Cell Division
An essential part of the life cycle of most cells, whether they be prokaryotic or eukaryotic, is division. This is a process through which a cell divides and produces two new cells. There are a few main reasons why a cell would want to do this:
Reproduction: The most basic reason for cells to divide is because they want to make more cells! Cell division is an essential part of reproduction, whether it be a yeast cell dividing and becoming two yeast cells, or a red panda producing a sperm cell to be used for reproduction later.
Growth: Think about you now compared to when you were seven years old. You most likely have grown significantly since then, but it's not because your cells got bigger. You just have a heck of a lot more of them.
Differentiation: Gorgonzola, my beloved bearded dragon, was once a single cell. He now has specialized cells for carrying oxygen in his blood, his muscles, his immune system, his brain, etc. During development, not only is growth important, but the differentiation of cells is as well. Cell division is an essential part of cell differentiation.
Repair: Once, when I was fidgeting, I accidentally stapled my thumb. Many cells died as the holes were made for that staple, but those holes aren't there anymore. The reason is that the other cells in my thumb divided and replaced those that had been damaged.
Some of those reasons don't apply very much to single-celled organisms like prokaryotes. Most of these species divide using a process known as binary fission. In this process, the cell grows, replicates its DNA, moves the DNA to opposite ends of the cell, and then splits into two. The vast majority of species do so through the use of the protein FtsZ. An FtsZ ring forms in the center of the cell and recruits other proteins that will aid in the division process. Some species reproduce via budding where cell division occurs at one specific spot on the original cell, with the new cell forming as a smaller outgrowth or "bud".
We don't do either of those. When most people use the phrase "cell division" they are referring to how human body cells grow, which is the mitotic cell cycle. We also use the cell division process of meiosis for the production of specialized sex cells, sperm and egg.
The Cell Cycle
The basic process through which cells undergo mitosis and divide is known as the cell cycle. This cycle can be broken up into two major parts: Interphase (getting ready to divide) and M Phase (actually dividing). This process is used by and for the production of our somatic cells (body cells - anything but sperm and egg).
Interphase
G1
The first step in Interphase is G1. Just think of "G" for "growth". In this phase, the cell grows as it prepares to divide. It also produces more organelles, and synthesizes mRNA and proteins that will be required for the next step.
S
If one cell is going to become two cells, it needs to have enough DNA to support both cells. S Phase is where new DNA is synthesized. Through a highly regulated and conserved process, DNA Replication occurs and copies the DNA.
G2
The final step of Interphase is G2. Once again, the cell will grow and produce proteins as it finishes preparing to divide.
Are cells always in the cell cycle?
No. Cells are capable of leaving the cell cycle and entering something known as G0. Some cells are in a permanent G0 where they will no longer divide, such as fully differentiated neurons. Other cells are in a quiescent G0 that is temporary and reversible upon the right stimuli, such as satellite cells - skeletal muscle stem cells that are usually dormant, but can be activated by muscle damage or exercise.
M Phase
After Interphase is M Phase, where the cell actually divides. It consists of two parts: nuclear division (mitosis) and cytoplasmic division (cytokinesis). Some specialized cells, such as skeletal muscle, will skip cytokinesis and result in multinucleated cells.
Prophase
As we begin mitosis, we have twice the amount of DNA, but it's all inside the nucleus. In addition, that DNA is in the form of chromatin (kind of like thin, stringy fibers of DNA wrapped around some proteins). In order to separate that DNA into two separate cells, it needs to be accessed. In this first step, Prophase, the nucleus begins to breakdown. Chromatin will condense into chromosomes (the more tightly-wound "X" that you probably think of as existing in the nucleus). Centrosomes, as they move to opposite ends of the cell, will begin the production of spindle fibers.
A helpful way to remember this step is that "PRO" rhymes with "NO" and this is where the nucleus begins to go away.
Prometaphase
In prometaphase, the nucleus finishes breaking down. The kinetochore, a protein complex, forms at the centromere of each chromosome. Spindle fibers attach to these kinetochores and will be essential in moving the chromosomes.
Metaphase
The motor proteins associated with the spindle fibers move the chromosomes such that they line up single-file along the metaphase plate at the center of the cell (at its equator).
Just remember M for middle - at this step the chromosomes line up in the middle.
Anaphase
Each replicated chromosome up until this stage looked like an X - it consisted of two sister chromatids, which are identical to each other. During this phase, the sister chromatids are pulled apart and moved to opposite ends of the cell. Once separated, each sister chromatid counts as its own chromosome, meaning the chromosome number in the cell doubles.
Think A for apart - the sister chromatids are pulled apart!
Telophase
This is the final step of mitosis, and is almost like the opposite of prophase and prometaphase. During telophase, a new nucleus forms around each set of chromatids. The chromosomes decondense back into chromatin. The spindle fibers dissemble.
Remember T for two - two nuclei form here.
Cytokinesis
The final step of the cell cycle actually usually begins in late anaphase and continues throughout telophase. This is the step where the cytoplasm divides. In animal cells, a cleavage furrow forms and the cytoplasm pinches in until the cell is divided. In plant cells, a cell plate forms and separates the two cells, creating the new sections of the cell walls.
At the end of the process, there will be two daughter cells that are both genetically identical to the original cell. If the original cell was diploid, as it is in our somatic cells, the daughter cells will be as well.
Regulation of the Cell Cycle
While cells dividing are important for a variety of life processes, you don't want cells dividing all of the time. It is a tightly regulated process with a variety of checkpoints. These checkpoints ensure that things are occurring properly, and are capable of preventing division so that cells do not divide when they are not supposed to, whether that be due to it not being the proper time to divide, or the cell being damaged in some way.
G1 Checkpoint: This checkpoint ensures that the cell has grown enough, has the proper amount of nutrients, that external signals such as growth factors are present, and that it does not have any DNA damage before allowing the cell to continue to divide.
G2 Checkpoint: At this checkpoint, the DNA is once again checked. Did replication happen completely? Did it happen accurately?
Spindle Checkpoint: This checkpoint ensures that the chromosomes are properly attached to spindle fibers at the metaphase plate.
One of the major factors driving the continuation of the cell cycle are cyclin-dependent kinases (Cdks). These are highly evolutionarily conserved enzymes that phosphorylate target proteins and cause the cell cycle to continue (such as by promoting DNA replication, activating enzymes to break down the nucleus, etc.). These enzymes are "cyclin-dependent" and rely on a specific class of proteins known as cyclins. We have four different cyclins: G1 cyclin, G1/S cyclin, S cyclin, and M cyclin.
The cyclins are produced at higher levels at different points of the cycle depending on when they are needed. Cyclins bind to Cdks to activate them and cause the cell cycle to progress.
When a cell fails a checkpoint, the cell cycle will be stopped. One such example is with p53, the "guardian of the genome". If p53 detects DNA damage at the G1 checkpoint, it triggers the production of Cdk Inhibitor which will bind to the cyclin-Cdk complex and prevent its functioning, pausing the cell cycle. In this situation, enzymes will be activated to attempt to repair the DNA. If not possible, apoptosis will be triggered.
What if the checkpoints don't work?
When cells are dividing even when they aren't supposed to, this can result in cancer - a group of diseases characterized by uncontrolled cell growth. Cancer is caused by a series of mutations that prevents the proper functioning of the cell cycle checkpoints (and up to 30% of human cancers have mutations in the Ras signaling pathway).
Normally, cells experience something known as density-dependent inhibition. Via juxtracrine signaling, cells will signal each other not to divide. The cell cycle will stop once a certain density is reached. Cancer cells have mutations allowing them to ignore this, resulting in the formation of tumors.
Also mediated through juxtracrine signaling, many cell types must be in contact with a solid surface (typically another layer of cells) in order to divide. This is known as anchorage-dependent inhibition. When anchorage-dependent cells detach from the surrounding extracellular matrix, they undergo a type of programmed cell death known as anoikis. When cancer cells become anchorage-independent, metastasis - the spreading of cells from a tumor to another area of the body - becomes possible.