Structure of Chromosomes

Structure of Chromosomes
Structure of a metaphase chromosome. The sister chromatids are still attached at their centromere. Each chromatid has a kinetochore, to which the kinetochore fibers are attached. Kinetochore microtubules from each chromatid run to one of the centrosomes, which are located at opposite poles.
Figure 3-21 Structure of a metaphase chromosome. The
sister chromatids are still attached at their centromere.
Each chromatid has a kinetochore, to which the
kinetochore fibers are attached. Kinetochore microtubules
from each chromatid run to one of the centrosomes, which
are located at opposite poles.
As mentioned earlier, DNA in eukaryotic cells occurs in chromatin, a complex of DNA with histone and nonhistone protein. Chromatin is organized into a number of discrete bodies called chromosomes (color bodies), so named because they stain deeply with certain biological dyes. In cells that are not dividing, chromatin is loosely organized and dispersed, so that individual chromosomes cannot be distinguished (Principles of Genetics:A Review). Before division the chromatin condenses, and chromosomes can be recognized and their individual morphological characteristics determined. They are of varied lengths and shapes, some bent and some rodlike. Their number is constant for the species, and every body cell (but not the germ cells) has the same number of chromosomes regardless of the cell’s function. A human, for example, has 46 chromosomes in each somatic cell.

During mitosis (nuclear division) chromosomes shorten and become increasingly condensed and distinct, and each assumes a shape partly characterized by the position of a constriction, the centromere (Figure 3-21). The centromere is the location of the kinetochore, a disc of proteins specialized to bind with microtubules of the spindle fibers during mitosis.

The problem of packaging the cell’s DNA so that the genetic instructions are accessible during the transcription process is formidable. Transcription is the formation of messenger RNA from nuclear DNA (Principles of Genetics:A Review).