The Cell Cycle
Let us think about the cell cycle from a personal point of view. Your age plus about nine months ago you were a zygote, a single cell formed when the sperm and egg from your biological parents fused in a fallopian tube (or in a test tube if in vitro fertilization factored into your birth). You have come a long way since then, progressing one cell cycle at a time. The cell cycle is the life cycle of a single cell. We depict the cell cycle in a circular diagram although the cells in your body do not actually go around in circles. The main idea is that when new cells are made from existing cells, the new cells start their lifecycle, and the old cells end theirs. The two new cells have the same genetic makeup as the old cell.
Figure 1. The cell cycle depicts the stages in the life of a cell. Those phases are interphase (with the G1, S phase, and G2 phase) and mitosis (prophase, metaphase, anaphase, telophase). Cells are in interphase the majority of the time as depicted with interphase being about 4/5 of the circle. Cell Cycle by NIH-NHGRI, n.d. Public Domain.
The first part of the cell cycle is the G1 phase (Fig. 1). Cells can go through growth and development in this phase. Some cells differentiate into specialized cells and never leave this phase. However, a zygote does not grow in size, but instead continues the cell cycle so it can quickly give rise to more cells.
The second part of the cell cycle is the S phase (synthesis phase, Fig. 1). Here, the cell replicates its chromosomes so it has a copy of each chromosome to pass on to daughter cells. The third part of the cell cycle is the G2 phase where the cell prepares for division. The G1, S and G2 phases together are called interphase (Fig. 1). The M phase completes the cell cycle (Fig. 1). ’M’ could be mitosis or meiosis depending on the type of cell. For the zygote, the goal is to make more somatic cells. Therefore, it goes through mitosis and gives rise to two daughter cells. This completes the life cycle of the zygote and starts the lifecycle of the new cells. Rounds of the cell cycle continued over and over to form the body you have today. As long as you live, some of your cells must be able to complete the cell cycle.
From a cytogenetics point of view, you have two types of cells in your body. You have cells with 46 chromosomes (somatic or body cells) and cells with 23 chromosomes (gamete or sex cells) (Fig. 2). Since you started as a single cell with 46 chromosomes, there must be two types of cell division taking place in your body to accommodate both somatic and gamete cells. Mitosis and meiosis are the two types of cell division.
Figure 2. Multicellular organisms that sexually reproduce have diploid and haploid cells. Diploid somatic cells have two of each kind of chromosome. In this case, two chromosomes with the D,d and B,b genes (in orange) and two chromosomes with the E,e gene (in green). Haploid gametes have one of each chromosome (only one orange chromosome with the D,d and B,b genes and one green chromosome with the E,e gene). Image by M. Hanneman, 2021. CC BY-NC-SA 4.0.