The molecule now known as DNA was first identified in the 1860s by a Swiss
physician, Johann Miescher. Miescher was researching the key components of
white blood cells, which are part of our body’s immune system. The main
source of these cells for his experiments was pus-coated bandages collected
from a nearby medical clinic. During his work, he discovered a protein
substance and he realized that it had unexpected properties different to those of
the other proteins he was familiar with. Miescher called this mysterious
substance ‘nuclein,’ because he believed it had come from the cell nucleus.
Meischer had discovered the molecular basis of all life – DNA. He published
his findings in 1874, but his work was largely ignored. Scientists continued to
believe that proteins were the molecules that held all of our genetic material.
They believed that nuclein simply wasn’t complex enough to contain all of the
information needed to make up a genome. Surely, one type of molecule could
not account for all the variation seen within species.
Albrecht Kossel was a German biochemist who took Meischer’s research, and
made great progress in understanding the basic building blocks of nuclein. In
1881 Kossel identified nuclein as a nucleic acid and provided its chemical name,
deoxyribonucleic acid (DNA). He also isolated the five nucleotide bases that are
the building blocks of DNA and RNA: adenine (A), cytosine (C), guanine (G),
thymine (T) and uracil (U). This work was rewarded in 1910 when Kossel
received the Nobel Prize in Physiology or Medicine.
Phoebus Levene was a colleague of Kossel, and in 1919, his research identified
the phosphate, base, and sugar nucleotide monomer of DNA & RNA. He
discovered that ribose was the sugar in RNA, and deoxyribose was the sugar in
DNA.
The Story of DNA
DNA’s structure and function was discovered by the work of many scientists
over about 100 years.
For a long time, the connection between DNA and genes was not known. In
1944, the scientists Oswald Avery, Colin MacLeod, and Maclyn McCarty
published their findings from their experiments. They had managed to transfer
the ability to cause disease from one strain of bacteria to a harmless bacteria.
Furthermore, they discovered these previously harmless bacteria could pass this
trait (the ability to cause disease) on to the next generation. This research
proved that genes were made of nucleic acids.
In 1949, Erwin Chargaff
did studies that showed
that the amount of
adenine always equals
the amount of thymine,
and the amount of
guanine always equals
the amount of cytosine
in every organism. The
reason for this was unknown at this time. In 1952, Alfred Hershey and Martha
Chase confirmed that DNA is the genetic material.
Now the scientific community knew what DNA was, and what it was
responsible for, but they still had no idea what it looked like.
Rosalind Franklin was a scientist working on x-ray
diffraction in the 1950’s. She took pictures of
DNA (pictured to the left) and discovered that
DNA had 2 helices, but she didn’t want to publish
this finding until all her research was complete. She
gave public lectures about her findings, however,
and James Watson attended one of them.
James Watson and Francis Crick (right) had been working for years to figure
out the structure of DNA, and when they saw the x-rays from Franklin’s
research, Watson and Crick were finally able to figure out the structure of
DNA. Watson and Crick are the two scientists credited with discovering the
structure of DNA. Watson went on to discover the base pairing rules: adenine
always pairs with thymine, and cytosine always pairs with guanine. In 1962,
Watson & Crick received the Nobel Prize in Physiology or Medicine. They
proposed that the DNA molecule is a double helix shape, which resembles a
twisted ladder. The “ladder rails” of DNA is made of alternating units of
phosphate and sugar, and the “steps” of the DNA ladder are made of a
nitrogen base pair.
Questions
Answer these questions in complete sentences on page _______ of your NB.
1.
2.
3.
4.
5.
6.
What are the two ways to describe the shape of the DNA molecule?
What is the monomer for nucleic acids, and what are the three parts to that
monomer?
What is the sugar in DNA? What is the sugar in RNA?
If a strand of DNA has 47 molecules of adenine, how many molecules of
thymine would it have? How do you know?
Why is DNA important?
What can you conclude about the nature of scientific discovery, based on
the amount of scientists involved and the time it took to discover the
structure of DNA?