Difference Between Cell Cycle and Cell Division

Main Difference – Cell Cycle vs Cell Division

Cell cycle and cell division consist of a series of events taking place sequentially in a cell’s life. Cell cycle includes the whole series of events, interphase of the cell which is followed by the mitotic phase, which is then followed by cytokinesis. The interphase of the cell cycle can be divided into three sequential phases: G₁, S, and G₂. Cell division occurs during the mitotic and cytokinetic periods of the cell cycle. The mitotic period can be divided into four phases: prophase, metaphase, anaphase and telophase. The cytokinesis is the division of the cytoplasm. The main difference between cell cycle and cell division is that cell cycle is the series of periods in the cell’s life whereas cell division is the series of phases where the cell splits to increase its number in the population.

This article explains,

1. What is Cell Cycle
      – Phases, Characteristics, Regulation
2. What is Cell Division
      – Phases, Characteristics, Regulation
3. What is the difference between Cell Cycle and Cell Division

What is Cell Cycle

The cell cycle is the series of events taking place during the cell’s life. The eukaryotic cell cycle is mainly composed of three sequential periods: interphase, mitotic phase, and the cytokinesis. During the interphase, the cell growth occurs through the synthesis of required proteins for the cell’s future stages and the replication of DNA to carry out the cell division. During the mitotic phase, the nucleus is divided into genetically identical two daughter nuclei, initiating the cell division. Cytokinesis is the division of the cytoplasm of the parent cell. Cell cycle checkpoints ensure the proper division of the eukaryotic cells.

Prokaryotic cell cycle can be divided into three sequential periods: B, C, and D. DNA replication is initiated at the B period and continued during the C period. It ends at the D period. Bacterial cell also splits into to daughter cells during the D period.

Cell Cycle Periods

The eukaryotic cell cycle is composed of three major sequential phases known as interphase, M phase, and cytokinesis. The interphase is the initial phase of the cell cycle in eukaryotes. Before entering into cell division, the cell prepares for the division by up taking all requiring nutrients into the cell, protein synthesis and replication of DNA during the interphase. Interphase takes about 90% of the total time of the cell cycle.

The interphase can be divided into three phases, which occur one after the other. They are G₁ phase, S phase, and G₂ phase. Before entering into the G₁ phase, a cell normally exists at the G₀ phase. The G₀ phase is the resting phase where the cell leaves the cell cycle and stops its division. Generally, non-dividing cells of the multicellular organisms, which are at G₁ phase enter this quiescent G₀ phase. Some cells like neurons remain dormant permanently. Some cells like kidney, liver and stomach cells remain semi-permanently in the G₀ phase. Some cells like epithelial cells do not enter G₀ phase. Entering of the cells into G₀ phase is shown in figure 1.

Figure 1: Entrance to the G0 phase

The G₁ phase or the growth phase is the first phase of the cell cycle. The biosynthetic activities of the cell take place rapidly during the G₁ phase. Synthesis of the proteins, as well as the increase of the number of organelles like mitochondria and ribosomes, occur at the G₁ phase, growing the cell in its size. G₁ phase is followed by the S phase. Replication of DNA commences and completes during the S phase, forming two sister chromatids per single chromosome. The ploidy of the cell remains unchanged by the doubling of the amount of DNA during the replication. The S phase is completed within a short period of time in order to rescue DNA from external factors like mutagens. S phase is followed by the G₂ phase. The G₂ phase is the second growth phase of the interphase which allows the cell to complete its growth prior to its division.

Regulation of Cell Cycle by Cyclin-CDK Complexes

The occurrence of the cell cycle in a sequential manner is regulated by two classes of regulatory molecules: cyclins and cyclin-dependent kinases (CDKs). Cyclins produce regulatory subunits while CDKs produce catalytic subunits. Both cyclins and CDKs work in an interactive manner. The preparation of the cell for S phase which is at the G₁ phase is done by the G₁ cyclin-CDK complex by promoting the expression of transcription factors which promotes the S cyclins. G1 cyclin-CDK complex also degrades the S phase inhibitors.

The timing of the G₁ phase is regulated by cyclin D-CDK4/6, which is activated by G₁ cyclin-CDK complex. The cyclin E-CDK2 complex pushes the cell from G₁ to S phase (G₁/S transition). Cyclin A-CDK2 inhibits the DNA replication of the S phase by disassembling the replication complex. A large pool of cyclin A-CDK2 activates the G₂ phase. Cyclin B-CDK2 pushes G₂ phase to the M phase (G₂/M transition). 

Regulation of Cell Cycle through Checkpoints

Two checkpoints can be identified during the interphase: G₁/S checkpoint and G2/M checkpoint. The transition of G₁/S, is the rate-limiting step of the cell cycle which is known as the restriction point. By the G₁/S checkpoint, the presence of the sufficient raw materials for the DNA replication is checked. The simultaneous replication of DNA in a growing embryo is checked by G₂/M checkpoint, obtaining a symmetrical cell distribution in the embryo.

Figure 2: Cell Cycle with Cyclin-CDKs and checkpoints

What is Cell Division

Cell division is the splitting of a parent cell into two daughter cells. This includes two periods of the cell cycle: mitotic division and the cytokinesis.

Cell Division Periods

The four phases in the mitotic division are prophase, metaphase, anaphase and the telophase. During prophase, chromatids are condensed into chromosomes, exhibiting short and thick thread-like structures. These chromosomes are aligned in the equatorial plate of the cell by the formation of a spindle apparatus. Spindle apparatus is composed of three components: spindle microtubules, kinetochore microtubules, and kinetochore protein complexes. The kinetochore protein complexes are attached to the centromeres of each chromosome. All the microtubules in a cell are controlled by two centrosomes arranged at the opposite poles of the cell, forming the spindle apparatus. Spindle microtubules are connected to each of the two centrosomes by their two ends. Kinetochore microtubules, starting from one centrosome, are attached to the centromere through the kinetochore protein complex.  

During metaphase, kinetochore microtubules are contracted, aligning the individual bivalent chromosomes on the cell equator. Tension is generated on the centromere which holds the two sister chromatids together at the anaphase by further contracting the kinetochore microtubules. This tension leads to the cleavage of cohesin protein complexes in the centromere, separating the two sister chromatids apart, producing two daughter chromosomes. During telophase, these daughter chromosomes are pulled towards the opposite poles by the contraction of the kinetochore microtubules.

After completing the mitotic phase, parent cell undergoes cytoplasmic division, resulting in genetically identical two separated cells. The cytokinesis is initiated at the late anaphase. During cytokinesis, organelles, along with the cytoplasm, is divided between two daughter cells by the cell membrane in an approximately equal manner. Plant cell cytokinesis takes place through the formation of a cell plate in the middle of the parent cell. Animal cell cytokinesis takes place by the cleavage furrow formed by the cell membrane. The difference between plant and animal cell cytokinesis is the requirement of a formation of new cell wall surrounding the plant cell.

Phases of Cell Division

Regulation of Cell Division by Cyclin-CDK Complexes and Checkpoints

The cyclin B-CDK2 complex controls the timing of the G₂ phase, entering the mitotic division. A single, but critical checkpoint can be identified. It is known as the metaphase checkpoint since it takes place at the late metaphase. During the metaphase checkpoint, the aligning of all individual, bivalent chromosomes on the cell equator is checked. The metaphase checkpoint allows the equal segregation of chromosomes between daughter cells. The dividing cell at late metaphase should pass the mitotic checkpoint in order to enter to the anaphase.

Difference Between Cell Cycle and Cell Division

Definition

Cell Cycle: The cell cycle is the series of periods of the cell’s life.

Cell Division: Cell division is the splitting of a cell into two daughter cells, increasing the cell number in the population.

Periods

Cell Cycle: Cell cycle is composed of three periods: interphase, mitotic division, and cytokinesis.

Cell Division: Cell division occurs in the last two periods of the cell cycle, mitotic division and the cytokinesis.

Regulation through Cyclin-CDK Complexes

Cell Cycle: Cyclin D-CDK4/6, cyclin E-CDK2, cyclin A-CDK2 and cyclin B-CDK2 are involved in the regulation of cell cycle.

Cell Division: Cyclin B-CDK2 are involved in the regulation of cell division.

Regulation through Checkpoints

Cell Cycle: Two checkpoints can be identified during the interphase: G₁/S checkpoint and G₂/M checkpoint. 

Cell Division: The mitotic checkpoint is involved in the regulation of cell division.

Conclusion

Both cell cycle and cell division contain different but sequential periods of the cell’s life. The cell cycle is composed of three periods. They are interphase, mitotic phase, and the cytokinesis. Mitotic division and cytokinesis are collectively called as the cell division. The interphase of the cell cycle is composed of G₁, S and G₂ phases. Mitotic division is composed of four phases: prophase, metaphase, anaphase and the telophase. Telophase is followed by the cytokinesis. The main difference between cell cycle and the cell division is the fact that the cell division is a part of the cell cycle.

Reference:
1.”Cell cycle.” Wikipedia. Wikimedia Foundation, 08 Mar. 2017. Web. 10 Mar. 2017.

Image Courtesy:
1. “0329 Cell Cycle”By OpenStax – (CC BY 4.0) via Commons Wikimedia
2. “0332 Cell Cycle With Cyclins and Checkpoints” By OpenStax –  (CC BY 4.0) via Commons Wikimedia
3. “Mitosis cells sequence” By LadyofHats – Own work. (Public Domain) via Commons Wikimedia