Mitosis

The M phase of the cell cycle is the process known as mitosis. Some scientists classify cytokinesis as separate from mitosis, but still in the M phase. Mitosis is split into 4 distinct phases plus cytokinesis.

Prophase is the longest phase of mitosis. Chromatin condenses to form chromosomes, centrioles move to the poles, spindles form, the chromosomes attach to the spindle fibers at the centromere and the nuclear envelope breaks down.

Metaphase is usually brief. Chromosomes line up across the center of the cell and microtubules connect the centromere of each chromosome to the two spindles at the poles.

Anaphase is the next phase. The spindles split the sister chromatids and when they reach the poles and stop moving, anaphase is over.

Telophase if the final phase. The distinct, condensed chromosomes disperse, the nuclear envelope reforms around the chromosomes, and the spindles break apart.

Cytokinesis is the last step of cell division. This involves the division of cytoplasm and the cell membrane pinches shut, separating the two daughter cells.

This Krispy Kreme illustration is actually a great example of what this looks like:

Click through to see it full size.

Cell Division – An Intro

When cells grow, the run into a problem.  As the cell grows in size, their surface area to volume ratio changes.  As it turns out the higher the volume is in relation to the surface area, the more difficult it becomes to diffuse enough material into the cell.  Think of a city with 1 road.  As the town grows, the main road doesn’t change much.  Sure you may be able to add in a few extra lanes, but the town can grow rapidly in every direction but the road can only grow a little bit.  So when before a cell gets too big to function properly, the cell divides.

In order for both of the daughter cells (the new cells) to be able to survive and work properly, the cell needs to do a little prep work.  In eukaryotes like your venus fly trap, a wild capybara, and even you & me the process of dividing is commonly called mitosis.  More accurately, however, the process is called the M stage of the cell cycle.  In the M stage, two things occur, 1) mitosis, in which the cell’s chromosomes are divided and 2) cytokinesis, in which a cell’s cytoplasm divides in half.  (In prokaryotes, like amoeba, cell division is usually referred to as asexual reproduction, but it is still the same process.)  The actual prep work of division, though, occurs in the rest of the cell cycle.

The Cell Cycle

The cell cycle can be easily broken into two parts: 1) cell division, and 2) interphase.  A great deal happens in both of these parts of the cycle, so we’ll look at them separately.  Today I give you interphase.  Cell division will be in my next post.