dna replicates in a cell cycle during synthesis.
dna replicates in a cell cycle during synthesis.
when does the dna replicate in a cell cycle?
synthesis
each process activated by the proteins of the cell cycle engine involve a cascade of many reactions. the longest subsystem cascade is dna replication. in "caulobacter" cells, replication of the chromosome involves about 2 million dna synthesis reactions for each arm of the chromosome over 40 to 80 min depending on conditions. while the average time for each individual synthesis reaction can be estimated from the observed average total time to replicate the chromosome, the actual reaction time for each reaction varies widely around the average rate. this leads to a significant and inevitable cell-to-cell variation time to complete replication of the chromosome. there is similar random variation in the rates of progression of all the other subsystem reaction cascades. the net effect is that the time to complete the cell cycle varies widely over the cells in a population even when they all are growing in identical environmental conditions. cell cycle regulation includes feedback signals that pace progression of the cell cycle engine to match progress of events at the regulatory subsystem level in each particular cell. this control system organization, with a controller (the cell cycle engine) driving a complex system, with modulation by feedback signals from the controlled system creates a closed loop control system.
caulobacter crescentus
multiparameter analysis of the cell cycle includes, in addition to measurement of cellular dna content, other cell cycle related constituents/features. the concurrent measurement of cellular dna and rna content, or dna susceptibility to denaturation at low ph using the metachromatic dye acridine orange, reveals the g, g, and g cell cycle compartments and also makes it possible to discriminate between s, g and mitotic cells. the cells in g are quiescent, temporarily withdrawn from the cell cycle (also identifiable as g), the g are in the growth phase while g are the cells just prior entering s, with their growth (rna and protein content, size) similar to that of the cells initiating dna replication. similar cell cycle compartments are also recognized by multiparameter analysis that includes measurement of expression of cyclin d1, cyclin e, cyclin a and cyclin b1, each in relation to dna content concurrent measurement of dna content and of incorporation of dna precursor 5-bromo-2'-deoxyuridine (brdu) by flow cytometry is an especially useful assay, that has been widely used in analysis of the cell cycle in vitro and in vivo. however, the incorporation of 5-ethynyl-2'-deoxyuridine (edu), the precursor whose detection offers certain advantages over brdu, has now become the preferred methodology do detect dna replicating (s-phase) cells.
cell cycle analysis