The Cell Cycle & Mitosis

Phases of The Cell Cycle

• The cell cycle is like the lifecycle of a cell – from the beginning of a new cell, through growth, and finally division.
• Some cells are capable of dividing many times, while others are not.
• In order to divide, a cell must go through its "stages of life". In this time, the following things will happen:
• The cell will grow;
• It will replicate its DNA;
• It will produce new organelles;
• As it nears cell division, it will sort the DNA so that each daughter cell receives one copy of each chromosome;
• Split into two cells (twin daughter cells).

• Interphase: Cell growth and DNA replication.
• G1 phase (1st gap)
• S phase (Synthesis)
• G2 phase (2nd gap)
• Mitotic (M) Phase: Replicated DNA and cell contents are separated.
• Mitosis
• Cytokinesis (partitioning of the cytoplasm)

Interphase

1. G1 phase: Gap phase 1.
2. The cell is stocking up on nucleotides, proteins and energy to replicate its DNA. That is, it increases abundance of molecules involved in replication.
3. S phase: Synthesis phase.
4. The cell replicates its DNA to form sister chromatids, and replicates the microtubules organizing center called the centrosome.
5. DNA remains in semi-condensed chromatin form.
6. G2 phase: Gap phase 2.
7. The cell may grow in size and increases abundance of proteins involved in mitosis (i.e. chromosome movement and manipulation).
8. It also replenishes energy stores and some organelles are duplicated.

• There may also be a G0 phase, in which the cells exist in a quiescent state (quiet) and they are not in the process of dividing or preparing to divide. This can sometimes be seen as an extended G1 phase.
• May be temporary (e.g. due to lack of nutrients);
• May be permanent (e.g. adult heart cells usually do not divide).

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.