Phases of Cell Cycle, Overview of Cell Cycle, Phases, and Significance

The phases of the cell cycle are essential steps that a cell goes through to grow and divide, creating two new cells. These phases ensure that the genetic material is copied correctly and distributed equally between the new cells. Understanding these phases is important for students preparing for the NEET Exam, as they form a key part of the NEET biology syllabus .

What are the Phases of Cell Cycle?

Cell division is a key process for all living organisms. During this process, a cell grows, copies its DNA , and divides into two new cells. The cell cycle is the series of steps a cell follows to duplicate its DNA, make the necessary cell parts, and divide. Although cell growth happens continuously, DNA replication occurs at a specific stage. This division ensures that the new cells (called daughter cells) receive complete and accurate genetic information. The entire process is controlled by genes.

• Interphase
• M Phase (Mitosis)

Interphase

• G1 Phase (Gap 1): In this phase, the cell grows and performs its normal functions, such as producing proteins and energy. However, no DNA replication occurs yet.
• S Phase (Synthesis) : During the S phase, the cell duplicates its DNA. This means the amount of DNA in the cell doubles, but the number of chromosomes stays the same.
• G2 Phase (Gap 2): In the G2 phase, the cell continues to grow and prepares for the next phase, which is mitosis. Important proteins for cell division are produced, ensuring the cell is ready for the division process.
• Quiescent Stage (G0) : Some cells, like heart muscle cells in adult animals, do not divide and enter a resting phase called the G0 phase. In this phase, the cells remain active but do not divide unless needed for repair, such as after an injury.

M Phase (Mitosis)

Two Main Parts of the M Phase:

• Karyokinesis : Division of the nucleus.
• Cytokinesis: Division of the cytoplasm.

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Stages of Karyokinesis

Prophase

• Chromosomes condense and become visible as compact structures, each made up of two chromatids connected at the centromere.
• Centrosomes move to opposite poles, radiating microtubules (asters), forming the mitotic spindle.
• The Golgi complex, endoplasmic reticulum (ER) , nucleolus, and nuclear envelope disappear by the end of prophase.

Metaphase

• Chromosomes align along the equatorial plane of the cell, known as the metaphase plate.
• Spindle fibers attach to the kinetochores, located at the centromeres of each chromosome.
• One chromatid from each chromosome is connected to spindle fibers from one pole, while the sister chromatid is attached to fibers from the opposite pole.

Anaphase

• Centromeres divide, and the sister chromatids separate, becoming individual daughter chromosomes.
• The chromosomes move toward opposite poles, with centromeres leading and arms trailing.

Telophase

• Chromosomes gather at the poles and start to decondense, losing their clear shape.
• Nuclear envelopes form around each cluster of chromosomes, creating two daughter nuclei.
• The nucleolus, Golgi complex, and ER reappear, signaling the end of karyokinesis.

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Cytokinesis

• In Animal Cells :
  • A cleavage furrow forms in the plasma membrane.
  • The furrow deepens, pinching the cell membrane inward until the cytoplasm is divided into two daughter cells.
• In Plant Cells :
  • Cytokinesis begins with the formation of a cell plate at the center of the cell.
  • The cell plate grows outward, merging with the existing cell walls to form a new cell wall, thus separating the daughter cells.
• Organelle Distribution : During cytokinesis, organelles such as mitochondria and plastids are distributed between the two daughter cells to ensure they function properly.
• Variations : In some organisms, karyokinesis occurs without cytokinesis, resulting in a multinucleate condition called a syncytium, such as in the liquid endosperm of coconuts.

Significance of Phase Cell Cycle

• The G1 phase is essential for cell growth and development.
• S phase ensures that the genetic material is duplicated, providing identical sets of chromosomes for each daughter cell.
• During the G2 phase, the cell continues to grow and prepares for mitosis by synthesizing proteins and organelles required for division.
• The M phase is essential for nuclear division, as it accurately segregates duplicated chromosomes into two daughter nuclei, maintaining genetic consistency.
• Cytokinesis is important for dividing the cytoplasm, completing the cell division process, and ensuring that both daughter cells receive the necessary organelles and resources.
• The phases of the cell cycle are critical for the growth and development of organisms, allowing them to transition from a single cell to complex multicellular structures.
• They contribute to cellular repair and maintenance, replacing damaged or dead cells to maintain tissue homeostasis.

MCQs of Phase of Cell Cycle

Q1. G1 phase is:-

1. End of G2 phase.
2. End of S phase and the start of mitosis.
3. Start of S phase and the end of mitosis.
4. End of mitosis and the start of S phase.

Q2. In the ’S’ phase of the cell cycle:

1. The amount of DNA is reduced to half in each cell.
2. The amount of DNA doubled in each cell.
3. The amount of DNA remains the same in each cell.
4. The chromosome number has increased.

Q3. Cells at the end of prophase, when viewed under the microscope, do not show:-

1. Golgi body and ER
2. Nucleolus
3. Nuclear envelope
4. All of these

Answers of MCQs of Phases of Cell Cycle