What is Cell Cycle?
The cell cycle is a series of events that occur during the life of a cell. Interphase, mitotic phase, and cytokinesis are the three stages that make up the eukaryotic cell cycle. Cell growth occurs during the interphase through the synthesis of needed proteins for the cell's subsequent stages, as well as the replication of DNA for cell division. The nucleus is divided into two genetically identical daughter nuclei during the mitotic phase, resulting in cell division. Cytokinesis is the division of the parent cell's cytoplasm.
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Stages of Cell Cycle
A cell must perform many crucial activities in order to divide: it must grow, duplicate its genetic material (DNA), and physically split into two daughter cells. The cell cycle is a sequence of phases in which cells perform various functions in an orderly and predictable manner. The cell cycle is a cycle rather than a linear pathway because each daughter cell can also undergo the same processes.
There are two key phases in the cell cycle of eukaryotic cells, or cells containing a nucleus, namely, interphase and mitotic (M) phase.
During interphase, the cell divides and duplicates its DNA.
The cell splits its cytoplasm and divides its DNA into two sets during the mitotic (M) phase, resulting in the production of two new cells.
Cell Cycle Interphase
Let's start the cell cycle from the beginning when a cell divides from its mother cell. There are mainly three steps to prepare for division, as discussed below.
G1 Phase
The cell becomes physically larger, duplicates organelles, and makes the chemical building blocks that will be needed in later processes during the G1 phase, also known as the first gap phase.
S Phase
The cell synthesizes a full copy of DNA in its nucleus during the S phase. The centrosome, a microtubule-organizing structure, is also copied. During the M phase, the centrosomes assist in the separation of DNA.
G2 Phase
The cell grows, produces proteins and organelles, and begins to restructure its contents in preparation for mitosis during the second gap phase, or G2 phase. When mitosis commences, phase G2 comes to an end.
Interphase is the combination of the G1, S, and G2 phases. The prefix inter-means between, reflecting the fact that interphase occurs between mitotic (M) phases.
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M Phase
During the mitotic (M) phase, the cell divides its copy DNA and cytoplasm to generate two new cells. During the M phase, two distinct division-related mechanisms occur mitosis and cytokinesis.
Mitosis
During mitosis, the cell's nuclear DNA condenses into visible chromosomes, which are pulled apart by the mitotic spindle, a specialised structure made up of microtubules. Mitosis is divided into four stages: prophase (sometimes known as early prophase and prometaphase), metaphase, anaphase, and telophase.
Cytokinesis
During cytokinesis, the cell's cytoplasm is split in half, resulting in the production of two new cells. Cytokinesis usually begins immediately as mitosis is concluding, with a little overlap. Importantly, cytokinesis happens in different ways in animal and plant cells.
Cell division occurs in mammals when the contractile ring, a band of cytoskeletal fibres, contracts inward and pinches the cell in half, a process known as contractile cytokinesis. The cleavage furrow is the indentation created as the ring contracts inward. As animal cells are delicate and squishy, they can be pinched in half.
Plant cells are substantially stiffer than animal cells, with a strong cell wall surrounding them and high internal pressure. Plant cells divide in half as a result of this, with a new structure forming in the middle of the cell. The cell plate is a structure that divides the cell in two and is made up of plasma membrane and cell wall components transported in vesicles. Plant cells have a stiff cell wall and a high internal pressure, making them much tougher than animal cells. As a result, cytokinesis happens differently in plant and animal cells.
Duration of the Cell Cycle
The cell cycle takes varying amounts of time for different cells. While a typical human cell divides in roughly 24 hours, fast-cycling mammalian cells, such as those that line the gut, can complete a cycle in as little as 9-10 hours when cultivated in vitro. Different cell types divide their time between cell cycle phases in various ways. For example, in early frog embryos, cells spend very little time in G1 and G2, preferring instead to cycle quickly between S and M phases, resulting in the division of one large cell, the zygote, into many smaller cells.
Some of the important differences between cell cycle and cell division are tabulated below.
What is the Difference Between Cell Cycle and Cell Division?
Did you Know?
Which is the Longest Cell Phase?
The longest cell phase is interphase, during which the cell goes through regular growth processes while also preparing for cell division. It is the longest phase of the cell cycle, with the cell spending over 90% of its time here.
FAQs on Cell Cycle
1. What is the cell cycle and why is it considered a cycle rather than a linear process?
The cell cycle is a series of organised events that a cell undergoes to grow and divide into two genetically identical daughter cells. It is called a cycle because, after division, each daughter cell can independently repeat the entire process, creating a continuous sequence of growth and division, which is essential for development and tissue repair.
2. Which are the main phases of the cell cycle, and what happens in each?
The main phases of the cell cycle are:
- Interphase: Consists of G1, S, and G2 phases, where the cell grows, duplicates its DNA, and prepares for division.
- Mitotic (M) phase: Involves mitosis (prophase, metaphase, anaphase, telophase) where the nucleus divides, and cytokinesis, where the cytoplasm splits to form two new cells.
3. Why is interphase considered the most important and longest phase of the cell cycle?
Interphase accounts for over 90% of the cell cycle’s duration. It is crucial because during this time, the cell grows, performs its regular functions, replicates DNA, and prepares all components necessary for division. Without successful interphase, mitosis cannot occur properly.
4. What is the functional significance of the G1, S, and G2 phases during interphase?
Each interphase stage plays a vital role:
- G1 phase: Cell grows and synthesises proteins and organelles.
- S phase: Entire DNA content is precisely duplicated.
- G2 phase: Additional growth and final production of molecules essential for mitotic division occur.
5. In what ways do mitosis and cytokinesis differ, and why is this difference important for cell division?
Mitosis is the division of the cell’s nucleus, ensuring equal DNA distribution to daughter nuclei. Cytokinesis is the separation of the cell's cytoplasm. Their distinction is crucial because it ensures genetic content and cell components are evenly split, leading to two functional daughter cells.
6. How does cytokinesis differ in plant and animal cells, and what causes this difference?
In animal cells, cytokinesis occurs by the formation of a contractile ring creating a cleavage furrow to pinch the cell. In plant cells, a cell plate forms in the centre due to the rigid cell wall, and this plate develops into a new wall separating the daughter cells. The difference is because of the presence of a strong, inflexible cell wall in plant cells.
7. Why do different cell types have variable durations for their cell cycles?
Cells spend different amounts of time in the phases of the cell cycle based on their function. For example, fast-dividing cells like those in embryos or gut lining prioritise rapid cycles, often minimising G1 and G2 phases, while differentiated cells may enter a resting stage (G0) or have longer cycles for specialised functions.
8. What is the biological significance of regulating the cell cycle strictly in multicellular organisms?
Strict cell cycle regulation ensures proper growth, maintenance, and repair of tissues. It prevents uncontrolled cell division which could lead to tumours or cancer, maintaining organism health and genomic stability as per the CBSE 2025–26 curriculum.
9. What would happen if a cell skipped the S phase but continued to divide?
If the S phase is skipped, the cell’s DNA would not replicate, resulting in daughter cells with incomplete or missing genetic material. Such cells are typically nonviable or dysfunctional, highlighting the necessity of DNA replication before cell division.
10. How does knowledge of the cell cycle help in understanding diseases such as cancer?
Understanding the cell cycle is key to identifying how unregulated division occurs in cancer. Cancer cells often lose normal control over the cell cycle, leading to unchecked growth. Many cancer treatments aim to target rapidly dividing cells by interrupting specific cycle phases.
