The Mitotic Cell Cycle

Functions of Cell Division Reproduction—some unicellular organisms divide to form duplicate offspring Growth—multicellular organisms grow and develop from single cell (fertilize egg) Repair—replace cells that die from normal wear & tear or accidents Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 12.1

Cell Division Genome—a cell’s total hereditary endowment of DNA –Genome is specific to species Human DNA extends about 3 meters, so how is it possible to copy all of it and ensure cells get even distribution? -DNA molecules are packaged into chromosomes which are more manageable

Every eukaryotic organism has a characteristic number of chromosomes –Human somatic cells (all body cells except reproductive cells) contain 46 chromosomes (23 pairs) –Human reproductive cells, gametes— sperm and egg cells—have 23 chromosomes Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 12.2

Chromosome Each duplicated chromosome consists of two sister chromatids which contain identical copies of the chromosome’s DNA. As they condense, the region where the strands connect shrinks to a narrow area, is the centromere. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 12.3

Mitotic Cell Cycle In a dividing cell, the mitotic phase (M) phase alternates with interphase, a growth period. –Mitotic phase— usually the shortest part of cell cycle –Interphase— accounts for –90% of the cycle

Interphase Subphases G1 phase (first gap)—cell grows by producing proteins and cytoplasmic organelles S phase (synthesis of DNA)—cell continues to grow as in G1 phase, while duplicating chromosomes G2 phase (second gap)—grows more as it completes preparations for cell division

Mitosis Prophase Metaphase Anaphase Telophase

G2 of Interphase Nucleus well-defined and bounded by nuclear envelope Contains one or more nucleoli. 2 centrosomes (with centriole pairs) visible Chromosomes duplicated –Still seen as chromatin (DNA + protein) –No individual chromosomes seen

Prophase Chromatin fibers become more tightly coiled, condensing into discrete chromosomes Nucleoli disappear Chromosomes appear as 2 identical sister chromatids joined together by centromere Mitotic spindle begins to form (made of microtubules), radiating from centrosomes Centrosomes move to opposite poles

Late Prophase/Prometaphase Nuclear envelope fragments-disintegrates Microtubules of spindle extend from poles and invade nucleus and interact with chromosomes Kinetochore forms on chromatids Some spindle fibers connect with kinetochores; some attach to opposite pole

Metaphase Centrosomes at opposite poles of cell Chromosomes convene on the metaphase plate Centromeres of all chromosomes are aligned with one another, and sister chromatids straddle metaphase plate Mitotic spindle completely formed

Anaphase Paired centromeres of each chromosome separate Each chromatid is now considered a full- fledged chromosome and move to opposite poles as kinetochore microtubules shorten

Telophase and Cytokinesis Nonkinetochore microtubules elongate the cell Daughter nuclei form at two poles of cell Nuclear envelopes arise from fragments of parent cell’s nuclear envelope and other portions of endomembrane system Chromatin fibers become less tightly coiled Cytokinesis—division of cytoplasm –Separate from mitosis –Formation of cleavage furrow, which pinches cell in two

Cytokinesis in Plants No cleavage furrow During Telophase, vesicles derived from Golgi apparatus move along microtubules to middle of cell producing cell plate Cell plate enlarges until its surrounding membrane fuses with the plasma membrane

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig left

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig right

The mitotic spindle = fibers composed of microtubules and associated proteins As the spindle assembles during prophase, the elements come from partial disassembly of the cytoskeleton. The spindle fibers elongate by incorporating more subunits of the protein tubulin. The mitotic spindle distributes chromosomes to daughter cells: a closer look Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Assembly of the spindle microtubules starts in the centrosome. –The centrosome (microtubule-organizing center) of animals has a pair of centrioles at the center Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 12.6a

As mitosis starts, the two centrosomes are located near the nucleus. As the spindle fibers grow from them, the centrioles are pushed apart. By the end of prometaphase they develop as the spindle poles at opposite ends of the cell. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Each sister chromatid has a kinetochore of proteins and chromosomal DNA at the centromere. The kinetochores of the joined sister chromatids face in opposite directions. During prometaphase, some spindle microtubules attach to the kinetochores. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 12.6b

When a chromosome’s kinetochore is “captured” by microtubules, the chromosome moves toward the pole from which those microtubules come. When microtubules attach to the other pole, this movement stops and a tug-of-war ensues. Eventually, the chromosome settles midway between the two poles of the cell, the metaphase plate. Other microtubules from opposite poles interact as well, elongating the cell. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

One hypothesis for the movement of chromosomes in anaphase is that motor proteins at the kinetochore “walk” the attached chromosome along the microtubule toward the opposite pole. –The excess microtubule sections depolymerize. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 12.7a

Experiments support the hypothesis that spindle fibers shorten during anaphase from the end attached to the chromosome, not the centrosome. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 12.7b

Nonkinetichore microtubules are responsible for lengthening the cell along the axis defined by the poles. –These microtubules interdigitate across the metaphase plate. –During anaphase motor proteins push microtubules from opposite sides away from each other. –At the same time, the addition of new tubulin monomers extends their length. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Onion Root Tip Rapidly dividing cells give us the opportunity to study the various stages of cell division. Notice that these plant cells, unlike animal cells, have cell walls. Bar = 100 Microns

30 microns

50 microns

Prokaryotes reproduce by binary fission, not mitosis. Most bacterial genes are located on a single bacterial chromosome which consists of a circular DNA molecule and associated proteins. While bacteria do not have as many genes or DNA molecules as long as those in eukaryotes, their circular chromosome is still highly folded and coiled in the cell. Mitosis in eukaryotes may have evolved from binary fission in bacteria Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

In binary fission, chromosome replication begins at one point in the circular chromosome, the origin of replication site. These copied regions begin to move to opposite ends of the cell. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig

The mechanism behind the movement of the bacterial chromosome is still an open question. –A previous hypothesis proposed that this movement was driven by the growth of new plasma membrane between the two origin regions. –Recent observations have shown more directed movement, reminiscent of the poleward movement of eukaryotic chromosomes. –However, mitotic spindles or even microtubules are unknown in bacteria. As the bacterial chromosome is replicating and the copied regions are moving to opposite ends of the cell, the bacterium continues to grow until it reaches twice its original size. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Cell division involves inward growth of the plasma membrane, dividing the parent cell into two daughter cells, each with a complete genome. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig

It is quite a jump from binary fission to mitosis. Possible intermediate evolutionary steps are seen in the division of two types of unicellular algae. –In dinoflagellates, replicated chromosomes are attached to the nuclear envelope. – In diatoms, the spindle develops within the nucleus. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

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