Partitioning, of chromosome

Colchicine is a specific spindle poison, which binds to tubulin, and inhibits its polymerization. Consequently, colchicine blocks mitosis, causing aneuploidy, the unequal partition of chromosomes, and metaphase arrest. 

The unequal partition of chromosomes or nondisjunction is a serious effect if the affected daughter cells survive. Down syndrome or Trisomy 21 is the result of chromosome nondisjunction in humans, those affected having 47 chromosomes instead of 46. This unequal partitioning of chromosomes can occur at mitosis in germ cells or during meiosis in the production of sperm or ova. 

Thus there are two types of mutagen those acting directly on DNA and those acting on the replication or the partition of chromosomes. Mutagens of the latter type may therefore only be effective at certain times in the cell cycle. Also, some mutagens may not be able to cross the nuclear membrane and are therefore only active at mitosis, when the nuclear material is in the cytoplasm. 

Eukaryotic ceils possess a discrete, membrane-bounded nucleus, the repository of the cell s genetic material, which is distributed among a few or many chromosomes. During ceil division, equivalent copies of this genetic material must be passed to both daughter ceils through duplication and orderly partitioning of the chromosomes by the process known as mitosis. Like prokaryotic... 

Pharmacology The fluoroquinolones are synthetic, broad-spectrum antibacterial agents that inhibit DNA gyrase and topoisomerase IV. DNA gyrase is an essential enzyme that is involved in the replication, transcription, and repair of bacterial DNA. Topoisomerase IV is an enzyme known to play a key role in the partitioning of the chromosomal DNA during bacterial cell division. 

Eukaryotic cells are more complex than prokaryotic cells and usually contain more DNA, which is partitioned between several chromosomes. In both prokaryotes and eukaryotes, almost all cells of the same organism contain the same number of chromosomes. In eukaryotes most of the chromosomes are localized in the nucleus. Thus the DNA is isolated from the main body of the cytoplasm—a unique feature of eukaryotes and the primary distinction between prokaryotes and eukaryotes. Some organelles, notably the mitochondria and the chloroplasts, contain a single circular chromosome. 

Ultimately a plasmid is defined by its mode of DNA repHcation. DNA repHcation is initiated at a single, characteristic sequence, termed the origin. The origin sequence determines the copy number of the plasmid relative to the host chromosome and the host enzymes that are involved in plasmid repHcation. Two different plasmids that contain the same origin sequence are termed incompatible. This term does not refer to the active exclusion of one plasmid by another from the ceU but rather to a stochastic process by which the two plasmids are partitioned differentially into progeny ceUs. A ceU which contains two plasmids of the same incompatibiHty group segregates two clonal populations, each of which has one of the two plasmids in it. 

Lica LM, Narayanswami S, Hamkalo BA 1986 Mouse satellite DNA, centromere structure, and sister chromatid pairing. J Cell Biol 103 1145-1151 Lin DC, Grossman AD 1998 Identification and characterization of a bacterial chromosome partitioning site. Cell 92 675-685... 

These results were obtained by coupling a genetic algorithm for descriptor and calculation parameter (PC, bins) selection to PCA-based partitioning. In these calculations, descriptors were chosen from a pool of approx 150 different ones, and both the number of PCs and bins were allowed to vary from 1 to 15. An initial population of 300 chromosomes was randomly generated with initial bit occupancy of approx 15%. Rates for mutation and crossover operations were set to 5% and 25%, respectively. After PCA-based partitioning, scores were calculated for the following fitness function ... 

RGURE 25-18 Chromosome partitioning in bacteria, (a) All replication is carried out at a central replication factory that includes two complete replication forks, (b) The two replicated copies of the bacterial chromosome are extruded from the replication factory into the two halves of the cell, possibly with each newly synthesized origin bound separately to different points on the plasma membrane. Sequestering the two chromosome copies in separate ceil halves facilitates their proper segregation at ceil division. 

The cell cycle in eukaryotes is divided into four phases, Glt S. G2, and M (Fig. 16-4). The cycle starts at the beginning of G, (G = gap), which often constitutes the major phase in duration. DNA replication commences at the beginning of the S phase and is completed before entry into G2. Mitosis, or the M phase, is relatively short and includes those steps that lead to chromosome segregation and partitioning into two daughter cells. 

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