Why describe and explain the various stages of cell division?

Cell division is key to life: from the moment we are first conceived, we are continually changing and growing. In order for our bodies to grow and develop, they must produce new cells—and allow for the death of old cells. Cell division is also an essential component of injury repair. If our cells couldn’t divide and create new cells, our bodies could never produce new skin cells to heal road rash, or grow a fingernail back. However, when cell division goes awry, dramatic results may occur. Without sufficient cellular oversight, repeated rounds of unregulated cell division can lead to a minor condition like psoriasis or a life-threatening disease like cancer. Cell division occurs by a strict cycle, with multiple stages and checkpoints to ensure things don’t go awry.

Perhaps most importantly, without cell division, no species would be able to reproduce—life would simply end (or would have ended a long time ago). Every human, as well as every sexually reproducing organism, begins life as a fertilized egg (embryo) or zygote. Trillions of cell divisions subsequently occur in a controlled manner to produce a complex, multicellular human. In other words, that original single cell is the ancestor of every other cell in the body. Single-celled organisms use cell division as their method of reproduction.

Figure 1. A sea urchin begins life as a single cell that (a) divides to form two cells, visible by scanning electron microscopy. After four rounds of cell division, (b) there are 16 cells, as seen in this SEM image. After many rounds of cell division, the individual develops into a complex, multicellular organism, as seen in this (c) mature sea urchin. (credit a: modification of work by Evelyn Spiegel, Louisa Howard; credit b: modification of work by Evelyn Spiegel, Louisa Howard; credit c: modification of work by Marco Busdraghi; scale-bar data from Matt Russell)

Learning Outcomes

• Understand chromosome structure and organization in eukaryotic cells
• Identify the stages of the cell cycle, by picture and by description of major milestones
• Identify and explain the important checkpoints that a cell passes through during the cell cycle
• Identify the stages of meiosis by picture and by description of major milestones; explain why meiosis involves two rounds of nuclear division
• Describe and explain a range of mechanisms for generating genetic diversity
• Examine karyotypes and identify the effects of significant changes in chromosome number