The Structure of Chromosomes:- Chromatin Consists of DNA and Proteins
The eukaryotic cell cycle produces remarkable changes in the structure of chromosomes (Fig. 1). In nondividing eukaryotic cells (in G0) and those in interphase (G1, S, and G2), the chromosomal material, chromatin, is amorphous and appears to be randomly dispersed in certain parts of the nucleus. In the S phase of interphase the DNA in this amorphous state replicates, each chromosome producing two sister chromosomes (called sister chromatids) that remain as sociated with each other after replication is complete. The chromosomes become much more condensed during prophase of mitosis, taking the form of a species-specific number of well-defined pairs of sister chromatids .
Chromatin consists of fibers containing protein and DNA in approximately equal masses, along with a small amount of RNA. The DNA in the chromatin is very tightly associated with proteins called histones, which package and order the DNA into structural units called nucleosomes (Fig. 2). Also found in chromatin are many nonhistone proteins, some of which help maintain chromosome structure, others that regulate the ex pression of specific genes. Beginning with nucleosomes, eukaryotic chromosomal DNA is packaged into a succession of higher-order structures that ultimately yield the compact chromosome seen with the light microscope. We now turn to a description of this structure in eukaryotes and compare it with the pack aging of DNA in bacterial cells.
FIGURE 1 Changes in chromosome structure during the eukaryotic cell cycle. Cellular DNA is uncondensed throughout interphase. The interphase period can be subdivided into the G1 (gap) phase; the S (synthesis) phase, when the DNA is replicated; and the G2 phase, in which the replicated chromosomes cohere to one another. The DNA undergoes condensation in the prophase of mitosis. Cohesins (green) and condensins (red) are proteins involved in cohesion and condensation (discussed later in the chapter). The architecture of the cohesin condensin-DNA complex is not yet established, and the interactions shown here are figurative, simply suggesting their role in condensation of the chromosome. During metaphase, the condensed chromosomes line up along a plane halfway between the spindle poles. One chromosome of each pair is linked to each spindle pole via microtubules that extend between the spindle and the centromere. The sister chromatids separate at anaphase, each drawn toward the spindle pole to which it is connected. After cell division is complete, the chromosomes decondense and the cycle begins anew.
FIGURE 2 Nucleosomes. Regularly spaced nucleosomes consist of histone complexes bound to DNA. (a) Schematic illustration and (b) electron micrograph.
