Physical Basis of Cell Division

CELL REPRODUCTION - "all living cells arise from pre-existing cells"

    METHODS of CELL REPRODUCTION               

        Fission - binary = 2 equal halves (bacteria)
                        unicellular organism divides into two or more
                        independently maturing daughter cells (see 17.2)

        Budding - (yeasts)                                       
                    bud…. an asexual reproductive structure,
                        in yeast or hydra... an outgrowth detached
                        that develops into a new individual.

Additional Methods of Cell Division

Sexual - Meiosis : produces sperm & egg

Asexual - Mitosis : makes identical copies of cells
               liver cells - only 1 time per year vs.  epithelial cells - usually once per day
       requirements     - must faithfully duplicate DNA
                               - separate duplicate chromosomes
                                  - double cell mass

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ASEXUAL CELL CYCLE - Life Cycle of a Cell        (see fig 17.1 & 17.4)

    [ 5 Stages ]  --->   G1 – S – G2    –    PMAT    –    cytokenesis
                                         interphase     –    mitosis   –    cytokenesis

                Cell Cycle times (see overhead)

Cell Cycle Stages                         

Interphase     - between successive divisions (3 parts)
                            G1, before DNA synthesis (S), & G2 period after
Cytokenesis    - physical division of cell into two parts
MITOSIS     - Nuclear Division Phase of the cell-
                            1st visible signs = condensation of chromatin

Genration time:    time from one cell division cycle to next
            often is species and environmentally dependent see fig

E.coli (basal media 370C)         45 min	Paramecium aurela                      6 hr
E.coli (enriched media 370C)     20 min	Allium cepa (onion root tip)           12 hr
sea urchin eggs (warm habitat)   30 min	Physarium polycephalum (euc)           8 hr
sea urchin eggs (cool habitat)    60 min	rat liver parenchymal cells             1x/yr
neurons (some muscle cells) - no division after maturity

  DNA Content during Mitosis & Meiosis

        Technique to Determine Duration of Cell Cycle Time

                PULSE-CHASE Experiment
                            - a group of synchronously dividing cell 
                                         given 1 hr pulse of radioactive thymidine 
                            - cells withdrawn @ intervals and
                            - a plot of… % labelled cells vs. time       

                DNA amounts per cell vs time mitosis & meiosis  
                             See handout  
                    

the Nuclear Division phase is…          Mitosis
                     1st visible sign   -->    the condensation of chromatin
      Stages of Mitosis         and some  Electron Micrographs

Interphase	DNA (chromatin) duplicates;   cell doubles in size
Prophase	chromatin condenses;   chromosomes become visible      - each homolog has 2 chromatids      - mitotic spindle forms from 2 centrosomes
Prometaphase	nuclear membrane disintegrates      - chromatids kinetochores bind to spindle MT
Metaphase	chromosome align at equator of cell       - homologs align independently of each other
Anaphase	chromatids of a homolog separate      - MT attached to kinetochore    (pull - push chromatids apart)
Telophase	2 sets chromosomes at now opposite poles      - nuclear envelope reforms       - chromosome may decondense
Cytokenisis	physical division of cell via contractile ring

Asexual Cell Reproduction...  Role of Cytoskeleton             
        CYTOSKELETON... is an intricate network of protein filaments 
             which runs throughout cytoplasm (euc's only) providing support... (16.1)
                     - dynamic - continuously reorganizes changing shape
                     - responsible for intracellular movement (organelles & chromosomes)
            made of 3 types of filaments 
               1) microtubules, 2) microfilaments, 3) intermediate filaments  - fig 16.2

        MICROTUBULES (MT's)
                - long, hollow-stiff tubes made of 2 globular proteins [ a -ß tubulin]
                - subunits linked by non-covalent forces into a ring of
                         13 parallel proto-filaments (alternating a -ß )    fig 16.9 
                - each end of MT's exhibits a polarity...
                             plus + end is alpha        &   minus - end is beta 
                - half of all tubulin is free in cytoplasm  &  half is complexed into MT's
                            results in dynamic assembly & disassembly process
        - MT's grow from one end (mostly + end)

    ...TO EFFICIENTLY SEPARATE REPLICATED CHROMOSOME
                requires 2 structures…         see fig 17.5

        1.  MITOTIC SPINDLE –  an apparatus of cytoskeletal elements...
                    an array of MT's   formed in late G2 after centrosome duplicates
                            one end of MT is anchored in centrosome
                            grows out & crosslinks to MT's of other centrosome

        kinetochore spindle fibers – attach centrosome to kinetochore        (17.9)
        polar spindle fibers – attach centrosome to medial region                (17.7)
        astral spindle fibers – unattached; radiate 360 ^o into space            (17.10)

2.  CONTRACTILE Ring - actin & myosin filaments responsible 
                                                        for cytokinesis                            (17.16)

    CENTROSOME is MTOC    -    MicroTubule Organizing Center

centrosome in animal cells - located near nucleus
        MTOC anchors (-) end of MT's, which grow outward            fig 16.10

g-tubulin (Ring Tubulin)... in MTOC… is origin site of new MT's
        contains centrioles - cylindrical array of MT's
        9 triplets of MT's, idential to basal body

Cilia & Flagella = 9 doublets around 2   (w dynein)

        MT growth from ring tubulin is dynamic ---> in/out
                 drug inhibited:   colchicine  figure - binds to free tubulin = no assembly
                                        taxol figure          - binds to MT's - favors disassembly

MT Dynamic Instability

    Alternating growth & shrinkage of MT's                 fig 16.12
             Tubulin possesses GTP hydrolytic activity [GTP --> GDP + P]
          Tubulin-GTP complex forms rapidly...   promotes growth at + end
              subunits can assemble faster than the GTP can be split...
              this GTP cap prevents depolymerizing, thus promotes growth of MT
              when tubulin hydrolyzes the GTP -->  tubulin-GDP becomes unstable
              tubulin-GDP disassembles and promotes autocatalytic shrinkage of MT

    Net Result: MTOC (centrosome) continually forms new MT's
            growth occurs in all direction (360 ^o)             fig 16.10
            until MT hits something - and attaches... leads to formation of cytoskeleton
            the framework for holding cell organelles & highway for intracellular movement             an analogy = flycasting for fish.          

Role of Mitotic Spindle                see panel 17.1

        MITOTIC SPINDLE:  [ fig ] 2 centrosomes each with a centriole, 
                    polar MT's, kinetochore MT's, & spindle (unattached) MT's

in late S – centrosome duplicates                panel 17.1.9    

in prophase –                                            panel 17.1.1            
        2 centrosomes begin to migrate to opposite poles
        MT's grow from each centrosome   at rates = up 20x faster than normal
         grow in all directions = spindle (astral) MT's
         MT's from each pole bind together forming polar MT's

        in prometaphase –                                    panel 17.1.2
                nuclear envelope disintegrates
                spindle MT's bind to kinetochore - (a protein complex found in centromere)

in Anaphase – (panel 17.1.4)
        proteolytic enzymes sever protein links between sister chromatids  (fig17.6)
        chromatids are pulled apart toward poles by MT's of mitotic spindle
                            rate = 1 um per min

2 forces at work here….   (fig 17.13)

ANAPHASE A - kinetochore MT shorten     [G. Borisy exp - fluorescent quench]
        depolymerize @ kinetochore end [fig] ... moves chromatids poleward
        role of kinetochore & motor proteins that "walk" chromosome down MT's

ANAPHASE B - spindle poles move apart
            push - elongation of polar MT's (polymerize @ + end)
            pull - unattached MT's pull pole toward cell outer cortex

Cytokenesis...    Defined as : the clevage of cytoplasm into 2 halves

ANIMAL cells - cell membrane furrows around cell's equator
                         perpendicular to long axis of mitosis spindle     fig17.15 

        CONTRACTILE RING             fig 17.16
                - overlapping arrays of actin/myosin proteins
                - actin filaments slide against myosin as in MUSCLE
                - RING becomes smaller as it closes...  like a camera diaphram
                - ultimately contractile ring disassembles

PLANT cells - mitotic spindle & polar MT's     fig 17.18

        form a PHRAGMOPLAST at equator of cell
                Golgi vesicles filled with cell wall polysaccharide &
                        glycoprotein accumulate at center 
                forms new cell wall - grows outward to plasma membrane

  MEIOSIS – sexual cell nuclear division  -  (1883, Gk - diminution, lessening)
                converts a diploid cell (2n) into a haploid gametes (1n)
                a special cell division, which halves chromosome number

       Primary Diffrences : compared with mitosis   
            Maternal & Paternal homologs pair forming bivalents of 4 chromatids
                    (in mitosis homologs act independently )                  fig 17.21

            Synaptonemal complex (EM's) - protein core that links homologs together 
                    allows recombination to occur (cross-over exchange)

Where?   does meiosis occur in sexual cell reproduction  [ life cycle ]
                    fertilization --->  diploid (chrm # = 46) 
                    meiosis --------->  haploid (half chrm # = 23)

    Stages of Sexual Cell Division

nomenclature is same as asexual division   ( interphase, nuclear, cytokinesis )
        but, 2 Divisions...  Meiosis I   &   Meiosis II
                                     1 cell = 2 cell = 4 cells

Stages are similar  &   have analogous functions 
    Prohase I...        chromosomes condense
        composite:     leptotene, zygotene, pachytene, diplotene, diakinesis
        drawings:      leptotene,   zygotene,   pachytene,   diplotene, & diakinesis
    Metaphase I..     chromosomes align at equator
            homologs PAIR together   and  crossover exchange (chiasmata) is complete
    Anaphase I...      chromosomes migrate toward poles
    Telophase I...     chromosome at poles    meiosis drawing

Summary of Meiosis

Meiosis I        --->   separates homologs of homologous pair
                                   daughter cells diploid, but not genetically same
Meiosis II      ---->  just like mitosis ---> separates chromatids
                                    of one homolog of a homologous pair

Significant Events of Meiosis -        see fig 17.23 p566
        Independent Assortment - random alignment homologs
        Crossing Over - exchange of chromosome material

        Summary of major meiotic events : 
                1. Nuclear division phase of sexual cell reproduction
                2. Two successive divisions, results in 4 daughter cells...  Meiosis 1 & Meiosis 2
                3. Reduction/division occurs... cells halve the number of parent cell chromosomes
                            diploid ---- > haploid

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