DNA Dynamics and Types of Division
Understanding the cycle is based on changes in the amount of DNA, denoted by the Latin letter n (the haploid set, typical of mature gametes). Most somatic cells in our body are diploid (2n).
There are two main types of cell division:
- Mitosis. The standard process by which somatic cells and early germline precursors multiply. The cell prepares by doubling its DNA to a tetraploid state (4n) and then divides, producing two identical copies, each with a 2n set.
- Meiosis. A specialized process occurring exclusively during gametogenesis (the final stages of oocyte and spermatocyte development). The first meiotic division is similar to mitosis (yielding two 2n cells from a 4n cell). The key difference lies in the second division: it starts without prior DNA replication, resulting in four haploid cells (n).
Periods of the Mitotic Cycle
For cells capable of dividing, the life cycle is identical to the mitotic cycle. It consists of the division phase itself (mitosis) and a prolonged preparation phase — interphase.
Interphase includes three phases:
- $G_1$ (presynthetic phase). Begins immediately after mitosis. The cell grows and restores cytoplasmic volume. At the end of the phase is the "restriction point": if the cell receives external signals (mitogens) and passes this point, it irreversibly commits to the next division. DNA set — 2n.
- $S$ (synthetic phase). The main event is the replication of DNA and chromosomal proteins. The DNA content increases from 2n to 4n. Centrioles duplicate near the nucleus. Centromeric regions of chromosomes are not copied in this phase, holding sister chromatids together until the start of mitosis anaphase.
- $G_2$ (postsynthetic phase). A relatively short stage before division. DNA set — 4n. The cell actively synthesizes substances for mitosis, primarily the protein tubulin, which builds the mitotic spindle.
Classification of Cells by Proliferative Capacity
Based on proliferative activity, all cells of the adult body can be divided into three functional groups:
- Mitotic cells. Continuously cycle through the cell cycle. Examples: basal layer cells of the epidermis, intestinal epithelium, early hematopoietic progenitor cells.
- Conditionally postmitotic cells. Normally reside in a resting phase ($G_0$) while performing their specific functions. However, when necessary (e.g., for organ regeneration), they can re-enter the cycle and begin dividing. These include hepatocytes, fibroblasts, lymphocytes, and stem cells of bone and muscle tissues.
- Postmitotic cells. Have undergone terminal differentiation and permanently lost the capacity to divide. They function for a long time and then die. Prominent examples include neurons, cardiomyocytes, muscle fibers, and cells of the upper epidermal layers.
Resting Phase ($G_0$) and Cell Fate
If a cell leaves the mitotic cycle at the $G_1$ stage, it enters the resting phase — $G_0$.
Exit to $G_0$ can occur for various reasons: temporary "dormancy" of stem cells, resource depletion, the need to repair DNA damage, or the initiation of specialization (differentiation).
For some cells (conditionally postmitotic), this exit is reversible: upon receiving specific signals, they can return directly from $G_0$ to the $S$ phase.
For others (postmitotic), specialization means an irreversible exit from the cycle. For example, in the epidermis, cells shift from the basal layer to the spinous, granular, and translucent layers, where they function but no longer divide, and eventually die in the cornified layer. Long-lived postmitotic cells (neurons, cardiomyocytes) function throughout the organism's entire lifespan and die either from aging or damaging factors.