Cell Division

The cell cycle refers to the sequence of events throughout a typical cell's life - involving growth, DNA replication, and preparation for cell division. For sexually-reproducing organisms, life begins as a zygote, a fertilized egg. Over time, that original single cell grows and divides in a controlled manner to produce a multicellular, complex individual. Cells continue this process, especially to maintain and repair tissues during aging.

Now let's take a closer look. In eukaryotes, double-stranded DNA is specially organized within a membrane-bound nucleus to accommodate the cell's limited space. At the first level of compaction, DNA is wrapped tightly around specific proteins called histones. This combination of DNA protein particles is then repeated and packed into arrays known as nucleosomes, which, along with linker DNA, form coiled chromatin fibers. Finally, additional fibrous proteins compact the chromatin even further, packing long lengths of DNA into tightly-condensed units. Recognized as chromosomes, depending on the phase of cell division.

To prepare for division, cells must go through interphase, which is divided into three stages. G1, S, and G2. In G1, the first gap phase, a newly-generated daughter cell grows in size and prepares for DNA duplication in the next phase. Now in S, the synthesis phase, cells duplicate their nuclear DNA, which remains packaged as chromatin. Cells also duplicate the centrosomes, the microtubule-organizing structures which form the mitotic spindle apparatus. Finally, in G2, the second gap phase, cells continue to grow, multiply organelles and proteins that are required for mitosis, and replenish their energy stores. The cell is now ready to enter the first stage of mitosis.

Comprised of five unique stages, mitosis is a form of division where a cell's genetic material is partitioned between two daughter cells. First during prophase in humans, nucleic chromatin condenses into X-shaped chromosomes, composed of sister chromatid pairs attached at centromere junctions. Concurrently outside the nucleus, centrosomes migrate to opposite sides of the cell. As they do so, microtubule rods begin to grow from each. Either towards the cell's interior or exterior, forming a web-like spindle apparatus. Next, the nuclear envelope dissolves during prometaphase, exposing the chromosomes to the cell's other contents. Protein structures also appear on both sides of the centromeres, one for every chromatid. Once these kinetochores form, extending interior microtubules fasten to them, with each sister chromatid being tethered to a different pole.

Mitosis then progresses to metaphase, where the spindle apparatus rearranges the chromosomes so that they are similarly oriented in a fixed row along the cell's equator. During anaphase, kinetochore-affixed microtubules shorten. And sister chromatids, now individually referred to as chromosomes, are dragged apart. These and other microtubule dynamics also elongate the cell. Finally, the chromosomes land at opposite cell sides during telophase. And the spindle apparatus disbands. The genetic material loosens, and two nuclear envelopes, one around each chromosome set, arise. During telophase, a distinct process, not technically a stage of mitosis, called cytokinesis, also divides the cell. Thus, the end result of mitosis is a cell pair genetically identical to their precursor.

Unfortunately, mutations can cause damage to genes controlling cell cycle regulation, which leads to cell division proceeding unchecked. In this case, each successive cell division produces daughter cells with even more damage…and the faulty growth regulation ultimately leads to the formation of cell masses called tumors.

In this lab, you will examine the different stages of mitosis using onion root tip cells. And then examine what happens when cell cycle control is lost.