In interphase cells, as noted previously, chromosome territories can be visualized with chromosome-specific fluorescently labeled hybridization probes (see Figure 1), but the detailed structure of individual chromosomes cannot be observed, even with the aid of electron microscopy. During mitosis and meiosis, however, the chromosomes con dense and become visible in the light microscope. Therefore, almost all cytogenetic work (i.e., studies of chromosome morphology) has been done with condensed metaphase chromosomes obtained from dividing cells—either somatic cells in mitosis or dividing gametes during meiosis.

EXPERIMENTAL Fig1. During interphase, human chromosomes remain in non-overlapping territories in the nucleus. (a) Fixed interphase human fibroblasts were hybridized in situ to fluorescently labeled probes specific for sequences along the full length of human chromosomes 7 (cyan) and 8 (purple). DNA is stained blue with DAPI. In the diploid cell, each of the two chromosome 7s and two chromosome 8s is restricted to a territory or domain within the nucleus, rather than stretching throughout the entire nucleus. (b) This image from a fixed interphase fibroblast from a human male was made with a method similar to that used in (a), except that chromosome paint probes specific for each chromosome were hybridized to the cell to reveal the locations of nearly all the chromosomes. Some of the chromosomes are not observed in this confocal slice through the nucleus. [Part (a) courtesy of Dr. Irina Solovei. Part (b) from A. Bolzer et al., 2005, Three-dimensional maps of all chromosomes in human male fibroblast nuclei and prometaphase rosettes. PLoS Biol. 3:826.]

The condensation of metaphase chromosomes probably results from several orders of folding of 30-nm chromatin fibers (see Figure 2). At the time of mitosis, cells have already progressed through the S phase of the cell cycle and have replicated their DNA. Consequently, the chromosomes that become visible during metaphase are duplicated structures. Each metaphase chromosome consists of two sister chromatids, which are linked at a constricted region, the centromere (see Figure 3).

Fig2. Model for mitotic chromosome condensation. (a) Stages of chromosome condensation during mitosis. Changes in large-scale chromatin folding (blue) versus distribution of Smc2, a subunit of condensin (red), from early prophase 1 to middle prophase 2 to late prophase 3 to metaphase 4. (b) Transmission electron micrograph of immunogold staining of Smc2 in a section through a metaphase chromosome reveals axial staining of Smc2 of about 0.15–0.2 μm in width. (c) “Hierarchical folding, axial glue” model of metaphase chromosome structure. (Left) 30-nm fiber folds into 100–130-nm chromonema fiber, which folds into 200–250-nm middle prophase chromatid, which folds into 500–750-nm metaphase chromatid. Only one chromatid is shown. (Right) Axial condensin distribution (red) occupies approximately one-third of the chromatid diameter, acting as a cross-linking “glue” to stabilize the structure of the metaphase chromosome. [Part (b) © 2004 Kireeva et al., The Journal of Cell Biology. 166:775-785. doi: 10.1083/ jcb.200406049.]

Fig3. Typical metaphase chromosome. As seen in this scanning electron micrograph, the chromosome has replicated and comprises two chromatids, each containing one of two identical DNA molecules. The centromere, where the chromatids are attached at a constriction, is required for their separation late in mitosis. Special telomere sequences at the ends function in preventing chromosome shortening. [Andrew Syred/Science Source.]

The number, sizes, and shapes of the metaphase chromosomes constitute the karyotype, which is distinctive for each species. In most organisms, all somatic cells have the same karyotype. However, species that appear quite similar can have very different karyotypes, indicating that similar genetic potential can be organized on chromosomes in very different ways. For example, two species of small deer—the Indian muntjac and Reeves muntjac—contain about the same total amount of genomic DNA. In one species, however, this DNA is organized into 22 pairs of homologous autosomes and two physically separate sex chromosomes. In contrast, the other species contains the smallest number of chromosomes of any mammal, only three pairs of autosomes; one sex chromosome is physically separate, but the other is joined to the end of one autosome.

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مواضيع ذات صلة

The TATA Box, Initiators, and CpG Islands Function as Promoters in Eukaryotic DNA

Chromosome Painting and DNA Sequencing Reveal the Evolution of Chromosomes

During Metaphase, Chromosomes Can Be Distinguished by Banding Patterns and Chromosome Painting

Origins and Types of Mutations

Regulation of Gene Expression

The Human Genome

The Polymerase Chain Reaction (PCR)

Additional Nonhistone Proteins Regulate Transcription and Replication

Nonhistone Proteins Organize Long Chromatin Loops

Modifications of Histone Tails Control Chromatin Condensation and Function

Chromatin Exists in Extended and Condensed Forms

Fetal DNA Sequencing and Fetal Genome Analysis