Chrmsme - a linear DNA mlecule Hmlgus chrmsmes - chrmsmes that have the same kind f genes in the same rder 1 cpy frm father, 1 cpy frm mther humans have 46 chrmsmes with 23 hmlgus pairs als knw as sister chrmsmes Chrmatid - after a chrmsme has replicated in S phase, there are tw chrmatids. Each chrmatid pssess an identical sequence f DNA. Centrmere - special regin where spindle fibers attach Grand Scheme Mitsis Mitsis Meisis 2N diplid cell replicates divides - tw diplid daughter cells 2N diplid cell replicates divisin - tw daughter cells, each 2N divisin (withut replicatin)- fur daughter cells, each N (haplid) Cell Cycle - phases Interphase G0 - Gap 0 phase, a resting stage, lw levels f MPF (Maturatin Prmting Factr), 2N G1 - Gap 1 phase, cell enters mitsis, lw levels MPF, 2N S - DNA synthesis ccurs, lw levels f MPF, 4N G2 - Gap 2 phase, time befre metaphase, MPF is lw at the start and increases at the end f G2, 4N M - M phase Prphase - chrmsmes cndense, nuclear membrane disappears, MPF is increasing Metaphase - chrmsmes line up n spindle, MPF is at its peak level Anaphase - chrmsmes begin t pull apart, MPF declines t initiate anaphase Telphase - spindles disappear, MPF is lw, nuclear membranes begin t reappear Cytkinesis - cell divisin, MPF lw Sequence f Events GO resting cell is triggered t enter cell cycle prgresses thrugh G1, S, G2 enters M phase shrt prphase metaphase nuclear membrane r envelpe brakes-dwn chrmsmes attach t spindles via the centrmeres spindles are micrtubules made f tubulin hmlgus chrmsmes align alng side f each ther MPF activity is very high anaphase chrmatids pull apart MPF activity decreases telphase micrtubules disappear
nuclear envelpe refrms cytkinesis cell divisin Meisis Sequence f Events Interphase - I G0 t G2 are the same as meisis except this is in a primary spermatcyte r primary cyte imprtant t remember that S phase has ccurred and gamete is 4N Prphase - I very lng in meisis, in spermatcyte might be days while in the cyte it can be years r decades lepttene chrmsme cndense zygtene hmlgus chrmsmes pair nte that this is NOT sister chrmatids called bivalents r tetrad in the literature pachytene pairing is cmplete crssing ver ccurs between hmlgus chrmsmes NOT sister chrmatids dipltene cytes stp here befre puberty (dictyate arrest) hmlgus chrmsmes pull apart but remain attached at crssver pints (chiasma) nte that chiasma are nt the centrmeres RNA synthesis can ccur up t this pint diakinesis RNA synthesis stps the 4 chrmatids f each pair f hmlgus chrmsmes becme visible hmlgus chrmsmes remain linked at chiasma chrmatids are linked at centrmere Metaphase - I nuclear membrane breaks dwn spindles attach t centrmeres hmlgus chrmsmes are initially attached at chiasma hmlgus chrmsmes g t ppsite ples and then pair acrss frm ne anther in the center f the metaphase plate Anaphase - I hmlgus chrmsmes pull apart chiasma separate hmlgus chrmsmes mve t ppsite ples Telphase - I micrtubules disappear and nuclear envelpes begin t reappear Cytkinesis - I divisin t frm secndary spermatcyte r secndary cyte and plar bdy - I Interphase - II shrt N DNA synthesis Prphase - II brief Metaphase - II (same as mitsis) cyte stps here until vulated and resumes nly if fertilized
Anaphase - II (same as mitsis) Telphase - II (same as mitsis) Cytkinesis - II (same as mitsis) results is haplid daughter cells remember fr cyte this stage cell never exists because meisis is nly cmpleted after penetratin f the cyte by a sperm Mitsis and Meisis What is Mitsis? Mitsis prduces tw daughter cells that are identical t the parent cell. If the parent cell is haplid (N), then the daughter cells will be haplid. If the parent cell is diplid, the daughter cells will als be diplid. N N 2N 2N This type f cell divisin allws multicellular rganisms t grw and repair damaged tissue. Click here t g t the chapter n Mitsis. Summary f the Phases f Mitsis The drawings belw shw chrmsme mvement and alignment in a cell frm a species f animal that has a diplid number f 8. As yu view the drawings, keep in mind that humans have a diplid number f 46. Interphase (G 1 and G 2) Chrmsmes are nt easily visible because they are
Prphase The chrmsmes begin t cil. The spindle apparatus begins t frm as centrsmes Prmetaphase The nuclear membrane disintegrates. Kinetchres frm n the chrmsmes. Kinetchre micrtubules attach t the chrmsmes Metaphase The chrmsmes becme aligned n a plane.
Anaphase The chrmatids separate (The number f chrmsm Telphase The nuclear membrane reappears. The chrmsmes uncil. The spindle apparatus breaks dwn. The cell divides int tw. G 1 Interphase The chrmsmes have ne chrmatid.
G 2 Interphase The chrmsmes are replicated. Each ne has tw s chrmatids. What is Meisis? Meisis prduces daughter cells that have ne half the number f chrmsmes as the parent cell. 2N N Meisis enables rganisms t reprduce sexually. Gametes (sperm and eggs) are haplid. Meisis invlves tw divisins prducing a ttal f fur daughter cells. Click here t g t the chapter n meisis. Summary f the Phases f Meisis A cell underging meisis will divide tw times; the first divisin is meisis 1 and the secnd is meisis 2. The phases have the same names as thse f mitsis. A number indicates the divisin number (1st r 2nd): meisis 1: prphase 1, metaphase 1, anaphase 1, and telphase 1 meisis 2: prphase 2, metaphase 2, anaphase 2, and telphase 2 In the first meitic divisin, the number f cells is dubled but the number f chrmsmes is nt. This results in 1/2 as many chrmsmes per cell. The secnd meitic divisin is like mitsis; the number f chrmsmes des nt get reduced.
Prphase I Hmlgus chrmsmes becme paired. Crssing-ver ccurs between hmlgus chrmsmes. Crssing ver Metaphase I Hmlgus pairs becme aligned in the center f the cell. The randm alignment pattern is called independent assrtment. Fr example, a cell with 2N = 6 chrmsmes culd have any f the alignment patterns shwn at the left..
Anaphase I Hmlgus chrmsmes separate. Telphase I This stage is absent in sme species Interkinesis Interkinesis is similar t interphase except DNA synthesis des nt ccur. Prphase II
Metaphase II Anaphase II Telphase II Daughter Cells Mitsis 1. It ccurs in the vegetative r smatic cells. 2. After this divisin tw daughter cells are prduced. 3. Daughter cells prcess the same chrmsme number as that f parent cell. 4. It is f shrt duratin. 5. Daughter cells are similar t parent cell genetically. 6. There is n crssing ver in mitsis. 7. With the splitting f centmere, chrmatids are pulled apart twards the respective ples. Each chrmatid behaves as an independent chrmsme. 8. This is an equatinal divisin. 9. Hmlgus chrmsmes are nt arranged in pairs in the equatrial plate.
10. Genetic variatin des nt ccur between daughter cells. 11. DNA synthesis is cmpleted in the interphase. 12. In metaphase, the centrmeres are lined up n the equatrial plane and the arms extend int the eytplasm. Meisis 1. It ccurs in the reprductive cells. 2. Fur daughter cells are prduced after meltic divisin. 3. Chrmsme number f daughter cells is reduced t half. 4. It is f lng duratin. The cmplete prcess is divided in t tw divisins. 5. Daughter cells have genetic differences frm the parent cell. 6. Crssing ver takes place between the nnsister chrmatids. 7. Whle chrmsme mves apart in anaphase I f meisis because there is n splitting f centrmere. 8. In meisis, the first divisin is reductinal and the secnd ne is equatinal. 9. Hmlgus chrmsmes are arranged in pairs in the equatrial plate. 10. Exchange between maternal and paternal chrmsmal segments des nt render the daughter cell identical. 11. DNA synthesis is nt cmpleted in the interphase. 12. In metaphasei, the centrmeres f the hmlgus chrmsmes lie twards the tw ppsite ples and their arms extend twards the equatr. Regulatin f the Cell Cycle The cell cycle is cntrlled by a cyclically perating set f reactin sequences that bth trigger and crdinate key events in the cell cycle The cell-cycle cntrl system is driven by a built-in clck that can be adjusted by external stimuli (chemical messages) Checkpint - a critical cntrl pint in the cell cycle where stp and g-ahead signals can regulate the cell cycle Animal cells have built-in stp signals that halt the cell cycles and checkpints until verridden by g-ahead signals. Three Majr checkpints are fund in the G1, G2, and M phases f the cell cycle The G1 checkpint - the Restrictin Pint The G1 checkpint ensures that the cell is large enugh t divide, and that enugh nutrients are available t supprt the resulting daughter cells. If a cell receives a g-ahead signal at the G1 checkpint, it will usually cntinue with the cell cycle If the cell des nt receive the g-ahead signal, it will exit the cell cycle and switch t a nndividing state called G0 Actually, mst cells in the human bdy are in the G0 phase The G2 checkpint ensures that DNA replicatin in S phase has been cmpleted successfully. The metaphase checkpint ensures that all f the chrmsmes are attached t the mittic spindle by a kinetchre.
Cyclins and Cyclin-Dependent Kinases - The Cell-Cycle Clck Rhythmic fluctuatins in the abundance and activity f cell-cycle cntrl mlecules pace the events f the cell cycle. Kinase - a prtein which activates r deactivates anther prtein by phsphrylating them. Kinases give the g-ahead signals at the G1 and G2 checkpints The kinases that drive these checkpints must themselves be activated The activating mlecule is a cyclin, a prtein that derives its name frm its cyclically fluctuating cncentratin in the cell Because f this requirement, these kinases are called cyclin-dependent kinases, r Cdk's MPF - Maturatin Prmting Factr (M-phase prmting factr) Cyclins accumulate during the G1, S, and G2 phases f the cell cycle By the G2 checkpint (the red bar in the figure), enugh cyclin is available t frm MPF cmplexes (aggregatins f Cdk and cyclin) which initiate mitsis MPF apparently functins by phsphrylating key prteins in the mittic sequence Later in mitsis, MPF switches itself ff by initiating a prcess which leads t the destructin f cyclin Cdk, the nn-cyclin part f MPF, persists in the cell as an inactive frm until it assciates with new cyclin mlecules synthesized during interphase f the next rund f the cell cycle PDGF - Platelet-Derived Grwth Factrs - An Example f an External Signal fr Cell Divisin PDGF is required fr the divisin f fibrblasts which are essential in wund healing When injury ccurs, platelets (bld cells imprtant in bld cltting) release PDGF Fibrblasts are a cnnective tissue cells which pssess PDGF receptrs n their plasma membranes The binding f PDGF activates a signal-transductin pathway that leads t a prliferatin f fibrblasts and a healing f the wund Density Dependent Inhibitin Cells grwn in culture will rapidly divide until a single layer f cells is spread ver the area f the petri dish, after which they will stp dividing If cells are remved, thse brdering the pen space will begin dividing again and cntinue t d s until the gap is filled - this is knwn as cntact inhibitin Apparently, when a cell ppulatin reaches a certain density, the amunt f required grwth factrs and nutrients available t each cell becmes insufficient t allw cntinued cell grwth Anchrage Dependence Fr mst animal cells t divide, they must be attached t a substratum, such as the extracellular matrix f a tissue r the inside f the culture jar Anchrage is signaled t the cell-cycle cntrl system via pathways invlving membrane prteins and the cytskeletn Cells Which N Lnger Respnd t Cell-Cycle Cntrls - Cancer Cells Cancer cells d nt respnd nrmally t the bdy's cntrl mechanism.
They divide excessively and invade ther tissues If left unchecked, they can kill the rganism Cancer cells d nt exhibit cntact inhibitin If cultured, they cntinue t grw n tp f each ther when the ttal area f the petri dish has been cvered They may prduce required external grwth factr (r verride factrs) themselves r pssess abnrmal signal transductin sequences which falsely cnvey grwth signals thereby bypassing nrmal grwth checks Cancer cells exhibit irregular grwth sequences If grwth f cancer cells des cease, it des s at randm pints f the cell cycle Cancer cells can g n dividing indefinitely if they are given a cntinual supply f nutrients Nrmal mammalian cells grwing in culture nly divide 20-50 times befre they stp dividing