A cell's genetic information, in the form of DNA, is stored in the nucleus.
The space inside the nucleus is limited and has to contain billions of
nucleotides that compose the cell's DNA. Therefore, the DNA has to be highly
organized. There are several levels to the DNA packaging.
Diagram showing the different levels of DNA packaging.
Notice the DNA starts out as single strand double helices and continues to be condensed until
it reaches the chromosomal level.
At the finest level, the nucleotides are organized in the form of linear
strands of double helices. As you zoom out, the DNA strand is wrapped around
histones, a form of DNA binding proteins. Each unit of DNA wrapped around a
histone molecule is called a nucleosome. The nucleosomes are linked together by
the long strand of DNA.
To further condense the DNA material, nucleosomes are compacted together to
form chromatin fibers. The chromatin fibers then fold together into large
looped domain. During the mitotic cycle, the looped domains are organized
into distinct structures called the chromosomes.
Chromosomes are also used as a way of referring to the genetic basis of an
organism as either diploid or haploid. Many eukaryotic cells have
two sets of the chromosomes and are called diploid. Other cells that only
contain one set of the chromosomes are called haploid.
The chromosome also plays an important role in cell death-related aging phenomena. At
the tips of chromosomes are segments called telomeres. As a cell's DNA is
damaged, the telomeres are shortened. Once the telomeres have been reduced to a
level, the cell decides that it can no longer repair itself and initiates apoptosis,
the cellular death process. Today, much research efforts have been devoted to
elucidating the specific mechanisms by which telomeres cause cell death.