Discovery of DNA
Solving the Structure of
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Long before the discovery of deoxyribonucleic acid (DNA), mankind observed
that certain human characteristics are passed on from parents to their
offspring. For a long time this concept was used for breeding animals or
cultivating plants with distinctive characteristics. The science behind
hereditary processes would, however, remain unknown for quite some time.
In 1865, Gregor Mendel discovered that inherited traits follow certain rules.
Mendel's work was largely ignored until it was rediscovered in the early 20th
century by Hugo de Vries and Carl Correns. This rediscovery of Mendel's work
laid the foundation for modern genetics and the concept of genes as building
blocks of hereditary information.
Discovery of DNA
DNA was found to be the material that the genetic code was written in.
Although its composition was analyzed early on, the exact organization and
3-dimensional structure of DNA was a highly debated topic. It wasn't until 1953,
when James Watson and Francis Crick formulated their double-helix model, that
the real structure of DNA and the mystery of the genetic code was solved. DNA is
a polymer chain that stores hereditary information, and is composed of two
strands of repeating elements (nucleotides) that are folded in a double helix.
The genetic code is embedded in the sequence of small groups of three
nucleotides. Depending on the organism, DNA molecules contain up to hundreds of
millions of nucleotides, and can therefore be anywhere between several
micrometers (bacteria) and meters (mammals) in length.
Solving the Structure of DNA
X-ray diffraction data of Rosalind Franklin has played an important part in
solving the structure of DNA, and this technique remains important even today,
where X-ray crystallography and electron microscopy are the main tools used to
image DNA at high resolution. Recently, however, scanning probe techniques have
gained importance due to their high signal to noise ratio, the absence of a
necessity to crystallize the DNA prior to imaging, and the possibility to study
protein-DNA interactions and processes as they occur.
All measurements were performed with a Nanosurf
easyScan 2 FlexAFM (100 µm scan range) operated in Dynamic mode in air
Atomic Force Microscopy (AFM) is used to image DNA with high accuracy and
under physiological conditions. In order to be imaged by AFM, samples must be
immobilized onto a flat surface. The negatively charged backbone of the DNA can
be utilized for immobilization onto charged substrates by means of electrostatic
interactions. Freshly cleaved muscovite mica is often used for this purpose.
Divalent cations can be used as a bridge to immobilize charged DNA molecules
onto mica surfaces. The cations used should be soluble in water and bind tightly
to both the backbone of the DNA and to the mica. Nickel (Ni2+) and
magnesium (Mg2+) have these characteristics and are therefore often
used. After the DNA and the cations have adsorbed onto the mica, the surface is
washed, air-dried, and promptly imaged.
Figure 1. Linearized Plasmid DNA (pGem7zf+ from Promega)
adsorbed onto muscovite mica. Image scan area corresponds to 2 µm × 2 µm.
Figure 2. Close-up of the same DNA on mica preparation.
Image scan area corresponds to 250 nm × 250 nm.
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