MITOSIS Nuclear Cell Division

Parts of a chromosome 4 Arms 

Mitosis has 4 (5?) phases 1. INTERPHASE: the chromatin amount doubles. Interphase has been called the “resting phase” because you don’t see anything happening. Interphase happens during three segments 1. G1: the period of time before the chromatin doubles 2. S: period of time during which the chromatin doubles 3. G2: period of time after the chromatin doubles but before it condenses

2. PROPHASE: A. the nucleoli disappear B. the chromatin condenses into visible chromosomes C. Centrioles move toward poles as the mitotic spindle forms

Prometaphase Between prophase & metaphase Nuclear envelope fragments Chromosomes have kinetochores form Some microtubules connect to kinetochores

3. METAPHASE chromosomes line up along the equator (middle of the cell)

4. ANAPHASE Spindle fibers pull the chromotids apart from the centromere and continue to pull individual chromosomes toward each pole of the cell.

As anaphase occurs the kinetochore microtubules shorten at their kinetochore ends, not at their spindle pole ends. Thus the chromosome tracts along a microtubule as the microtubule depolymerizes at its kinetochore end.

5. TELOPHASE A. The chromosomes reach the poles B. The nuclear membrane reforms C. Cytokinesis occurs (the cytoplasm splits separating the cell into two “daughter” cells)

There are three major checkpoints during the cell cycle which are critical control points where stop and go-ahead signals can regulate the cycle. The most important one in mammal cells is the G1. If the go-ahead signal is not received the cell will switch to a G0 inactive stage.

Protein kinases (enzymes) give the go ahead signals at the checkpoints Protein kinases (enzymes) give the go ahead signals at the checkpoints. In order to be active kinases must attach to a cyclin protein. These are known as cyclin dependent kinases or CDKs. CDKs function by phosphorylating substrate proteins that then affect steps in the cell cycle. The M checkpoint holds between metaphase and anaphase to make sure the kinetochores are all attached to the spindle before proceeding.

Cyclin (a protein) is synthesized during the S phase Cyclin (a protein) is synthesized during the S phase. This cyclin then combines with CDK to produce a molecule known as MPF (maturation promoting factors). MPF causes phosphorylation of various proteins and results in: fragmentation of nuclear envelope, chromosome condensation, spindle fiber formation as well as the beginning and ending of mitotic phases.

Rate of mitosis is controlled by both internal and external signals. Internal signals include proteins known as growth factors, more than 50 of which are known. When growth factors are produced, they stimulate mitosis. External signals include “density dependent inhibition” which cause crowded cells to stop dividing. Most cells will grow to form a single solid layer with is attached to a substratum (anchorage dependence). If a cell is bound to surface proteins of its neighbor a growth inhibiting signal is sent and the cell cycle comes to a halt.

In plant cells there is no pinching in of the cytoplasm due to the cell wall. Instead a cell plate develops from vesicles derived from the Golgi apparatus that splits the cell into two cells during telophase. Plant cells also lack centrioles so other microtubules aid the division of chromatids during mitosis.

Karyotypes

centromere chromosome Nuclear memebrane

centromere telophase chromosome Nuclear memebrane prophase

Nuclear membrane telophase centromere anaphase chromosome