Mitotic Phase of Cell Cycle

Mitotic Phase

• Recall that the M Phase of the cell cycle can be subdivided into mitosis and cytokinesis.
• The first portion is also called karyokinesis, which literally just means division of the nuclear contents.
• The second portion, cytokinesis, means separation of the cytoplasmic components into two daughter cells.
• Most of the cell division in our bodies occurs through mitosis.
• Mitosis is for growth, and to replace old and dying cells.

Phases of Mitosis

1. Prophase

• Chromosomes start to condense.
• Centrosome begins to form the mitotic spindle structure composed of microtubules which organize chromosomes and move them to the appropriate locations during mitosis.
• The spindle gets longer as the centrosomes move to opposite poles of the cell.
• The nucleolus disappears and nuclear envelope breaks down.

1. Prometaphase

• Chromosomes continue to further condense.
• The mitotic spindle grows and begin to attach to the kinetochore of the chromosomes' centromere.
• The kinetochore is a protein located at the centromere of the sister chromatids.

1. Metaphase

• Chromosomes align at the metaphase plate (also sometimes called the equatorial plane).
• The two kinetochores of each chromosome (one for each sister chromatid) are attached to microtubules – each attached to an opposite centrosome.
• Maximal condensation of chromosomes, but chromatids are still attached to one another.

1. Anaphase

• Sister chromatids separate and are pulled towards opposite ends of the cell towards the two centrosomes.
• Now they are each one chromosome!
• The microtubules that are not attached to chromosomes push apart, causing the cell to elongate and the poles to move away from each other.

1. Telophase

• The mitotic spindle is broken down into tubulin monomers that will form the cytoskeleton of daughter cells.
• Chromosomes start to decondense.
• Nuclear envelope begins to form.

Cytokinesis

• Physical separation of cytoplasmic components occurs in this phase.
• Usually starts to occur in late anaphase or telophase.

In animal cells

• The cell "pinches in" at the metaphase plate due to a band of filaments that act like a drawstring. These filaments are composed of a protein called actin.
• The pinching forms a crease called the cleavage furrow.
• Two daughter cells are formed when cytokinesis is complete.

In plant cells

• Remember that plants have cell walls, composed of cellulose, so a new one must form between the daughter cells.
• Vesicles formed from the Golgi apparatus that contain the components necessary for this process (e.g. glucose) go to the metaphase plate.
• A cell plate forms down the middle, splitting the cell into two and forming a cell wall between them.

Ploidy & C-value coefficients

• Ploidy = n = # of chromosomes in a single set
• Example: Humans
• Have 23 distinct chromosomes
• Have 2 sets of chromosomes
• 2n = 2(23) = 46 total chromosomes
• C-value = C = the physical amount of DNA across a single set of chromosomes in pg
• Example: Human cell at Interphase
• Has 23 distinct chromosomes that holds "C" pg of DNA
• Have 2 sets of chromosomes
• 2C = total amount of DNA within the cell