DISSERTATION

______________________________________________________________________ Introduction
1.2 DNA
Deoxyribonucleic acid (DNA) is the carrier of genetic instructions used for the proper
development and functioning of all living organisms. It was isolated for the first time by
Friedrich Miescher in 1869. Since then, the “molecule of life” has evoked curiosity from many
researchers around the world. The defining moment in nucleic acid research was a paper
published in The Journal of Experimental Medicine in 1944 by Oswald Avery, Colin MacLeod
and Maclyn McCarty, where they show for the first time that DNA, and not proteins, is the
material of inheritance 25 .
The controversy about the discovery of the DNA structure still remains, as it is debatable who
should get the credit for it. In 1953 five papers were published in the journal Nature, describing
and providing evidence for the double helix structure of the DNA. James Watson and Francis
Crick first suggested the correct double helix model 26,27 , based on an X-ray diffraction image
taken by Rosalind Franklin and her student Raymond Gosling. Previously, J. Watson listened
R. Franklin’s lecture about the DNA structure in 1951, and got access to her progress report
without her knowledge. In the same issue of the journal, Maurice Wilkins with his colleagues 28
and R. Franklin with R. Gosling 29,30 provided experimental evidence supporting the Watson
and Crick model. In 1962, after R. Franklin’s death, Watson, Crick and Wilkins received
together the Nobel Prize in Physiology or Medicine. Nobel Prizes are awarded only to living
recipients and it remains unknown whether Rosalind Franklin should have received the prize
as well.
DNA consists of two strands, where each strand is composed of subunits called nucleotides. A
nucleotide consists of three components – a nucleobase, a sugar (deoxyribose) and a phosphate
group (Figure 1.3). Within the same strand, nucleotides are covalently bound to each other in a
chain by connecting the sugar of one nucleotide to the phosphate group of the next one via a
phosphodiester bond, creating a sugar-phosphate backbone. Each phosphate group is bound to
the 3' carbon of the previous deoxyribose, and to the 5' carbon of the following sugar ring. The
ends of a strand are then commonly designated as 3' end or 5' end if the chain finishes with an
unbound 3' or 5' carbon, respectively. Moreover, the connection between nucleotides of two
complementary DNA strands occurs through the nucleobases via formation of hydrogen bonds.
The nucleobases are divided in two groups – pyrimidines (thymine and cytosine) and purines
1.2 DNA 6