Chromatin consists of DNA and all of the associated proteins involved in the organization and function of DNA. In humans, each cell’s chromatin (except that of eggs and sperm) is divided among 46 chromosomes (23 pairs). After DNA replication but before cell division, each chromosome consists of two identical chromatin units called chromatids held together by complexes of cohesin proteins.

DNA of each human cell is approximately 2 m long, with 3.2 billion base pairs (bp), and therefore must be extensively packaged within the nucleus. This occurs initially by the DNA associating with sets of small basic proteins called histones. The structural unit of DNA and histones is called the nucleosome (Figures1 and 2), which has a core of eight histones (two copies each of histones H2A, H2B, H3, and H4), around which is wrapped about 150 bp of DNA. Each nucleosome also has a larger histone (H1) associated with both the wrapped DNA and the surface of the core. In the EM the series of nucleosomes on DNA resembles “beads on a string,” with 50-80 bp of linker DNA separating each bead. Nucleosomes are structurally dynamic; modification and rearrangement of the histones allows temporary unwrap ping of the DNA and arrival of enzymes and other proteins required for replication and gene transcription.

Fig1. Components of a nucleosome.

Fig2. From DNA to chromatin.

DNA wrapped around nucleosomes is coiled further for greater compaction within the nucleus and for general regulation of gene activity. The 10-nm fiber of nucleosomes and DNA undergoes helical folding to yield a fiber with a diameter of 30 nm (Figure 2). Beyond the 30-nm fiber chromatin structure is much less well understood, but it does form nearly ubiquitous kilobase-sized and larger loops of coiled DNA, some of which are unstable and may reflect gene activity rather than a distinct hierarchical level of chromatin structure. Many such loops appear tethered to central scaffold-like arrays containing large protein complexes (condensins) capable of compacting chromatin. Further packaging during the first phase of cell division causes chromosomes to become visible as discrete structures by light microscopy (Figure 2).

Microscopically two categories of chromatin can be distinguished in nuclei of most nondividing cells. Euchromatin is visible as finely dispersed granular material in the electron microscope and as lightly stained basophilic areas in the light microscope. Heterochromatin (Gr. heteros, other + chroma, color) appears as coarse, electron-dense mate rial in the electron microscope and as intensely basophilic clumps in the light microscope.

DNA in the more open structure of euchromatin is rich with genes, although not all of the genes are transcribed in all cells. Heterochromatin is always more compact than euchromatin, shows little or no transcriptional activity, and includes at least two types of genomic material called constitutive and facultative heterochromatin. Constitutive heterochromatin is generally similar in all cell types and contains mainly repetitive, gene-poor DNA sequences, including the large chromosomal regions called centromeres and telomeres, which are located near the middle (most often) and at the ends of chromosomes, respectively. Facultative heterochromatin contains other regions of DNA with genes where transcription is variably inactivated in different cells by epigenetic mechanisms and can undergo reversible transitions from compact, transcriptionally silent states to more open, transcriptionally active conformations.

The ratio of heterochromatin to euchromatin seen with nuclear staining can provide a rough indicator of a cell’s metabolic and biosynthetic activity. Euchromatin predominates in active cells such as large neurons, while heterochromatin is more abundant in cells with little synthetic activity such as circulating lymphocytes. Facultative hetero chromatin also occurs in the small, dense “sex chromatin” or Barr body which is one of the two large X chromosomes present in human females but not males. The Barr body remains tightly coiled, while the other X chromosome is uncoiled, transcriptionally active, and not visible. Cells of males have one X chromosome and one Y chromosome; like the other chromosomes, the single X chromosome remains largely euchromatic.

Although much heterochromatin tends to be concentrated near the nuclear lamina, evidence for spatial organization of chromatin is not normally seen. Recent in situ hybridization studies of cultured human fibroblast nuclei, using differently labeled fluorescent probes for sequences on each individual chromosome, have revealed that these structures occupy discrete chromosomal territories within dispersed chromatin (Figure 3). Such studies show further that chromosomal domains with few genes form a layer beneath the nuclear envelope, while domains with many active genes are located deeper in the nucleus.

Fig3. Chromosome territories of a human fibroblast nucleus.

The X and Y sex chromosomes contain genes determining whether an individual will develop as a female or a male. In addition to the pair of sex chromosomes, cells contain 22 pairs of autosomes. Each pair contains one chromosome originally derived from the mother and one from the father. The members of each chromosomal pair are called homologous because, although from different parents, they contain forms (alleles) of the same genes. Cells of most tissues (somatic cells) are considered diploid because they contain these pairs of chromosomes. Geneticists refer to diploid cells as 2n, where n is the number of unique chromosomes in a species, 23 in humans. Sperm cells and mature oocytes (germ cells) are haploid, with half the dip loid number of chromosomes, each pair having been separated during meiosis (described later in the chapter).

Microscopic analysis of chromosomes usually begins with cultured cells arrested in mitotic metaphase by colchicine or other compounds that disrupt microtubules. After processing and staining the cells, the condensed chromosomes of one nucleus are photographed by light microscopy and rearranged digitally to produce a karyotype in which stained chromosomes can be analyzed (Figure 4).

Fig4. Human karyotype.

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مواضيع ذات صلة

Nucleolus

Nuclear Envelope

Prokaryotic Cell Structure: The Plasma Membrane

Regulation of Cellular and Systemic Iron Homeostasis

Hemoglobin Function

Hemoglobin Structure

Phagocytes Ingest Target Cells

Cellular and Molecular Mechanisms in Development: Morphogens and Cell-to-Cell Signaling

Thrombopoietin Signaling

Mechanisms of Endomitosis in Megakaryocytes

Megakaryocytopoiesis

Ontogeny of Erythropoiesis