funky experiments with pea plants that showed that the baby pea plants can inherit characteristics such as plant height, seed shape and colour, and flower position and colour from the parent pea plants. This showed something was being passed on to the next generation. Many people describe Mendel as the founder of genetics for this reason.
The next main dude was Darwin, who you might just have heard of before! He went one step further than Mendel and developed a theory, the theory of evolution. He also was not aware of DNA but described a unit that could be passed down generations (we now know as a gene) that when slight changes creep can cause a differences in the appearance or behaviour of the living thing. Darwin looked at birds (Finches) on the Galapagos islands and noticed that birds living on different islands had different beaks, and that they must have adapted to the environment on those islands. He wrote the Origin of the Species after this.
In the early 1900’s several scientists looked to see what this inherited material is. Frederick Griffith injected 2 types of pneumonia into mice, the R strain (so called because it looked rough under the microscope) and the S strain (called ‘S’ because it looks smooth under the microscope). The S bacteria kills the mouse but the R bacteria doesn’t. If you heat the R or S bacteria before injecting it into the mouse, the mouse lives. But if he mixed dead S cell parts (heat treated) with live R bacteria, the mouse strangely was seen to die. When he looked inside the dead mouse at the bacteria, under the microscope they looked smooth (S), showing the R bacteria had permanently become S bacteria. We now know that the R bacteria had took up the S bacteria DNA, by a process called ‘transformation’. Other scientists such as Avery took this further by separating out these cellular parts (by spinning them very fast-centrifugation) and finding out which bits were able to transform! For a while people thought that it was proteins that were the heritable material, but treating the cell extracts with an enzyme that destroys proteins. Two scientists called Hershey and Chase then did a neat experiment with phage, which are viruses that infect bacteria. They grew the bacteria in either radioactive phosphorus (labels the DNA) or radioactive sulphur (labels the protein) and looked to see which gets injected into the bacteria. They saw that it was DNA.
The next important people in the story are James Watson and Francis Crick. In the 1950’s these guys used a technique called X-ray crystallography to look at the structure of DNA and they found that it forms a double helix, by base pairing. A female scientist called Rosalind Franklin was also working on this but is not credited as much as she should be.
After this people worked on how the DNA code creates RNA and then protein, but that is another long story!
🙂