The cell cycle is crucial for life. Without it, we would just be a single cell with no function or purpose! 10% of this cycle is a process called “mitosis”. The remaining 90% is a phase called “interphase”.
So what happens during interphase? The answer is brief: the cell doesn’t divide. However, there are many processes happening within the cell to prepare it for mitosis. These processes can be divided into three sub-phases: G1, S, and G2.
G1 Phase
This is the growth sub-phase. The cell produces organelles and proteins required for survival, DNA replication, and cell division. This phase usually takes 5-6 hours. However, it varies across cells. For instance, in a human somatic cell, G1 lasts a whopping 18 hours! The G1 phase ends at a G1 checkpoint. At the G1 checkpoint, the cell checks whether cell division is necessary, whether the cell is large enough to divide, if sufficient nutrients are available for the synthesis and division of DNA, and whether growth factors are present. (Growth factors are proteins that stimulate cell division.) If all goes well, then the cell can enter the next sub-phase of interphase: S phase.
S Phase
“S” means “synthesis”. Hence, as the name suggests, DNA synthesis occurs during this sub-phase. To be more specific, bubbles open up between the chromosomes and they grow. Kind of like the gaps that form when you peel a sticker off it’s wax sheet. The bubbles merge with their neighbouring bubbles until all that’s left is one huge gap. This means the chromosomes have divided to form sister chromatids. Sister chromatids are essentially DNA wound around histones. The two sister chromatids are attached to each other by a centromere.
Now that the DNA has divided, the cell can enter the next sub-phase: G2 phase.
G2 Phase
G2 usually takes 4-6 hours. During this phase, the cell replicates its organelles and performs a type of “final call” to ensure everything is ready for mitosis. Furthermore, the cell checks for any deficiencies that may hamper mitosis. For instance, breaks in the DNA. If any deficiencies are present, the cell will stop the process of cell division.
G2 ends at the G2/M checkpoint. At this checkpoint, the cell checks whether the DNA has replicated correctly, the cell is large enough to divide, and sufficient amounts of cell organelles are present. If all goes well, mitosis begins!
Citations:
Campbell Biology, 11/e by Lisa A. Urry
Cover Image Credits: https://news.harvard.edu/wp-content/uploads/2017/03/human-cell-large_605.jpg?w=605
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