Interphase: Stages, Cell cycle, Diagram, Video

Cells continuously undergo cell division through which daughters cells arise which mature and also produce their own daughter cells and the cycle continues. The events of the cell cycle involve cell growth and cell division, of which the interphase defines the phase of cell growth where several metabolic reactions take place. The interphase is the preparation phase for mitosis and it is also the longest phase in the cell cycle. The interphase takes place in the cytoplasm and the cell nucleus.

Figure: The Stages of Interphase and the Cell Cycle. Image Source: OpenStax (Rice University).

Interphase stages: What happens during the interphase?

• Gap 1 (G1)
• Synthesis (S), and
• Gap 2 (G2).

G1 and G2 phase represents the time of growth and preparation for mitosis.

The synthesis (S) phase is the phase of cell copying or cell duplication of its DNA of its entire genome.

Gap 1 (G1)

• This is the phase in which the cell undergoes normal growth and cell function synthesizing high amounts of proteins.
• The cell increases in size and volume as more cell organelles are produced.
• The cell can only move to the next phase, S after synthesizing enough ribosomes.
• As the phase is almost done, the mitochondria of the cell fuse into a network of mitochondria for effective energy production for cell processes.
• If the cell can not divide again, it enters the G_0^.

Synthesis, S phase

• This is the phase when the cell synthesizes and doubles its DNA.
• During doubling, the number of chromosomes remains constant, a process is known as semiconservative replication.
• Additionally, new packaged proteins known as histones, wrap around the DNA copies to make it stable.
• During the S phase, more phospholipids are produced which make up the cell membrane and the membranes of the cell organelles.

Gap 2 (G2)

• During this phase, the cell continues its growth phase as it prepares for cell division.
• Mitochondria will divide and continue to grow until the start of mitosis.
• Also, the plant chloroplast also divides in this phase.

Controlling the interphase

• At the end of Gap 1 (G1), the cell is controlled by a checkpoint known as the G1/S whereby, the cell is scrutinized on whether to undergo replication or not. It is at this checkpoint that DNA damages or errors are checked to ensure that cell division thrives.
• During checking, various proteins interact with the DNA a mechanism known as molecular switching. 
• The molecular switch is an on and off process and it continues through the S phase which allows the damaged cells to undergo apoptosis (programmed cell death).
• At the G2 phase again, a second checkpoint takes place after the synthesis of DNA in the S phase.
• The second checkpoint uses kinase enzymes which control various stages in cell division cycles.
• A typical example of a kinase enzyme in cell division is the Cyclin-Dependent Kinase (CDK) which uses cell signals to switch cell mechanisms especially when a genetic mutation is detected.
• The Cyclin-Dependent Kinase is activated by regulatory protein complexes (tumor suppressors) that control cell growth and induce apoptotic mechanisms of errored cells.
• In the case of a genetic mutation in the regulatory proteins at the checkpoint, a permanent molecular switch is turned on leading to uncontrolled cell multiplication which may lead to carcinogenesis or development of tumors.
• Additionally, in case the cell misses on the second checkpoint, cancer cells may develop for example neoplasia develops due to unchecked cell division.

Interphase and the cell cycle

• The interphase prepares the cell for the subsequent phases in cell division such as mitosis and cytokinesis.
• Since interphase is a preparation phase for the cell division processes, it enables the cell to grow, synthesizing organelles that allow the cell to function adequately ones it matures.
• The stages of the interphase prepare the cell for mitosis by meeting the external and internal conditions for cell mechanisms.
• After the G2 stage, the cell proceeds to prophase or pre-prophase in plants.
• Prophase is the initiating phase of mitotic cell division.
• Note that, some cells undergo G0, a stage into the G1 stage of cell division, which does not allow cell division.
• Some cells which do not undergo cell division remain in the G0 permanently

Interphase in different cells

• Different cells may have varying interphase processing mechanisms.
• A typical eukaryotic cell undergoes the subsequent stages of the interphase i.e. G1, S, and G2. Cells that undergo cell division spend approximately 95% of their time in the interphase.
• Cells that do not undergo cell division (have no DNA replication) are permanently on the interphase stage such as the neuron cells while cells that are actively dividing such as skin cells which are continuously dividing, must enter the interphase where they accumulate all the necessary organelles while actively replicating during the interphase.
• Cancer cells develop due to errors that occur during the second checkpoint, G2.
• A mutation that defects the functioning of regulatory proteins which are responsible for activating the Cyclin-Dependent Kinase enzymes, leads to permanent molecular switching, which causes uncontrolled cell division leading to carcinogenesis or tumor development.
• Bacterial cells do not have the interphase stage of cell division because the mechanism of cell division is meiosis whereby the interphase is rather a special phase known as interkinesis, and there is not DNA replication occurring.

Interphase video animation (Khan Academy)

References and Sources

1. Microbiology by Prescott, 5th Edition
2. https://www.expii.com/t/what-is-interphase-overview-diagrams-10154
3. https://bio.libretexts.org/Bookshelves/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/10%3A_Cell_Reproduction/10.2%3A_The_Cell_Cycle/10.2B%3A_The_Mitotic_Phase_and_the_G0_Phase
4. Biology Dictionary/interphase
5. Teach me Physiology/ The Cell Cycle
6. https://sciencing.com/list-3-steps-occur-during-interphase-17577.html
7. https://ib.bioninja.com.au/standard-level/topic-1-cell-biology/16-cell-division/interphase.html
8. https://en.wikipedia.org/wiki/Interphase#:~:text=Interphase%20is%20the%20portion%20of,prepares%20for%20mitosis%20(G2).
9. https://www.gwisd.us/vimages/shared/vnews/stories/4ebbe99bda63a/Mitosis%20and%20Cell%20Division%20Text.pdf
10. https://www.cyclacel.com/research_science_cell-cycle.shtml