Each eukaryotic cell contains about 6 billion bits of information encoded in DNA structure, which has a total length of about 1.8 meter. The length of the DNA molecule is 100,000 times longer than the nuclear diameter. Therefore, the DNA must be highly folded and tightly packed in the cell nucleus. This is accomplished by the formation of a unique nucleoprotein complex called chromatin. The chromatin complex consists of DNA and structural proteins. Further folding of chromatin, such as that which occurs during mitosis, produces structures called chromosomes. Each human cell contains 46 chromosomes. Chromatin proteins include five basic proteins called histones along with other nonhistone proteins. A unique feature of chromatin packaging is that it permits the transcriptional machinery to access those regions of the chromosomes that are required for gene expression.
Sequencing human genome was successfully completed in 2003.
The human genome encompasses the entire length of human DNA that contains the genetic information packaged in all 46 chromosomes. Sequencing of the human genome took about 13 years and was successfully completed in 2003 by the Human Genome Project. The human genome contains a 2.85-billion base pair consensus sequence of nucleotides, which are arranged in about 23,000 protein-coding genes. For years it was thought that genes were usually present in two copies in a genome. However, recent discoveries have revealed that large segments of DNA can vary in numbers of copies. Such copy number variations (CNVs) are widespread in the human genome and most likely lead to genetic imbalances. For instance, genes that were thought to always occur in two copies per genome have sometimes one, three, or more copies. A previous definition of a gene as a segment of DNA involved in producing a polypeptide chain has been recently updated to now read as being a union of genomic sequences encoding a coherent set of potentially overlapping functional products.
In general, two forms of chromatin are found in the nucleus: A condensed form called heterochromatin and a dispersed form called euchromatin.
In most cells, chromatin does not have a homogeneous appearance; rather, clumps of densely staining chromatin are embedded in a more lightly staining background. The densely staining material is highly condensed chromatin called heterochromatin, and the lightly staining material (where most transcribed genes are located) is a dispersed form called euchromatin. It is the phosphate groups of the chromatin DNA that are responsible for the characteristic basophilia of chromatin (page 6). Heterochromatin is disposed in three locations (Fig. 3.1):  Marginal chromatin is found at the periphery of the nu cleus (the structure light microscopists formerly referred to as the nuclear membrane actually consists largely of marginal chromatin).  Karyosomes are discrete bodies of chromatin irregular in size and shape that are found throughout the nucleus.  Nucleolar-associated chromatin is chromatin found in association with the nucleolus.
Heterochromatin stains with hematoxylin and basic dyes; it is also readily displayed with the Feulgen procedure (a specific histochemical reaction for the deoxyribose of DNA, page 6) and fluorescent vital dyes such as Hoechst dyes and propidium iodide. It is the heterochromatin that accounts for the conspicuous staining of the nucleus in hematoxylin and eosin (H&E) preparations.
Euchromatin is not evident in the light microscope. It is present within the nucleoplasm in the “clear” areas between and around the heterochromatin. In routine electron micro-graphs, there is no sharp delineation between euchromatin and heterochromatin; both have a granular, filamentous appearance, but the euchromatin is less tightly packed.