Cell Cycle & Mitosis

Know these phases, what order they occur, and what happens in each.

The Cell Cycle

The cell spends most of it's time (about 90%) in Interphase. During interphase the cell goes through 3 main phases known as G1, S, and G2.

Cell division (in this case, mitosis) makes up the other 10% of the cell cycle. There are 4 major phases: Prophase, Metaphase, Anaphase, and Telophase (PMAT).

Cytokinesis runs concurrently with telophase and is the splitting of the cytoplasm, the final separation of the two cells.

Some cells that do not do mitosis also undergo a G0 phase [G-zero]. Examples include nerve cells which do not repair or replicate and therefore do not need to prepare for mitosis, as well as any cell that does not need constant replacement.

The Phases of mitosis:

Prophase:

Prior to prophase the DNA is uncoiled and loose in the nucleus (chromatin). Think of it like a bowl of spaghetti... That makes it difficult to sort into two groups so during prophase the DNA condenses by wrapping tightly around proteins (called histones) and then supercoiling into tightly packed chromosomes that is attached to it's duplicate strand (that was created during S phase of interphase so that each new cell can have a copy).
The nucleus breaks up and disappears so that the DNA can move around and separate. Often denoted by a dotted line.
Spindle fibers (microtubules) form from the centromeres at each end of the cell (aids in separating the chromosomes).

Metaphase:

In metaphase the spindle fibers attach to the centromere at the center of each chromosome and help move them to the center of the cell. Metaphase = middle

Anaphase:

Remember that we duplicated the DNA in the S phase of interphase so that both new cells can have a complete copy. The separation of these copies happens in anaphase. Anaphase = Apart
Sister chromatids are torn apart and sorted so that each end of the cell gets a complete copy of each chromosome. [each chromatid is a complete copy of genes (made of DNA) for that chromosome. Represented by 1/2 of the X. Sister chromatids refer to the original DNA and the copy]

Telophase:

Here we see the nuclear envelope (nucleus) return at both ends of the cell. This will form the new nucleus for each cell. This is also denoted by a dotted line (as we saw in prophase when it disappeared) but you know it is telophase if there are two of them.
Spindle fibers retract and disappear.
Evidence of Cytokinesis is apparent. Cytokinesis = separation of cytoplasm

Cytokinesis:

Separation of the cytoplasm.

Plant cells will form a cell plate in the middle that will grow to form the cell wall.

Animal cells will form a cleavage furrow that will pinch off until the cell becomes two.

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