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Components of Cells
The Macromolecules
Chromosomes
"colored bodies"
In living cells, DNA molecules are always protected and packaged into complex, multidimensional structures that become "colored bodies" when stained with dyes and observed under a light microscope.

From this property to take up dyes, we get the general term for these structures; chromosomes, a compound word derived from the words "chroma" meaning color, and "soma" meaning body.

In eukaryotic cells, the DNA molecules are enormously long and linear. These impossibly long thin threads are first wound twice around special proteins called histones to form a "spool" called a nucleosome.

Sets of these numcleosome "spools" are then packed together in a type of spiral arrangement or coil, to which extra packing proteins are added. This coil of nucleosomes and scaffolding proteins is called chromatin. The scaffolding proteins form a central "core" into and out from which the DNA fibers loop.

Stacked sections of protein and chromatin coil gently along the fully condensed "arms" of chromosomes that are so packed together they can be stained with dyes and seen under the light microscope.

But this is an extreme form of packing. When the eukaryotic cell is actively growing or metabolizing, the DNA is kept more accessible to the cellular machinery that reads the genetic messages. It is only when the cell is preparing for division (mitosis or meiosis) that fully condensed chromosomes are created.

Figure legend: The structure of chromosomes.

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© 2001, Professor John Blamire