Chapter 17The cell cycle17.1 Overview of the Cell CycleCell cycle– a orderly sequenceofevents that take place in acell leading to the duplicationofits DNA and cell division to produce two identical daughtercellsoIn unicellular species, such as bacteria and yeasts, each cell division produces a completenew organismoIn multicellular species, cell divisions are required to produce a functioning organism (i.e., needed to replace cells that die)Cell cycle:Phase DurationFunctionInterphase S phase (S for DNA synthesis)10-12 hr- occupies abouthalf of the cell-cycle timeChromosome duplicationG2 phase (gap phase)time needed for growth and doubling of the cell’s mass of proteins and organelles M phase (M for mitosis)~1 hr- chromosome segregation and cell division - M phase comprises two major events: nuclear division, or mitosis, during which the copied chromosomes are distributed into a pair of daughter nuclei; and cytoplasmic division, or cytokinesis, when the cell itself divides in twoInterphase G1 phase (gap phase)time needed for growth and doubling of the cell’s mass of proteins and organellesIn a typical human cell, interphase may occupy 23 hrs/24 hr cycle, with 1 hour for M phaseCell growth occurs throughout the cell cycle, except during mitosisThe length of the G1phase depends on external conditions and extracellular signals from other cells. If extracellular conditions are unfavorable, G1prohibits the progress into the M phase. G0phase– is a period in which cells exist in a quiescent (dormant/inactive) state.G0phaseis viewed as either an extended G1phase, but, the cell is neither dividing nor preparing to divideomany cells remain permanently in G0until they or the organism dies. If extracellular conditions are favorable cells in early G1 or G0progress through a commitment point near the end of G1known as Start. After passing this point, cells are committed to DNA replication, even if the extracellular signals that stimulate cell growth and division are removed.