Y khoa, y dược - Cells: the living units: Part D

3 Cells: The Living Units: Part DCell CycleDefines changes from formation of the cell until it reproducesIncludes:InterphaseCell division (mitotic phase)InterphasePeriod from cell formation to cell divisionNuclear material called chromatinFour subphases:G1 (gap 1)—vigorous growth and metabolismG0—gap phase in cells that permanently cease dividingS (synthetic)—DNA replicationG2 (gap 2)—preparation for divisionFigure 3.31G1GrowthSGrowth and DNAsynthesisG2Growth and finalpreparations fordivisionMG2 checkpointG1 checkpoint(restriction point)Figure 3.33Centrosomes(each has 2centrioles)NucleolusInterphasePlasmamembraneNuclearenvelopeChromatinInterphaseDNA ReplicationDNA helices begin unwinding from the nucleosomesHelicase untwists the double helix and exposes complementary chainsThe Y-shaped site of replication is the replication forkEach nucleotide strand serves as a template for building a new complementary strandDNA ReplicationDNA polymerase only works in one directionContinuous leading strand is synthesizedDiscontinuous lagging strand is synthesized in segmentsDNA ligase splices together short segments of discontinuous strandDNA ReplicationEnd result: two DNA molecules formed from the originalThis process is called semiconservative replicationFigure 3.32AdenineThymineCytosineGuanineOld (template) strandTwo new strands (leading and lagging)synthesized in opposite directionsDNA polymeraseDNA polymeraseLaggingstrandLeading strandFree nucleotidesOld strand acts as atemplate for synthesisof new strandChromosomeHelicase unwindsthe double helix andexposes the basesOld DNAReplicationfork