Stages Of Interphase: Detailed Analysis on Every Stage, Events And Facts

The interphase is the longest phase in the cell cycle and the stages of interphase include G1, S, G2 and in some special cases G0.

All cells remain in interphase till it is time for division in the form of mitosis or meiosis. Most cells must mature to a certain stage before being able to divide, this period is none other than interphase.

During interphase, the cell prepares for division by creating more cellular material- organelles, genetic material and stored food and energy and can become double its size. But there is no great molecular change that is recognizable under the microscope.

Some cells can also enter G0 phase or Gap 0 phase where G1 phase is extended basically the cell gets stuck in this phase because cell division is not required.

Stages of the interphase diagram:

Though interphase is called the in-between stage separating two cell division phases, it is during this phase that the cell is most active. Interphase can be divided into a total of 3 different stages- the G1 phase, S phase and the G2 phase. All these phases have their specific functions and requirements.

G1 PHASE:

The Growth-1 phase, or Gap-1 phase or simply the G1 phase is the first phase of the interphase of a cell. This is the phase during which various enzymes and nutrients are synthesized by the cell, which are eventually required for the replication of DNA and cell division.

Most of the proteins required for division are synthesized during this phase. The duration of the G1 phase can vary from cell to cell or also on the availability of nutrients.

Even though the G1 phase is a part of the interphase of the cell cycle, it can also be divided into four subphases: competence, entrance, advancement, and assembly.

  • The process through which a cell receives nutrients and substances from beyond the cell membrane is referred to as competence.
  • These elements are employed to help the cell expand during the progression subphase after they enter the cell during the entry subphase.
  • In the last subphase or assembly, all the cell’s components come together to complete the G1 process and the restriction point stage.

S PHASE:

The S phase (Synthesis Phase) of the cell cycle occurs between the G1 and G2 phases and is when DNA is duplicated. Because precise genome duplication is essential for effective cell division, the mechanisms that take place during S-phase are highly controlled and conserved.

  • During the S phase, a typical cell only copies its diploid genome once.
  • The synthesis phase of the cell cycle is crucial for accurately reproducing the genetic information stored in the cell’s nucleus.
  • The activation and signaling of a variety of particular components, including CDKs and Myc(a family of transcription factor-coding regulator genes and proto-oncogenes), is required for the G1 to S phase transition.
  • The G1 restriction point (R) controls entry into S-phase, committing cells to the rest of the cell cycle if nutrients and growth signals are present. This entry is one wat and the cell cannot return to the G1 phase.
  • To begin DNA replication, the cell transforms pre-RCs generated in M and G1 phases into active replication forks during S-phase.
  •  Cdc7 and different S-phase CDKs, both of which are increased upon S-phase entry, are required for this process.
  • During S-phase, the cell produces free histones, which are quickly integrated into new nucleosomes. This mechanism is inextricably linked to the replication fork since it occurs both in front of and behind the replication complex.

G2 PHASE:

  • The third and final sub phase of interphase in the cell cycle, right before mitosis, is known as the G2 phase, Gap 2 phase, or Growth 2 phase.
  •  G2 takes place after the S phase, during which the cell’s DNA is completely duplicated successfully
  • G2 phase ends when prophase (the first phase of mitosis) begins and the cell’s chromatin condenses into chromosomes.
  • As the cell prepares for mitosis, the G2 phase is marked by rapid cellular expansion and protein synthesis. Surprisingly, the G2 phase of the cell cycle is not a necessary requirement.
  • Breaks in one sister chromatid caused by double-strand chromatids formed in the S phase can be repaired by homologous recombinational repair utilizing the other intact sister chromatid as a template during the G2 phase.
stages-of-interphase
Cyclin B feed back loop that triggers the end of G2 phase and start of M phase Image: Wikipedia

What happens in the interphase?

Interphase is technically termed as the in-between phase between two cell division phases in the cell cycle.

Interphase or the intermining phase is when the cell prepares itself to undergo mitosis or meiosis. A time the cell takes to synthesize the required materials itself to sustain two cells in one to undergo division of these resources.

An illustration of interphase
Image: Wikipedia

This includes synthesis and replication of genetic materials, enzymes, energy resources and even cell organelles.

Unlike mitosis, though a cell does require to undergo interphase before meiosis is is technically a part of meiosis. This is probably because meiosis itself has to occur in 2 stages- meiosis 1 and meiosis 2.

Which set of symbols represents the stages of interphase?

The interphase stage of the cell cycle is made up of a total of three phases.

These stages are elaborated as -the Growth phase 1 or Gap phase 1, the Synthesis phase and finally the  Growth phase 2 or Gap phase 2. Biologically they are simply termed as G1 phase, S phase, and G2 phase.

Simple representation of the phases of the cell cycle
Image: Wikipedia

Each of these phases has its specific utilities, durations and functions. The reason they are usually represented in symbolic form is to make them easier to remember and also because the names can be quite misleading.

 Only the Synthesis phase makes some sense. But even then, the S phase has more function than just synthesizing of materials

Stage of interphase where DNA is replicated:

  • The S phase of the cell cycle is responsible for DNA synthesis or replication during interphase, before mitosis or meiosis. A cell’s genetic material is doubled in this way before starting mitosis or meiosis, allowing enough DNA to be divided into daughter cells.
Nucleosome replication in S phase
Image: Wikipedia
  • Since new DNA synthesis requires histones the first function of the S phase is histone synthesis.
  • These free histones are then rapidly incorporated into nucleosome or simply DNA replication that also occurs in the S phase.
  • This mechanism is inextricably linked to the replication fork since it occurs both in front of and behind the replication complex.
  • Chromatin assembly factors (CAFs), which are loosely connected with replication proteins, are responsible for nucleosome re-assembly following the replication fork.

The longest stage of the interphase:

The longest phase of cell division is also the S phase probably because the cell is most active during this stage.

DNA synthesis in S phase
Image: Wikipedia

The cell has to perform quite a long list of activities in this phase including:

  • Nucleosome or DNA replication
  • But before creating more DNA it also synthesizes more free histone proteins.
  • After replication of DNA is completed it is thoroughly checked and marked with checkpoints for further checking and also or chromatin division.
  • The last activity that occurs is the re-establishment of the chromatin domains in the cell nucleus.

Since such a large amount of actions occur in this stage it takes the longest to progress to the next.

During what phase of the cell cycle does cell division occur?

Cell division occurs in what is called as the M-phase.

The cell cycle is divided mainly into 2 phases-interphase and the M phase or mitotic phase. Meiosis also has interphase but it is not included in the meiotic cell cycle.

Illustration of cytokinesis Image: Wikipedia

During cell division, the mother or original cell is divided into two or four daughter cells in mitosis and meiosis respectively. This divides the genetic material and cellular components equally among the daughter cells.

Stages of interphase in meiosis:

Unlike Mitosis Meiotic division occurs twice to produce four daughter cells instead of two. Interphase (divided into G1, S, and G2 phases), prophase 1, metaphase 1, anaphase 1, telophase 1, prophase 2, metaphase 2, anaphase 2, and telophase 2 are the general processes of meiosis.

Interphase in meiosis also has the same stages of G1 phase, S phase and G2 phase.

  1. G1 PHASE: The chromosomes are contained within a nuclear membrane at this period. Cells proliferate and carry out many of their essential jobs. Producing proteins and delivering or receiving signals from other cells are examples of these tasks.
  2. S PHASE: Sister chromatids are identical twins that each chromosome produces. The twins are connected by a thick region known as a centromere. Sister chromatids are twin chromosomes that have been linked together. The nuclear envelope is still present throughout the S phase, and the chromatids are not distinguishable. During the S phase, a spindle grows in plant cells that will eventually pull the chromatids apart.
  3. G2 PHASE: The G2 phase, which is similar to the G1 phase, for the most part, is the final phase of the meiotic interphase. With the double chromosomes wrapped inside a nuclear membrane, the cell continues to expand and execute its biological functions. Centriole pairs, which are bundles of microtubules that replicate within the centrosome near the end of the G2 phase in animal cells, become well-defined.