This blog post explains in simple terms the role that invisible cells play as the basic units of life.
We often hear the term “cell.” Cancer cells have plagued us for a long time, and recently there has been controversy over stem cells. Cosmetics companies extract collagen cells from pigs and advertise that they are good for the skin. As such, cells seem to be commonly used and familiar in our daily lives. However, ordinary people cannot see cells directly. What are cells? We looked at what scholars have discovered about cells, focusing on their life cycle.
All living things are made up of cells. However, when we look at living things, we do not see their cells. This is because cells are very small, measuring only 1 to 100 μm, and are difficult to see with the naked eye. Until the 17th century, cells were not observed. There was no established understanding of cells, and based on Archimedes’ novel, it was believed that all things were composed of four elements: water, fire, earth, and air. This perception was completely changed in 1665 when scientist Robert Hooke observed dead cell tissue from a piece of cork using a microscope he invented himself and named it “cell.”
After that, with the invention of improved microscopes, such as Rayven Hooke’s 270x magnification microscope, discussions about what cells are began. In 1838, botanist Schleiden announced his plant cell theory, which stated that all plants are composed of cells, based on his observations. A year later, cell biologist Schwann announced his animal cell theory, which stated that animals, like plants, are composed of cells, in his paper “Microscopic Research on the Structure and Growth of Animals.” In 1855, Virchow completed the modern cell theory by asserting that all cells are created from existing cells. The contents of this cell theory are as follows.
1. All living things are made up of cells.
2. Cells are the structural and functional units of living things.
3. All cells are made from existing cells.
In other words, it was not until the late 19th century that the general public was able to move away from abstract concepts such as temple novels and think scientifically about what living things are based on cells.
Once cells were defined and became easier to observe with the development of microscopes, scholars became interested in their properties. Among these properties, scholars were most curious about the life cycle of cells. They soon realized that the life cycle of cells is closely related to the process of cell division.
The cell division process can be broadly divided into the interphase and the mitotic phase. The interphase is the preparatory stage for mitosis and accounts for 80% of the entire cell division process. At the end of mitosis, a single cell becomes two cells, so the amount of contents within the cell must also double. Cells perform this task during the interphase. The interphase can be further divided into the G1, S, and G2 phases. The G1 phase is when cells grow for the next division. Since cells repeat the division phase and the G1 phase, the G1 phase, which is the first part of the G1 phase, is also the first phase that comes after the division phase. During the G1 phase, cells increase the amount of contents within the cell through the transcription process of making RNA and the translation process of making proteins. The S phase stands for synthesis, and DNA replication occurs. Cells have a nucleus that contains all the genetic information of an organism and regulates the expression of traits, and inside the nucleus is DNA that preserves this information. DNA is essential for cells to carry out their life activities. Even after cell division is complete and two cells are formed, the genetic information of the organism must be preserved in each cell. Therefore, cells must undergo a separate S phase to double the amount of DNA. Like the G1 phase, the G2 phase is a period of preparation for division, but it is shorter than the G1 phase and transcription and translation activities are less active.
The division phase is divided into nuclear division and cytoplasmic division. Cells are composed of a nucleus and cytoplasm, which is the part excluding the nucleus. Nuclear division is the period when the nucleus divides, and cytoplasmic division is the period when the cytoplasm divides. Nuclear division is divided into the prophase, metaphase, anaphase, and telophase. In the prophase, the nuclear membrane surrounding the nucleus disappears, the chromosomes (a collection of DNA and proteins) condense into chromosomes, and spindle fibers involved in the movement of chromosomes are formed. In the metaphase, the spindle fibers connect to the chromosomes, and the chromosomes move and arrange themselves in the center of the cell, which is called the “equatorial plane.” In the late stage, the chromosomes on the equatorial plane separate completely and move to opposite poles. In the final stage, nuclear membranes form again on both sides, separating the chromosomes and creating two daughter nuclei. The final stage is immediately followed by the cytoplasmic division.
The method of cell division differs between animal cells and plant cells. Plant cells have a hard cell wall surrounding the cytoplasm, so a cell plate forms on the equatorial plane of the cell to divide the cytoplasm, and this cell plate later develops into a new cell wall. On the other hand, animal cells do not have cell walls, so the cytoplasm is separated by a process called cell membrane invagination, in which the cell membrane rolls inward from the outside. This process is a complex procedure involving various structures within the cell and complex regulatory mechanisms, requiring precise biological coordination rather than simple physical division.
So far, we have looked at the life cycle of cells. As mentioned earlier, cells are difficult to observe with the naked eye and require the use of a microscope. The academic study of invisible cells is an important turning point in human history, demonstrating our growing interest in the invisible world and the expansion of our intellectual curiosity.
