| Eukaryotic
chromosomes consist of protein and deoxyribonucleic acid (DNA), in about
equal amounts. DNA is the hereditary material that makes up chromosomes
and genes. DNA was first isolated by Friedrich Miescher, a German physician,
in 1869. The substance that he isolated was white, sugary, slightly acidic,
and contained phosphorus. Since he found that substance in the nuclei,
he called it "nuclein." Later on, Robert Feulgen, discovered that DNA was
present in all cells and was located in the chromosomes. However, it was
not until 1950s that scientists were able to understand some of the main
functions of DNA when the Watson-Crick model came out.
 Watson-Crick
model stated that DNA is an exceedingly long, entwined double helix. DNA
contains four nitrogenous bases: adenine (A), thymine (T), guanine (G),
and cytosine (C). These nitrogenous bases make up each of the double-strands
of DNA. The bases of the two strands are connected to each other via hydrogen
bonds. Each base is also covalantly bonded to a sugar-phosphate base.
Nucleotides along each strands could be assembled in any order: for example,
TTCGGGATCCATATG.
 Watson-Crick
model also explained how more strands of DNA could be produced in a process
called replication. A new DNA strand is always synthesized in one
particular direction. The strand that is continuously synthesized is called
the leading strand, while the lagging strand gets synthesized discontinuously
in short pieces running opposite to the direction of replication fork movement.
DNA
consists of 5 elements: nitrogen, carbon, hydrogen, oxygen, and phosphorous.
The best way to illustrate this essential organic molecule is to view DNA
as a rope that has two strands. These strands have locked and wrapped each
other in a way that if you twist the rope in one way, it becomes more concentrated,
and if you twist it in the other direction, it becomes less dense and less
concentrated. DNA molecules are very long, but have you ever wondered
how these long molecules are being fit in a tiny cell? The answer
is simple and yet profound. The DNA molecule twists itself with the
help of different enzymes in order to become denser and more concentrated.
When a DNA wants to go through a replication process; however, it must
go back to its untwisted form. This process once again requires different
enzymes.
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