Cell Cycle – Definition, Phases, Checkpoints & Importance

Are we aware of the tallest and smallest animal or human in the world? How they become so tall or so small. These are nothing but the result of the Cell Cycle and Cell Divisions. Cells are the basic units of all living things. Cells make up leaves, roots, stems, and flowers the same way cells are responsible for bones, muscles, skin, and blood. As the organism grows, the cells must reproduce.

What is Cell Cycle?

Cell Cycle is the sequence of series of events that occur in a cell, by which a cell duplicates its genome and synthesises the other cell constituents. These events include the duplication of its DNA and its organelles and subsequently the partitioning of its cytoplasm and other components into two daughter cells. The cell cycle is simpler in prokaryotic cells, but in the case of eukaryotic cells, the cell cycle is more complex.

Cell Cycle and Cell Division

A Cell Cycle is the series of events that occur in a cell that includes the division and growth of a cell. The division of a mother cell into two or more daughter cells is known as Cell Division. Cell division is the key to growth, repair, and reproduction. Every dividing cell is called a mother cell, and its descendants are named daughter cells. Cell division is generally referred to as cellular reproduction.

What are the Phases of Cell Cycle?

Cell Cycles consist of two main phases:

(A) Interphase

(B) M Phase (Mitotic Phase)

(A) Interphase

In Interphase, a newly formed cell and its nucleus enquiring a series of changes before it becomes capable of division again. The interphase called the resting phase; in the meantime, the cell is preparing for division by undergoing both cell growth and DNA replication in an orderly manner. Interphase lasts quite \(95\% \) of the cell cycle. It is further divided into four phases:

1. \({{\rm{G}}_{\rm{0}}}\) Phase (Quiescent Stage)
2. \({{\rm{G}}_1}\) Phase (Gap \(1\))
3. \({\rm{S}}\) Phase (Synthesis)
4. \({{\rm{G}}_2}\) Phase (Gap \(2\))

Fig: The Cell Cycle

1. \({{\rm{G}}_0}\) Phase (Quiescent Stage)
a) This is known as the resting or inactive or quiescent stage, as it neither divides nor grows.
b) This phase can divide further as an extended \({{\rm{G}}_1}\) phase or a separate phase-out of the cell cycle.
c) In this phase, cells remain metabolically active but do not undergo division.
d) These cells can resume division as and when required.
e) Nerve cells and muscle cells are examples of cells that enter the \({{\rm{G}}_0}\) phase when they reach maturity.
f) Sometimes, cells might enter the \({{\rm{G}}_0}\) phase from the checkpoint in the \({{\rm{G}}_1}\) phase because of the lack of growth factors or nutrients.
g) Some cells like the liver and kidneys’ parenchymal cells enter the \({{\rm{G}}_0}\) phase semi-permanently and can be induced to divide.
h) The \({{\rm{G}}_0}\) phase is often associated with senescence; the \({{\rm{G}}_0}\) phase is a reversible stage where a cell can enter the cell cycle again to divide.
i) The cells in the \({{\rm{G}}_0}\) phase have different regulators that ensure the proper functioning of the cell.

2. \({{\rm{G}}_1}\) Phase (Gap \(1\))
a) \({{\rm{G}}_1}\) phase is a part of the interphase.
b) This phase commences at the termination of the previous mitotic phase and continues till the starting of \({\rm{DNA}}\) replication, so it is known as the first growth phase or post-mitotic gap phase.
c) Throughout this phase, the cell is metabolically active and continuously grows.
d) In this phase, the transcriptions of all three sets of \({\rm{RNA}}\) \(\left( {{\rm{mRNA,}}\,{\rm{tRNA}}\,{\rm{and}}\,{\rm{rRNA}}} \right)\) and proteins are synthesised.
e) The duration of the \({{\rm{G}}_1}\) phase is also highly variable among different cells.
f) An important in the \({{\rm{G}}_1}\) phase is the \({{\rm{G}}_1}/{\rm{S}}\) checkpoint determining if the cell is ready to proceed into the division phase.
g) At this point, events like detecting \({\rm{DNA}}\) damage and nutrient concentration are performed to make sure that the cell has enough machinery to undergo cell division.

3. \({\rm{S}}\) Phase (Synthesis)
a) \({\rm{S}}\) phase is known as the synthesis phase. It takes place in between \({{\rm{G}}_1}\) and \({{\rm{G}}_2}\) phases.
b) In this phase, \({\rm{DNA}}\) replication takes place on the template of the existing \({\rm{DNA}}\) to form chromatin and chromatid.
c) The formation of histone proteins and other proteins are important in this phase as the newly replicated \({\rm{DNA}}\) molecules need histone proteins to form nucleosomes.
d) The entry into the \({\rm{S}}\) phase is regulated by the \({{\rm{G}}_1}/{\rm{S}}\) checkpoint that only enables cells with enough nutrients and healthy \({\rm{DNA}}\) to enter the next phase.
e) This phase is moderately long, occupying about \(30\%\) of the total cell cycle time.
f) Each chromosome carries a duplicate set of genes; a haploid cell becomes diploid, and a diploid cell becomes tetraploid at the end of the \({\rm{S}}\) phase.

4. \({{\rm{G}}_2}\) Phase
a) \({{\rm{G}}_2}\) is also known as the second growth phase or pre mitotic gap phase because the cell collects nutrients and releases proteins to prepare the cell for the \({\rm{M}}\) phase.
b) This phase is also essential as it checks for \({\rm{DNA}}\) damage (during replication) to make sure that the cell is in proper condition to undergo division.
c) In this phase, the synthesis of \({\rm{DNA}}\) stops, synthesis of \({\rm{RNAs}}\) and proteins continues.
d) Organelles and spindle formation take place.
e) The entry of the cell from the \({{\rm{G}}_2}\) phase to the \({\rm{M}}\) phase is regulated by the \({{\rm{G}}_2}\) checkpoint, where different proteins and complexes are involved.
f) In case of \({\rm{DNA}}\) damage or insufficient nutrients, the cell remains in the \({{\rm{G}}_2}\) phase and is not passed for cell division.

(B) \({\rm{M}}\) Phase (Mitotic Phase)

\({\rm{M}}\) phase is the most dramatic multi-step process in which actual cell division occurs. This phase starts with the nuclear division, corresponding to the separation of the daughter chromosome (Karyokinesis) and usually ends with the division of cytoplasm (Cytokinesis). It is also called an equational division because the number of chromosomes in parent and daughter cells remain the same.

Mitosis:
a) The cellular division is called somatic division.
b) Cell division resulting in the assembly of diploid cells for growth and development.
c) In plants, mitosis happens in both haploid, diploid cells.
d) Two major events happen in mitosis a) Karyokinesis and b) Cytokinesis.
e) Mitosis is accomplished during the expansion and development of the organisms.
f) Karyokinesis has four phases:

1. Prophase
2. Metaphase
3. Anaphase
4. Telophase

Fig: Stages of Mitosis

The four stages of karyokinesis are:

1. Prophase

a) It is the first stage of mitosis where the chromosomes become shorter and thicker and have clear visibility.
b) Chromosomes are identical to paired chromatids.
c) Sister chromatids link to each other at a little region called the centromere.
d) Centrioles begin moving apart and reach opposite poles.
e) Spindle fibres appear in the middle of daughter centrioles forming the achromatic spindle.
f) Various cell organelles as Golgi bodies and endoplasmic reticulum cannot be seen throughout this stage. Nucleolus and nuclear envelope also vanish from the site.

2. Metaphase

a) Each chromosome gets attached to the spindle by its centromere.
b) In each chromosome, both chromatids are connected to the spindle fibre from both poles.
c) Chromosomes line up in one plane at the equator.
d) The plane of alignment of chromosomes during this phase is called the metaphase plate.

3. Anaphase

a) Centromere attaching the two chromatids divides.
b) The two sister chromatids of each chromosome separate and are drawn apart towards opposite poles pulled by shortening of spindle fibres.

4. Telophase

a) Two sets of daughter chromosomes reach opposite poles.
b) Spindle fibres vanish from sight.
c) Chromatids thin get in the shape of chromatin fibres.
d) The nuclear envelope is created around the chromosome clusters. Nucleolus, Golgi complex and ER are formed.
e) The cleavage lines begin deepening within the animal cell.
g) Karyokinesis is followed by cytokinesis.

Cytokinesis

a) At the top of telophase, a furrow appears within the cell membrane in the middle, which deepens and eventually splits the cytoplasm into two, thus producing two new cells.
b) In plant cells, cell wall formation begins within the centre with the formation of the cell plate. This grows outwards to meet the existing lateral walls, and thus, the cytoplasm is split into two parts.
c) This cell plate becomes the centre lamellae for the plant cells.
d) In some organism’s cytokinesis is not immediately followed by karyokinesis , and therefore, the multinucleate stage is created, referred to as a syncytium, e.g., like fungi, algae, and plant cells.

Cell Cycle Checkpoints

Fig: Cell Cycle Checkpoints

Checkpoints	Characters
\({{\rm{G}}_1}\) Checkpoint	a) The availability of mitogen and energy-rich compounds decide whether the \({{\rm{G}}_1}\) phase will be arrested (\({{\rm{G}}_0}\) phase) or undergo the \({\rm{S}}\) phase. b) This point is named as \({{\rm{G}}_1}\) Checkpoint or \({{\rm{G}}_1}\) cyclin or \({{\rm{G}}_1}\) point. c) Once the checkpoint of the \({{\rm{G}}_1}\) phase crossed, the cell cycle will go on further division till the finish. d) Some cells also need mechanical cues like being attached to a supportive network called the extracellular matrix to divide. If a cell does not get the go-ahead cues it needs at the \({{\rm{G}}_1}\) checkpoint, it may leave the cell cycle and enter a resting state called \({{\rm{G}}_0}.\) e) Some cells stay permanently in \({{\rm{G}}_0}\) while others resume dividing if conditions improve.
\({{\rm{G}}_2}\) Checkpoint	a) For smooth cell division, the cell has an additional checkpoint before the \({\rm{M}}\) phase, called the \({{\rm{G}}_2}\) checkpoint. b) At this checkpoint, the cell will check for any damaged \({\rm{DNA}}\) and check whether \({\rm{DNA}}\) is completely copied during the \({\rm{S}}\) phase. c) If any errors or damage are detected, the cell will pause at the \({{\rm{G}}_1}\) checkpoint to repair. d) If the checkpoint mechanisms detect problems with the \({\rm{DNA}},\) the cell cycle is stopped, and the cell attempts to either complete \({\rm{DNA}}\) replication or repairs the damaged \({\rm{DNA}}.\) e) If the damage is irreparable, the cell may undergo apoptosis or programmed cell death. This self-destruction mechanism ensures that damaged \({\rm{DNA}}\) is not passed onto daughter cells and is essential in preventing cancer.
Metaphase Checkpoint	a) The \({\rm{M}}\) checkpoint is named the spindle checkpoint because in this checkpoint cell checks whether all the sister chromatids are correctly fixed to the spindle microtubules. b) Because the separation of the sister chromatids during anaphase is an irreversible step, the cycle will not proceed until all the chromosomes are firmly fixed at the two spindle fibres from opposite poles of the cell. c) It seems that cells do not scan the metaphase plate to confirm that all the chromosomes are there. d) Instead, they look for “straggler” chromosomes that are in the wrong place (e.g., floating around in the cytoplasm). d) Instead, they look for “straggler” chromosomes that are in the wrong place (e.g., floating around in the cytoplasm). e) If a chromosome is missing, the cell will pause mitosis, allowing time for the spindle to capture the stray chromosome.

Cell Cycle Importance

The importance of the Cell Cycle is given below:

a) The cell cycle repairs and controls damages caused to the cells.
b) The cell cycle helps in renewing damaged cells.
c) The cycle is important for the survival and growth of living organisms.
d) The cell cycle is essential to all organisms as if it is not present or stops suddenly, life on earth would completely stop.
e) The cell cycle provides a greater number of cells for growth and development.
f)  The cell cycle helps in the replication and reproduction of cells in both prokaryotes and eukaryotes.

Summary

Cell Cycle is a series of events in a cell at the time of its growth and division. These events include cell division, in which a parent cell divides into two or more daughter cells. The cell cycle consists of two stages interphase – the beginning of the cell division and the mitotic phase – which is the actual period of cell division.

Further interphase is subdivided into \({{\rm{G}}_{\rm{0}}}\), \({{\rm{G}}_{\rm{1}}}\), \({\rm{S}}\) and \({{\rm{G}}_{\rm{2}}}\) and Mitosis are also subdivided into four stages, namely Prophase, Metaphase, Anaphase and Telophase. The cell cycle is essential for all living organisms as it allows them to survive, replicate and reproduce. In brief, we can say that the cell cycle plays a significant role in the existence of life on earth.

Frequently Asked Questions (FAQs) on Cell Cycle

The commonly asked queries about cell cycle are answered here: