What is the cell cycle? Describe its various phases

The cell cycle refers to a series of events that take place in a cell as it grows and divides to form two daughter cells. The cycle is crucial for the growth, development and maintenance of all living organisms, ensuring that the cells replicate and divide properly. The concept of the cell cycle was first systematically described by Howard and Pelc in 1953. They observed the different stages of cell division and growth, which led to the understanding of the continuous process of the cell cycle. The cycle is highly regulated, and any errors in this process can lead to diseases such as cancer.

Phases of the Cell Cycle

The cell cycle is divided into two main stages:
  1. Interphase: The preparatory phase where the cell grows and DNA is replicated.
  2. Mitotic Phase (M phase): The phase where the actual division of the cell occurs.

1. Interphase:

Interphase is the longest phase of the cell cycle and is primarily involved in preparing the cell for division. It is divided into three sub-phases:
  1. G1 Phase (Gap 1):
    • This is the first phase of interphase and marks the cell's growth period. During G1, the cell synthesizes RNA and proteins necessary for DNA replication. It also performs its normal functions in terms of metabolism and energy production. If the cell checks and detects any errors in its environment or internal processes, it can delay further progress in the cycle to repair these issues. This is also the phase where cells decide whether to continue through the cycle or enter G₀ phase (G Zero Phase).
  2. S Phase (Synthesis):
    • In this phase, the cell's DNA is replicated. Each chromosome is duplicated, creating two sister chromatids joined at the centromere. This phase ensures that after cell division, each daughter cell will have an identical set of chromosomes.
  3. G2 Phase (Gap 2):
    • In the G2 phase, the cell continues to grow and prepares for the mitotic phase. During this stage, the cell synthesizes proteins required for mitosis and checks the replicated DNA for any errors. If any issues are found, the cell can halt the process to correct the mistakes before proceeding to division.
  • G₀ Phase (Resting Stage):
    • The G₀ phase is also referred to as the resting stage, but this doesn't mean that the cell is inactive. It's a phase where some cells exit the cell cycle after the G₁ phase and may stay in this phase temporarily or permanently. Cells in the G₀ phase do not divide but continue performing normal biological functions. For example, nerve cells and cardiac muscle cells often enter the G₀ phase and may never divide again.

2. Mitotic Phase (M phase):

The mitotic phase is the part of the cell cycle where the actual division occurs. This phase is further divided into Karyokinesis and Cytokinesis.

1. Karyokinesis (Nuclear Division):

Karyokinesis refers to the division of the cell's nucleus. This is broken down into four stages:
  1. Prophase:
    • This is the first stage. The chromatin inside the nucleus becomes thick and changes into visible chromosomes. Each chromosome has two sister chromatids joined at the centromere. The nucleolus becomes smaller and disappears. The nuclear envelope starts breaking. Outside the nucleus, centrosomes move to opposite sides and begin forming spindle fibers.
  2. Prometaphase:
    • The nuclear membrane breaks fully. Chromosomes become free in the cytoplasm. A structure called the kinetochore appears on each centromere. Spindle fibers from opposite poles attach to these kinetochores.
  3. Metaphase:
    • Chromosomes line up at the center of the cell on the metaphase plate. Their centromeres are aligned. Spindle fibers are fully attached. Chromosomes are most clearly seen in this stage.
  4. Anaphase:
    • The centromeres divide. Sister chromatids separate and move to opposite poles. They are now called daughter chromosomes.
  5. Telophase:
    • Daughter chromosomes reach the poles and turn back into chromatin. A new nuclear membrane and nucleolus form. Spindle fibers disappear.

2. Cytokinesis (Cytoplasmic Division):

Cytokinesis is the process where the cell's cytoplasm and other organelles are divided into two daughter cells. 
  • In animal cells, a contractile ring forms around the center of the cell, pinching the cell membrane and forming two distinct daughter cells.
  • In plant cells, a cell plate forms in the center of the cell, eventually turning into a new cell wall, thus dividing the cell into two.
The cell cycle refers to a series of events that take place in a cell as it grows and divides to form two daughter cells. The cycle is crucial for the growth, development and maintenance of all living organisms, ensuring that the cells replicate and divide properly.









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