BIOLOGY 101 CHAPTER 12: The Cell Cycle: The Key Rles f Cell Divisin
The Key Rles f Cell Divisin CONCEPTS: 12.1 Mst cell divisin results in genetically identical daughter cells 12.2 The mittic phase alternates with interphase in the cell cycle 12.3 The eukarytic cell cycle is regulated by a mlecular cntrl system Can yu describe r explain these features?
The Key Rles f Cell Divisin 12.1 Mst cell divisin results in genetically identical daughter cells In bth prkarytes and eukarytes, mst cell divisin invlves the distributin f identical genetic material DNA t tw daughter cells. The exceptin is meisis, the special type f eukarytic cell divisin that can prduce sperm and eggs. There is a high level f fidelity with which DNA is passed alng, withut dilutin, frm ne generatin t the next. A dividing cell duplicates its DNA, allcates the tw cpies t ppsite ends f the cell, and nly then splits int daughter cells
The Key Rles f Cell Divisin 12.1 Mst cell divisin results in genetically identical daughter cells The prcess f smatic cell divisin invlves three primary events: 1. DNA Replicatin 2. Srting f Chrmsmes int new nuclei 3. Cell divisin
The Key Rles f Cell Divisin 12.1 Mst cell divisin results in genetically identical daughter cells A cell s genetic infrmatin, packaged as DNA, is called its genme. In prkarytes, the genme is ften a single lng circular DNA mlecule. In eukarytes, the genme cnsists f a number f DNA mlecules (chrmsmes).
The Key Rles f Cell Divisin 12.1 Mst cell divisin results in genetically identical daughter cells A human cell must cpy r replicate abut 2m f DNA and separate the tw cpies s that each daughter cell ends up with a cmplete genme. DNA mlecules are packaged int chrmsmes. Every eukarytic chrmsme cnsists f ne lng, linear DNA mlecule assciated with many prteins called histnes.
The Key Rles f Cell Divisin 12.1 Mst cell divisin results in genetically identical daughter cells A human cell must cpy r replicate abut 2m f DNA and separate the tw cpies s that each daughter cell ends up with a cmplete genme. DNA mlecules are packaged int chrmsmes. Every eukarytic chrmsme cnsists f ne lng, linear DNA mlecule assciated with many prteins called histnes. Histnes wrapped in DNA are called nuclesmes Tgether, the cmplex is referred t as chrmatin. The prteins maintain the structure f the chrmsme and help cntrl the activity f the genes.
The Key Rles f Cell Divisin 12.1 Mst cell divisin results in genetically identical daughter cells A human cell must cpy r replicate abut 2m f DNA and separate the tw cpies s that each daughter cell ends up with a cmplete genme. DNA mlecules are packaged int chrmsmes. Every eukarytic chrmsme cnsists f ne lng, linear DNA mlecule assciated with many prteins called histnes. Histnes wrapped in DNA are called nuclesmes Regins f DNA that will be expressed have fewer nuclesmes than ther areas, allwing greater access fr building prteins
The Key Rles f Cell Divisin 12.1 Mst cell divisin results in genetically identical daughter cells Befre a cell can divide int tw daughter cells the DNA must be replicated Semicnservative DNA Replicatin creates identical cpies f chrmsmes
The Key Rles f Cell Divisin 12.1 Mst cell divisin results in genetically identical daughter cells Befre a cell can divide int tw daughter cells the DNA must be replicated Semicnservative DNA Replicatin creates identical cpies f chrmsmes After a cell duplicates all its chrmsmes, it must srt them int the tw daughter cells
The Key Rles f Cell Divisin 12.1 Mst cell divisin results in genetically identical daughter cells Each eukarytic species has a characteristic number f chrmsmes in each cell nucleus. Human smatic cells (all bdy cells except sperm and egg) have 46 chrmsmes, made up f tw sets (2N) f 23 - ne set frm each parent. Human reprductive cells r gametes (sperm r eggs) have ne set f 23 chrmsmes, half the number in a smatic cell. (1N) The number f chrmsmes in smatic cells varies widely amng species
Mst cell divisin results in genetically identical daughter cells 12.1 Chrmsmes are distributed during eukarytic cell divisin When a cell is nt dividing, each chrmsme is in the frm f a lng, thin chrmatin fiber.
Mst cell divisin results in genetically identical daughter cells 12.1 Chrmsmes are distributed during eukarytic cell divisin When a cell is nt dividing, each chrmsme is in the frm f a lng, thin chrmatin fiber. Befre cell divisin but after DNA replicatin, the chrmatin cndenses, ciling and flding t make a smaller package that is easier t manage and srt
Mst cell divisin results in genetically identical daughter cells 12.1 Chrmsmes are distributed during eukarytic cell divisin Humans are diplid, meaning that they have tw cpies f each chrmsme ne set cming frm each parent Theses sets frm each parent are called Hmlgus Chrmsmes. They are each parent s versin f the particular chrmsme. Humans have 23 sets f chrmsmes (46 ttal)
Mst cell divisin results in genetically identical daughter cells 12.1 Chrmsmes are distributed during eukarytic cell divisin Humans are diplid, meaning that they have tw cpies f each chrmsme ne set cming frm each parent Theses sets frm each parent are called Hmlgus Chrmsmes. They are each parent s versin f the particular chrmsme. Humans have 23 sets f chrmsmes (46 ttal) When chrmsmes are replicated, they are physically jined tgether at the centrmere
Mst cell divisin results in genetically identical daughter cells 12.1 Chrmsmes are distributed during eukarytic cell divisin Sister Chrmatids are identical cpies f a chrmsme made during DNA replicatin Centrmere is the part f the chrmsme where sister chrmatids are linked; the kinetchre is assciated with the centrmere Kinetchre is a prtein structure that mittic spindles cnnect with t pull sister chrmatids apart (they *cnnect* the spindles t the *cre* and are assciated with the centrmere)
Mst cell divisin results in genetically identical daughter cells 12.1 Chrmsmes are distributed during eukarytic cell divisin Sister Chrmatids are identical cpies f a chrmsme made during DNA replicatin The duplicated chrmatids are initially attached alng their lengths by prtein cmplexes called chesins. This attachment is knwn as sister chrmatid chesin. Each centrmere is a specialized regin f ne chrmatid with specific DNA sequences. An unduplicated chrmsme has a single centrmere, distinguished by the prteins that bind there, and tw arms.
Mst cell divisin results in genetically identical daughter cells 12.1 Chrmsmes are distributed during eukarytic cell divisin Mittic spindles are the fibers (micrtubules) made during mitsis that pull sister chrmatids apart. Centrsme is made up tw centriles and is the anchr fr the mittic spindles which are created. (It is the central bdy frm which the spindle riginates) Centrile is a small set f micrtubules that make up the centrsme which help t create mittic spindles fr cell divisin.
Mst cell divisin results in genetically identical daughter cells 12.1 Chrmsmes are distributed during eukarytic cell divisin Chrmsmes (Hmlgus Chrmsmes and Sister Chrmatids) Centrmeres Kinetchre Mittic Spindles Centrsme Centriles
The Key Rles f Cell Divisin 12.2 The mittic phase alternates with interphase in the cell cycle The MITOTIC (M) PHASE f the cell cycle, which includes mitsis and cytkinesis, alternates with the much lnger INTERPHASE. Interphase accunts fr abut 90% f the cell cycle. Interphase has three subphases: the G 1 phase ( first gap ), the S phase ( synthesis ), and the G 2 phase ( secnd gap ). During all three subphases, a cell that will eventually divide grws by prducing prteins and cytplasmic rganelles such as mitchndria and endplasmic reticulum. Chrmsmes are duplicated nly during the S phase
The Key Rles f Cell Divisin 12.2 The mittic phase alternates with interphase in the cell cycle A typical human cell might divide nce every 24 hurs. Of this time, the M phase wuld last less than an hur and the S phase might take 10 12 hurs, r half the cycle. The rest f the time wuld be divided between the G 1 and G 2 phases.
The Key Rles f Cell Divisin 12.2 The mittic phase alternates with interphase in the cell cycle Fr cnvenience, MITOSIS is usually divided int five subphases: prphase, prmetaphase, metaphase, anaphase, and telphase. CYTOKINESIS, verlapping with the latter stages, cmpletes the mittic phase.
The Key Rles f Cell Divisin 12.2 The mittic phase alternates with interphase in the cell cycle Fllwing Cytkinesis, new daughter cells immediately enter Interphase (G 1 ) In INTERPHASE (G 1 ), the cell is nt yet respnding t signals t begin dividing During this phase, cells carry ut their nrmal peratins During INTERPHASE (S), the cell duplicates cellular rganelles and structures Chrmsmes are duplicated using DNA Replicatin This frms tw Sister Chrmatids
The Key Rles f Cell Divisin 12.2 The mittic phase alternates with interphase in the cell cycle In late INTERPHASE (G 2 ), the chrmsmes have been duplicated but are nt cndensed. A nuclear membrane still exists. The centrsme has replicated t frm tw centrsmes. In animal cells, each centrsme features tw centriles
The Key Rles f Cell Divisin 12.2 The mittic phase alternates with interphase in the cell cycle In PROPHASE, the chrmsmes are tightly ciled, with sister chrmatids jined tgether. Chrmsmes (Sister Chrmatids) begin t cndense The mittic spindle begins t frm. It is cmpsed f centrsmes and the micrtubules that extend frm them. The centrsmes mve away frm each ther
The Key Rles f Cell Divisin 12.2 The mittic phase alternates with interphase in the cell cycle During PROMETAPHASE, the nuclear envelpe begins t break dwn Spindle micrtubules attach t the centrmere regin f sister chrmatids A kinetchre (prteins within a centrmere) is the actual site f attachment fr spindle micrtubules The spindle begins t push sister chrmatids twards the center f the cell
The Key Rles f Cell Divisin 12.2 The mittic phase alternates with interphase in the cell cycle
The Key Rles f Cell Divisin 12.2 The mittic phase alternates with interphase in the cell cycle The spindle fibers push the sister chrmatids until they are all arranged at the metaphase plate, an imaginary plane equidistant frm the ples, defining METAPHASE.
The Key Rles f Cell Divisin 12.2 The mittic phase alternates with interphase in the cell cycle At ANAPHASE, the centrmeres divide, separating the sister chrmatids. Each chrmatid is pulled tward the ple t which it is attached by spindle fibers. By the end, the tw ples have equivalent cllectins f chrmsmes.
The Key Rles f Cell Divisin 12.2 The mittic phase alternates with interphase in the cell cycle At TELOPHASE, daughter nuclei begin t frm at the tw ples. Nuclear envelpes arise frm the fragments f the parent cell s nuclear envelpe and ther prtins f the endmembrane system. The chrmsmes becme less tightly ciled and unpack. The spindle begins t break dwn
The Key Rles f Cell Divisin 12.2 The mittic phase alternates with interphase in the cell cycle CYTOKINESIS, divisin f the cytplasm, is usually well under way by late telphase. In animal cells, cytkinesis invlves the frmatin f a cleavage furrw, which pinches the cell in tw. In plant cells, vesicles derived frm the Glgi apparatus prduce a cell plate at the middle f the cell
The Key Rles f Cell Divisin 12.2 The mittic phase alternates with interphase in the cell cycle
The mittic phase alternates with interphase in the cell cycle 12.2 The mittic spindle distributes chrmsmes t daughter cells: a clser lk The mittic spindle, fibers cmpsed f micrtubules and assciated prteins, is a majr driving frce in mitsis. As the spindle assembles during prphase, the elements cme frm partial disassembly f the ther micrtubules f the cytskeletn. The spindle fibers elngate (plymerize) by incrprating mre subunits f the prtein tubulin and shrten (deplymerize) by lsing subunits
The mittic spindle distributes chrmsmes t daughter cells: a clser lk 12.2 Cytkinesis divides the cytplasm: a clser lk Cytkinesis, divisin f the cytplasm, typically fllws mitsis. In animal cells, cytkinesis ccurs by a prcess called cleavage The first sign f cleavage is the appearance f a cleavage furrw in the cell surface near the ld metaphase plate. On the cytplasmic side f the cleavage furrw is a cntractile ring f actin micrfilaments assciated with mlecules f the mtr prtein mysin. Cntractin f the ring pinches the cell in tw
REVIEW: Duplicate, Allcate, Separate Interphase: G 1 S G 2 M Phase: Prphase Prmetaphase Metaphase Anaphase Telphase Cytkinesis
REVIEW: Duplicate, Allcate, Separate Interphase: G 1 Fllws cytkinesis; Nrmal cell peratins; Cells signaled t divide S Organelles and chrmsmes duplicated (Sister Chrmatids) G 2 Cell prepares t enter M Phase M Phase: Prphase Chrmsmes cndense; Spindle frms; Centrsmes separate Prmetaphase Spindle attaches t Sister Chrmatids; Nuclear envelpe breaks dwn Metaphase Spindle aligns sister chrmatids at metaphase plate Anaphase Spindle separates sister chrmatids Telphase Cleavage furrw frms; Nuclear envelpe refrms; Spindle breaks dwn Cytkinesis Tw identical daughter cells are created
REVIEW: Duplicate, Allcate, Separate Interphase: G 1 S DUPLICATE Chrmsmes G 2 M Phase: Prphase Prmetaphase Metaphase Anaphase ALLOCATE Chrmsmes Telphase Cytkinesis SEPARATE int identical Daughter Cells
The Key Rles f Cell Divisin 12.3 The eukarytic cell cycle is regulated by a mlecular cntrl system The timing and rate f cell divisin in different parts f an animal r plant are crucial fr nrmal grwth, develpment, and maintenance. The frequency f cell divisin varies with cell type. Sme human cells divide frequently thrughut life (skin cells). Others human cells have the ability t divide but keep it in reserve (liver cells). Mature nerve and muscle cells d nt appear t divide at all after maturity. Investigatin f the mlecular mechanisms regulating these differences prvides imprtant insights int the peratin f nrmal cells and may als explain hw cancer cells escape cntrls
The eukarytic cell cycle is regulated by a mlecular cntrl system 12.3 Cytplasmic signals drive the cell cycle Fusin f an S phase cell and a G 1 phase cell induces the G 1 nucleus t start S phase Additinally, Fusin f a cell in interphase (even G1 phase) with a cell in Mitsis induces the interphase cell t enter mitsis QUESTION: What des this evidence suggest regarding cell cycle cntrl? ANSWER: The cell cycle appears t be driven by specific chemical signals present in the cytplasm.
The eukarytic cell cycle is regulated by a mlecular cntrl system 12.3 Cytplasmic signals drive the cell cycle The sequential events f the cell cycle are directed by a distinct cell cycle cntrl system. Cyclically perating mlecules trigger and crdinate key events in the cell cycle. A checkpint in the cell cycle is a cntrl pint where stp and g-ahead signals regulate the cycle. The signals are transmitted within the cell by signal transductin pathways.
The eukarytic cell cycle is regulated by a mlecular cntrl system 12.3 Cytplasmic signals drive the cell cycle The sequential events f the cell cycle are directed by a distinct cell cycle cntrl system. Cyclically perating mlecules trigger and crdinate key events in the cell cycle. A checkpint in the cell cycle is a cntrl pint where stp and g-ahead signals regulate the cycle. The signals are transmitted within the cell by signal transductin pathways. Animal cells generally have built-in stp signals that halt the cell cycle at checkpints until they are verridden by g-ahead signals. Checkpints als register signals frm utside the cell
The eukarytic cell cycle is regulated by a mlecular cntrl system 12.3 Cytplasmic signals drive the cell cycle Three majr checkpints are fund in the G 1, G 2, and M phases. Fr many cells, the G 1 checkpint, the restrictin pint in mammalian cells, is the mst imprtant. If the cell receives a g-ahead signal at the G 1 checkpint, it usually cmpletes the cell cycle and divides.
The eukarytic cell cycle is regulated by a mlecular cntrl system 12.3 Cytplasmic signals drive the cell cycle If the cell des nt receive a g-ahead signal, the cell exits the cycle and switches t a nndividing state, the G 0 phase S what is the difference between G 1 and G 0? G 1 is the nrmal state fr actively dividing cells G 0 is where mst adult cells exist in the human bdy. This is where quiescent cells reside that are nt destined fr cell divisin Liver cells can be called back t the cell cycle by external cues, such as grwth factrs released during injury. Highly specialized nerve and muscle cells never divide and remain in a G 0 state
Cytplasmic signals drive the cell cycle 12.3 The cell cycle clck is regulated by cyclins and cyclin-dependent kinases Specialized cntrl mlecules pace the events f the cell cycle. These regulatry mlecules are mainly prteins f tw types: prtein kinases and cyclins Prtein kinases are enzymes that activate r inactivate ther prteins by phsphrylating them. Specific prtein kinases give the g-ahead signals at the G 1 and G 2 checkpints The kinases that drive the cell cycle are present at cnstant cncentratins but require the assciatin f a secnd prtein, a cyclin, t becme activated. Because f the requirement fr binding f a cyclin, the kinases are called cyclin-dependent kinases, r CDKs CDK s carry ut different functins in the cell cycle depending n which cyclin they are assciated with
Cytplasmic signals drive the cell cycle 12.3 The cell cycle clck is regulated by cyclins and cyclin-dependent kinases Cyclins can activate CDK s (specifically, CDK1) When CDK-1 assciates with Cyclins it is called MPF (Mitsis Prmting Factr) G1-Cyclin assciates with CDK1 t bypass the G1 Restrictin Pint
Cytplasmic signals drive the cell cycle 12.3 The cell cycle clck is regulated by cyclins and cyclin-dependent kinases Cyclins activate CDK s (specifically, CDK1) When CDK-1 assciates with Cyclins it is called MPF (Mitsis Prmting Factr) G1-Cyclin assciates with CDK1 t bypass the G1 Restrictin Pint In S-Phase, G1-Cyclin is brken dwn Late in G2, M-Cyclin frms Active MPF, which allws the cell t prgress thrugh the M-Phase The active MPF allws the cell t cmplete mitsis, but als deactivates itself by breaking dwn M Cyclins
Cytplasmic signals drive the cell cycle 12.3 The cell cycle clck is regulated by cyclins and cyclin-dependent kinases Because MPF breaks dwn M-Cyclins it deactivates itself befre entering back int G 1 The nn-cyclin part f MPF, CDK1, persists in the cell until it assciates with new cyclin mlecules synthesized during the G 1 phase QUESTION: What wuld happen if MPF didn t break dwn M-cyclins? ANSWER: MPF wuld remain active and cells wuld cnstantly vercme their G 1 restrictin and cntinually divide
The cell cycle clck is regulated by cyclins and cyclin-dependent kinases 12.3 Internal and external cues help regulate the cell cycle The M phase checkpint ensures that the kinetchres f all the chrmsmes are prperly attached t the spindle at the metaphase plate befre anaphase This is nt regulated by a cyclin-cdk cmplex but an enzyme cmplex called separase Separase cleaves the chesins and allws the sister chrmatids t separate. This blck ensures that daughter cells d nt end up with missing r extra chrmsmes. QUESTION: What wuld happen if separase failed? ANSWER: Sister Chrmatids wuld never separate and the cell wuld freeze in metaphase
Internal and external cues help regulate the cell cycle 12.3 Cancer cells have escaped frm cell cycle cntrls Cells in culture that acquire the ability t divide indefinitely are said t have undergne transfrmatin, the prcess that causes them t behave like cancer cells. Nrmally, the immune system recgnizes and destrys transfrmed cells.
Internal and external cues help regulate the cell cycle 12.3 Cancer cells have escaped frm cell cycle cntrls Cancer cells that prliferate can frm a tumr, a mass f abnrmal cells. If the abnrmal cells remain at the riginating site, the lump is called a benign tumr. Mst benign tumrs d nt cause serius prblems and can be fully remved by surgery A malignant tumr includes cells whse genetic and cellular changes enable them t spread t new tissues and impair the functins f ne r mre rgans. An individual with a malignant tumr is said t have cancer.
Internal and external cues help regulate the cell cycle 12.3 Cancer cells have escaped frm cell cycle cntrls Cancer cells are abnrmal in many ways. Cancer cells may have an unusual number f chrmsmes, their metablism may be disabled, and they may secrete signal mlecules that cause bld vessels t grw tward the tumr. Cancer cells ften lse their attachment t nearby cells, are carried by the bld and lymph system t ther tissues, and start mre tumrs in an event called metastasis.
Internal and external cues help regulate the cell cycle 12.3 Cancer cells have escaped frm cell cycle cntrls Treatments fr metastasizing cancers include high-energy radiatin and chemtherapy with txic drugs. These treatments target actively dividing cells. Chemtherapeutic drugs interfere with specific steps in the cell cycle. Fr example, Taxl prevents micrtubule deplymerizatin, preventing cells frm prceeding past metaphase The side effects f chemtherapy are due t the drug s effects n nrmal cells. Fr example, nausea results frm chemtherapy s effects n intestinal cells, hair lss results frm its effects n hair fllicle cells, and susceptibility t infectin results frm its effects n immune system cells.