The Cell Cycle and Mitosis

The Cell Cycle and Mitosis • The eukaryotic cell cycle • Regulation of the cell cycle • Mitosis: an overview • Stages of mitosis • The spindle apparatus • Cytokinesis

A. The Eukaryotic Cell Cycle • Eukaryotic cell cycle: • A series of stages through which most eukaryotic cells pass during some time in their development • In single-celled eukaryotes, the cell cycle often represents the major mechanism for asexual reproduction of the species • In multicellular eukaryotes, the cell cycle is used for cellular reproduction in embryonic cells and stem cells

A. The Eukaryotic Cell Cycle • Key events of the cell cycle • Interphase: a period of extensive metabolic & synthetic activity • During this time the DNA (chromosomes) of the cell are replicated • Mitosis: the nucleus of the cell divides to form two identical nuclei, with each new nucleus having the same number of chromosomes as the parent nucleus • Cytokinesis: in most cases (with some exceptions), the cell’s cytoplasm divides to form two cells, each containing one of the new nuclei

A. The Eukaryotic Cell Cycle • Stages of the cell cycle • G1 (gap-1) is the period before chromosomal replication takes place • S (synthesis) is the period when the chromosomes are replicated • G2 (gap-2) is the period following chromosomal replication • M (mitosis) is the period of nuclear division

A. The Eukaryotic Cell Cycle • G1, S, and G2 are collectively called “interphase,” characterized by: • The presence of a well-defined nucleus • Individual chromosomes are not visible • Chromatin: chromosomes are dispersed in an uncondensed state called chromatin; “Grainy” or “sandy ” appearance • Nucleolus: a dark-staining spot sometimes (but not always) seen in the interphase nucleus; The site of ribosome synthesis

B. Regulation of the Cell Cycle • In multicellular eukaryotes, dividing cells are found: • In embryonic tissue; • In certain tissues that are constantly renewed, such as bone marrow stem cells or plant meristems; • In damaged tissue, such as cell division in damaged liver

B. Regulation of the Cell Cycle • Go (stationary) cells: cells that have reached terminal differentiation and no longer divide • Cells stop dividing before they enter “S” phase; Therefore, Go is similar in some respects to G1 except that the cell does not enter “S” phase • The cell cycle is regulated by several types of regulatory proteins that act at two major points: the G1 – S junction, and the G2 – M junction

C. Mitosis: An Overview • During G1 each chromosome is uncondensed and in an unreplicated state.

C. Mitosis: An Overview • In the S phase, each chromosome is replicated. The replicated chromosome consists of two sister chromatids attached at the centromere.

C. Mitosis: An Overview • At the beginning of mitosis, the chromosomes condense and become visible. Note that the chromosome is still in a replicated state, with a pair of chromatids attached at the centromere.

C. Mitosis: An Overview • During mitosis: the centromere splits, the chromatids separate and become daughter chromosomes, and the daughter chromosomes move to opposite sides of the cell.

C. Mitosis: An Overview • At the end of mitosis, the chromosomes on each side of the cell decondense and form two new, identical nuclei.

D. Stages of Mitosis • Prophase: • The nuclear membrane and nucleolus disperse • The chromatin condenses to form visible chromosomes • The mitotic spindle apparatus begins to form

D. Stages of Mitosis • Metaphase • The chromosomes line up along the equator of the spindle apparatus • Some writers use the term “prometaphase” to represent the transitional state between prophase & metaphase • Anaphase • The centromeres split and the daughter chromosomes move to opposite poles of the spindle apparatus

D. Stages of Mitosis • Telophase • At each pole of the spindle, the chromosomes decondense and return to the state of chromatin • New nuclear membranes and nucleoli form at each spindle pole, completing the formation of two new identical nuclei

E. The Spindle Apparatus • Composed of microtubules: fibers that are dynamically assembled from the protein tubulin • Centrosomes: • Found at the poles of the spindle in animal cells • Each centrosome consists of two centrioles: short tubulin structures that help to regulate tubulin polymerization • In plant cells: plant cells have “microtubule-organizing centers” (instead of centrosomes) at each pole

E. The Spindle Apparatus • Some fibers are attached to chromatids • Each attached fiber is connected to the kinetochore of the chromatid: a region near (or at) the centromere • The movement of chromatids is accomplished by shortening of the kinetochore fibers • This is done by depolymerization of the microtubules at the spindle poles

E. The Spindle Apparatus • Chemicals that inhibit microtubule formation (such as colchicine) cause mitosis to stop at metaphase • Colchicine and similar chemicals are used to arrest mitosis at metaphase for the purpose of chromosomal analysis (karyotyping)

F. Cytokinesis • In animal cells: • Cytokinesis is accomplished by the formation of the cleavage furrow • A contractile ring or “girdle” of actin fibers and myosin (a transient muscle-like assembly) forms just inside the plasma membrane, around the equatorial plate • The contractile ring pulls the plasma membrane together to pinch the cell into two cells

F. Cytokinesis • In plant cells: • Cytokinesis is accomplished by the formation of the cell plate • Cell plate: A new cell wall forming at the equatorial plate • The cell plate enlarges until it completely separates the cell into two cells

The Cell Cycle and Mitosis