Using in situ hybridization fluorescence microscopy techniques (FISH), the centromere region of human metaphase chromosomes can be distinguished from adjacent areas by selective staining with the appropriate fluorophores.

Incident light fluorescence microscopy is growing rapidly in importance as an investigational tool in the fields of medical and biological research. All photomicrographs in this gallery were taken with Olympus microscopes employing UIS optics and a PM-30 automatic camera system. The specimen is a spread of human chromosomes double-stained using FITC (centromeres) and propidium diiodide (PI; whole chromosomes). Photomicrographs were recorded utilizing a UPL 100x apochromatic objective coupled to a WIB dichroic filter combination. A 3.3x projection photo eyepiece was used to transfer light from the intermediate image plane to the photographic emulsion.

Packed inside the nucleus of every human cell is nearly 6 feet of DNA, which is divided into 46 individual molecules, one for each chromosome and each about 1.5 inches long. Packing all this material into a microscopic cell nucleus is an extraordinary feat of packaging. For DNA to function, it can't be crammed into the nucleus like a ball of string. Instead, it is combined with proteins and organized into a precise, compact structure, a dense string-like fiber called chromatin.

Each DNA strand wraps around groups of small protein molecules called histones, forming a series of bead-like structures, called nucleosomes, connected by the DNA strand. Under the microscope, uncondensed chromatin has a "beads on a string" appearance.

The string of nucleosomes, already compacted by a factor of six, is then coiled into an even denser structure, compacting the DNA by a factor of 40. This compression and structuring of DNA serves several functions. The overall negative charge of the DNA is neutralized by the positive charge of the histone molecules, the DNA takes up much less space, and inactive DNA can be folded into inaccessible locations until it is needed.

There are two types of chromatin. Euchromatin is the genetically active portion and is involved in transcribing RNA to produce proteins used in cell function and growth. Heterochromatin contains inactive DNA and is the portion of chromatin that is most condensed, since it not being used. Throughout the life of a cell, chromatin fibers take on different forms inside the nucleus. During interphase, when the cell is carrying out its normal functions, the chromatin is dispersed throughout the nucleus in what appears to be a tangle of fibers. This exposes the euchromatin and makes it available for the transcription process.

When the cell enters metaphase and prepares to divide, the chromatin changes dramatically. First, all the chromatin strands make copies of themselves through the process of DNA replication. Then they are compressed to an even greater degree than at interphase, a 10,000-fold compaction, into specialized structures for reproduction, termed chromosomes. As the cell divides to become two cells, the chromosomes separate, giving each cell a complete copy of the genetic information contained in the chromatin.