Cell Proliferation

Section 1: Cell Proliferation

Cell Growth and Cell Division

As multicellular organisms increase in size, known as growth, it relies not only on cell enlargement but also on cell division to increase the number of cells. In fact, the size of cells in similar organs or tissues of different animals (plants) is generally not significantly different. The size of organs mainly depends on the number of cells. The increase in cell number is achieved through cell division.

Cell Proliferation

The process by which cells increase in number through cell division is called cell proliferation. Single-celled organisms reproduce through cell proliferation. Multicellular organisms, starting from fertilized eggs, develop into adults through cell proliferation and differentiation. Cells in organisms also constantly undergo aging and death, needing to be replenished through cell proliferation. Therefore, cell proliferation is an important cellular activity that forms the basis for organism growth, development, reproduction, and inheritance.

Cell Division is not Simple Division

Imagine during the process of developing from a fertilized egg into an individual, if cell division were simply a matter of splitting into two, wouldn't the genetic material in cells diminish with each division? How does cell division ensure that the genetic material of daughter cells remains identical to that of the parent cell? Before cell division, cells must undergo certain material preparations, especially replication of genetic material. Cell proliferation includes two consecutive processes: "material preparation, division, material preparation, and division again..." Clearly, cell proliferation is cyclical.

Cell Cycle

For continuously dividing cells, from the completion of one division to the completion of the next, it constitutes one cell cycle. A cell cycle consists of two stages: interphase and mitotic phase.

Interphase: From the end of one cell division to just before the next, it is called interphase. Most of the cell cycle's time is spent in interphase (Table 6-1), occupying 90% to 95% of the cell cycle. During interphase, active material preparation for division occurs, including DNA molecule replication and synthesis of related proteins, while the cell moderately grows.

Mitotic Phase: After interphase ends, the cell enters the mitotic phase, initiating cell division. For eukaryotic organisms, mitosis is the primary method of cell division. After division, the resulting daughter cells can enter interphase again (Figure 6-1).

Mitosis

After entering the mitotic phase, how is the DNA replicated during interphase evenly distributed into two daughter cells? In eukaryotic cells, this is mainly accomplished through mitosis. Mitosis is a continuous process divided into four stages based on chromosome behavior: prophase, metaphase, anaphase, and telophase.

Next, let's take the example of higher plant cells to understand the basic process of mitosis.

Mitosis in Animal Cells

Animal cells also undergo mitosis (Figure 6-3). What are the similarities and differences between mitosis in animal cells and plant cells?

The process of mitosis in animal cells is fundamentally similar to that in plant cells. The differences lie in: firstly, animal cells have a pair of centrioles that constitute the centrosome, which doubles during interphase and forms two sets. After entering the mitotic phase, the two sets of centrioles move toward opposite poles of the cell. Around these two sets of centrioles, numerous radiating aster rays are emitted, and the aster rays between the two sets of centrioles form the spindle. Secondly, at the end of animal cell division, no cell plate is formed. Instead, the cell membrane pinches inward from the middle of the cell, ultimately cleaving the cell into two parts, each containing one nucleus. Thus, one cell divides into two daughter cells (Figure 6-4).

The significance of cell mitosis is to accurately distribute the chromosomes of parent cells, after replication (especially DNA replication), into two daughter cells. Since chromosomes carry genetic material DNA, mitosis ensures genetic stability between parent and offspring cells. Thus, cell mitosis is crucial for genetic inheritance.

Normal cell division is precisely controlled within the body. In a human's lifetime, somatic cells can generally divide 50 to 60 times. However, some cells, under the influence of carcinogenic factors, undergo genetic changes, becoming uncontrolled malignant proliferating cells that continuously divide. These cells are cancer cells.

Amitosis

The process of amitosis is relatively simple; generally, the nucleus of the cell first elongates, the center of the nucleus constricts inward, and the cell is cleaved into two nuclei. Subsequently, the entire cell cleaves into two parts from the middle, forming two daughter cells. Since there is no spindle fiber or change in chromosomes during division, it is called amitosis, such as the amitosis of frog red blood cells.