The Cell Cycle, Cellular Growth, and Cancer - Shmoop Introduction

Biology: it is all about birth, life, growth, rules, regulation and change. Yep, we are talking about the circle of life, but not in a Lion King kind of way. Think smaller—much, much smaller. Every cell in your body can trace its ancestry back to a one cell zygote, formed when a single sperm fertilized an egg. The specialized, organized cells of your body are the product of millions of cycles of cell growth and division. We call the process by which a cell grows, divides and returns to its normal working state the cell cycle.

We think of the cell cycle in terms of four stages:

• G₁: when a cell grows and carries out its regular cellular functions;

• S: when the cell replicates its DNA, resulting in duplicated chromosomes;

• G₂: when a cell continues to grow and prepares itself for cell division; and

• M: the phase where the cell divides itself into two genetically identical daughter cells. The M phase involves both a nuclear division (called mitosis) and a cytoplasmic division (called cytokinesis).

The orchestrated movements of chromosomes during mitosis are described as phases: prophase, prometaphase, metaphase, anaphase and telophase. In prophase, the nuclear envelope begins to break down, the nuclear material (or chromatin) condenses into rod-shaped chromosomes consisting of two sister chromatids, and the mitotic spindle begins to assemble. In prometaphase, the chromosomes begin to attach to the mitotic spindle and start to align along the center of the cell. Metaphase describes when all the chromosomes are aligned in a straight line down the center of the cell, with each sister chromatid attached to the mitotic spindle emanating from one of the cell's poles. In anaphase, the sister chromatids are pulled apart towards opposite poles. Finally in telophase, the separated chromatids decondense to their interphase-like structures and the nuclear envelope reforms. Cytokinesis physically divides the cell in half, resulting in the formation of two identical cells.

The movement of cells through the cell cycle is well regulated. Basically, a cell's progression from one phase to the other is carefully governed – in fact, you can think of it as each cell having its own set of traffic lights, telling it when to stop and when to go. This red light / green light type of system makes sure that cells don't do anything before they are supposed to, such as trying to separate their chromatids before they have finished replicating them. The system also tightly coordinates cell growth with cell division. This is a good thing too, because misregulation of the cell cycle or unchecked cellular growth and division can be catastrophic, leading to conditions like cell death or cancer.

Basically, a cell's life is a product of preparation, rules and regulation, and sometimes even sex, which is made possible by a special type of cell division called meiosis, where cells are created with half the complete set of chromosomes. Yeah, we know this particular circle of life is small and easy for us to ignore on a daily basis (mostly because there are way fewer singing animals), but the microscopic pathways inside of us are the secret of life. Occasionally things do go wrong, but on the whole our cells manage all the hard stuff without us ever having to give it a second thought. While it may be true that we've all got bigger things to deal with, don't forget to take a second, or at least a week when you are studying the cell cycle for your biology exam, to pay the cells in our bodies some respect for all that hard work they do whilst we are not paying attention. After all, it is because of them that we can take a hakuna matata, problem-free, philosophy to life.