Concept 12.2: The mitotic phase alternates with interphase in the cell cycle

If one cell division takes 24 hours, how long does each phase occupy?

G1 - 5 - 6 hours (most variable in length) S - 10 - 12 hours (half the cycle) G2 - 4 - 6 hours M - 1 hour

Summarize G1 - M

G1 - cell grows S - continues to grow as it copies its chromosomes G2 - grows more as it completes preparations for cell division M - divides

Centrosome

In animal cells, the assembly of spindle microtubules starts at the centrosome, a sub-cellular region containing material that functions throughout the cell cycle to organize the cell's microtubules. The centrosome replicates during interphase, forming two centrosomes that migrate to opposite ends of the cell during prophase and prometaphase

Metaphase

The centrosomes are now at opposite poles of the cell The chromosomes have all arrived at the metaphase plate, a plane that is equidistant between the spindle's two poles. The chromosomes' centromeres lie at the metaphase plate. For each chromosome, the kinetochores of the sister chromatids are attached to kineotchore microtubules coming from opposite poles

Telophase

Two daughter nuclei form in the cell. Nuclear envelopes arise from the fragments of the parent cell's nuclear envelope and other portions of the endomembrane system Nucleoli reappear The chromosome becomes less condensed Any remaining spindle microtubules are depolymerized Mitosis, the division of one nucleus into two genetically identical nuclei, is now complete.

Cytokinesis in Plant Cells

Vesicles derived from the Golgi apparatus move along microtubules to the middle of the cell, where they coalesce, producing a cell plate. Cell wall materials carried in the vesicles collect inside the cell plate as it grows. The cell plate enlarges until its surrounding membrane fuses with the plasma membrane along the perimeter of the cell. Two daughter cells result, each with its own plasma membrane. Meanwhile, a new cell wall arising from the contents of the cell plate forms between the daughter cells.

What provides the material used to construct the spindle?

While the mitotic spindle assembles, the other microtubules of the cytoskeleton partially disassemble, providing the material used to construct the spindle.

Many of the events in mitosis depend on the _______ _______, which begins to form in the ______ during _______.

mitotic spindle; cytoplasm; prophase

What are the five stages of mitosis?

prophase, prometaphase, metaphase, anaphase, and telophase (cytokinesis overlaps with the latter stages of mitosis)

Interphase accounts for ____ % of the cell cycle. What three phases is it divided into?

90; G1, S, and G2

What happens during all three stages of interphase?

A cell grows by producing proteins and cytoplasmic organelles such as mitochondria and endoplasmic reticulum.

kinetochore

A complex of proteins associated with the centromere of a chromosome during cell division, to which the microtubules of the spindle attach.

G2 of Interphase

A nuclear envelope encloses the nucleus The nucleus contains one or more nucleoli Two centrosomes have formed by duplication of a single centrosome. Centrosomes are regions in animal cells that organize the microtubules of the spindle. Each centrosome contains two centrioles.

Kinetochore

A specialized protein structure at the centromere

Mitotic Spindle

A structure that consists of fibers made of microtubules and associated proteins.

aster

An aster (a radial array of short microtubules) extends from each centrosome.

metaphase plate

An imaginary plane perpendicular to the spindle fibers of a dividing cell, along which chromosomes align during metaphase.

Separase

Anaphase begins suddenly when the coheins holding together the sister chromatids of each chromosome are cleaved by an enzyme separase. Once separated, the chromatids become individual chromosomes that move toward opposite ends of the cell along the kinetochore microtubules

Anaphase

Anaphase is the shortest stage of mitosis, often lasting only a few minutes Anaphase begins when the cohesin proteins are cleaved. This allows the two sister chromatids of each pair to suddenly part. Each chromatid thus becomes an independent chromosome. The two new daughter chromosomes begin moving toward opposite ends of the cell as their kinetochore microtubules shorten. Because these microtubules are attached at the centromere region, the centromeres are pulled ahead of the arms, moving at a rage of about 1 um/min The cell elongates at the nonkinetochore microtubules lengthen By the end of anaphase, the two ends of the cell have equivalent and complete collections of chromosomes

What occurs during the S phase?

Duplication of chromosomes

What occurs during the G phases?

Intense metabolic activity and growth occur throughout these phases.

Mitotic (M) phase

Mitosis and cytokinesis occurs during this phase; the shortest part of the cell cycle

Prophase

The chromatin fibers become more tightly coiled, condensing into discrete chromosomes The nucleoli disappear Each duplicated chromosome appears as two identical sister chromatids joined at their centromeres and, in some species, all along their arms by cohesins (sister chromatid cohesion) The mitotic spindle (named for its shape) begins to form. It is composed of the centrosomes and the microtubules that extend from them The centrosomes move away from each other, propelled partly by the lengthening microtubules between them

Cytokinesis in Animal Cells

The division of the cytoplasm is usually well underway by late telophase, so the two daughter cells appear shortly after the end of mitosis. In animal cells, cytokinesis involves the formation of a cleavage furrow, which pinches the cell in two.

Prometaphase

The nuclear envelope fragments The microtubules extending from each centrosome can now invade the nuclear area The chromosomes have become even more condensed A kinetochore has now formed at the centromere of each chromatid (two per chromosome) Some of the microtubules attach to the kinetochores, becoming "kinetochore microtubules," which jerk the chromosomes back and forth

Asters

The radial arrays of shorter microtubules that extend from the centromeres.

How do spindle microtubules elongate and shorten?

The spindle microtubules elongate (polymerize) by incorporating more subunits of the protein tubulin and shorten (depolymerize) by losing subunits.