When the Earth is viewed from space, massive continents and oceans are easily visible. However, from this distance the surface appears relatively inactive, with few exciting or visible changes. In contrast, we know from an insider’s view that the Earth is an extremely busy place. At any given moment, many activities are in process, from babies being born to plants processing sunlight energy to bacteria infecting animals. These activities are the processes of life on Earth, but none can be seen from space.
If we observe a series of human body cells through a microscope, the vast majority of the cells will appear static, as though nothing interesting is occurring. However, like the Earth, life inside a cell is always busy. Cells are constantly producing the proteins that function as enzymes, structural support, transporters, and facilitators of movement. Cells also require and produce new lipids, carbohydrates, and nucleic acids while degrading and recycling others. In a multicellular organism, cells frequently transmit or receive signals from one part of the organism to another and activate new processes while terminating others.
In cells, this busy stage of “living” is called interphase. Interphase is part of a cyclical growth process in all cells called the cell cycle. An average eukaryotic cell spends most of its life in the interphase part of the cell cycle, around 90% of its time, to be precise. However, the actual amount of time a specific cell spends in each stage is highly variable. For example, most human nerve cells spend their entire lives in interphase, never progressing forward into cell division. Because of this characteristic, injuries that destroy nerve cells in the spinal cord usually result in permanent paralysis because surviving nerve cells do not divide and produce new cells to replace those lost.
The scientists who first investigated cells under a microscope were excited about non-interphase stages of the cell cycle because significant changes were rapid and easily observed from their distant viewpoint. These scientists coined the term interphase (“between phase”) to indicate the time between the more interesting phases where change was more apparent. Therefore, interphase is sometimes misinterpreted as a “resting” stage of the cell cycle, but cells in interphase are doing anything but resting!
During eukaryotic interphase, DNA is organized and packaged with histones into chromatin, but is not completely condensed. In this form, genes are easily accessed and expressed to produce functional RNA and protein molecules as needed by the cell. Eukaryotic interphase includes three main sub-stages: G1 phase, S phase, and G2 phase.
When a daughter cell initially forms from a parent cell, the new cell begins its life in G1, the first sub-stage of interphase. G1 is the first “gap” phase, during which the cell may grow at a rapid rate, or it may grow very slowly or not at all. Like interphase as a whole, G1 is a very busy time where cells are continuously “living.” The term “gap” is historical, and a better memory trick might be to remember G1 as a “growth” phase. When a cell, such as a nerve cell, spends an extremely long time in G1 without moving forward, some scientists change the cell’s cycle designation to G0 phase to indicate that the cell is, or will be, non-dividing for a significant period of time.
If a cell receives a signal to initiate cell division, the second sub-stage of interphase begins. During this “synthesis,” or S, phase, the cell’s DNA is replicated to produce two identical copies of the entire genome. S phase is tightly regulated to check for DNA damage and proofread replication products to avoid creating mutations. DNA damage and replication errors are normally repaired before a cell is allowed to exit S phase.
When a chromosome is replicated, the two copies remain connected. A complex of proteins binds to both identical strands at a central region of DNA to form a centromere, which tightly connects the two strands. Maintaining a continuous connection between each pair of identical strands helps a cell to correctly separate the identical strands and produce two identical nuclei during mitosis. When two identical DNA strands are connected as one unit, the entire structure is considered one chromosome. Therefore, the two identical DNA strands connected by a centromere are called sister chromatids. During mitosis, the two sister chromatids will separate and each chromatid will become its own chromosome.
After DNA is replicated and checked for errors, S phase is complete and the cell enters G2. Like G1 phase, G2 is an active and busy time in the life of a cell. Although many of the normal activities of cellular life continue to occur, G2 also involves preparation for cell division because the cell has finished replicating its DNA.
When the cell is prepared, regulated signals initiate the start of cell division and the end of interphase. In eukaryotic cells, cell division includes two separate, but overlapping and regulated processes: mitosis and cytokinesis. Some textbooks refer to the part of the cell cycle devoted to cell division as M phase to highlight the importance of mitosis in eukaryotic cell division. In its active form, chromatin would be too long and extended to separate sister chromatids easily, leading to breakage. Therefore, at the end of interphase, condensins begin to pack the duplicated chromosomes more tightly, forming condensed chromosomes in preparation for division.
The Cell Cycle
Identify the cell cycle phases and events in this activity.