Problem 55 - Objective Biology for NEET

Question

The correct sequence of stages in cell cycle is (a) \(\mathrm{G}_{1}, \mathrm{~S}, \mathrm{G}_{2}, \mathrm{M}\) (b) \(\mathrm{G}_{1}, \mathrm{G}_{2}, \mathrm{~S}, \mathrm{M}\) (c) \(\mathrm{M}, \mathrm{S}, \mathrm{G}_{1}, \mathrm{G}_{2}\) (d) \(\mathrm{G}_{2}, \mathrm{G}_{1}, \mathrm{M}, \mathrm{S}\)

Step-by-Step Solution

Answer

The correct sequence of stages in the cell cycle is option (a): \(\mathrm{G}_{1}\), \(\mathrm{S}\), \(\mathrm{G}_{2}\), \(\mathrm{M}\).

Step 1: Understand the Cell Cycle

The cell cycle is a process that all cells go through, and it's how they reproduce. It consists of four main stages: \(\mathrm{G}_{1}\), \(\mathrm{S}\), \(\mathrm{G}_{2}\), and \(\mathrm{M}\). Respectively, these stand for: Growth 1, Synthesis, Growth 2, and Mitosis.

Step 2: Compare the Stages in Each Option

Compare the sequence of stages in each provided option to the actual sequence of the cell cycle. This will allow us to identify which option has the correct sequence.

Step 3: Select the Correct Sequence

\(\mathrm{G}_{1}\), \(\mathrm{S}\), \(\mathrm{G}_{2}\), and \(\mathrm{M}\) is the correct sequence of the cell cycle. Thus, option (a) is the correct answer.

Key Concepts

G1 Phase

The G1 phase, also known as the first gap phase, is the initial stage of the cell cycle. During this phase, the cell experiences significant growth as it prepares to replicate its DNA in the upcoming phase.
The primary activities during the G1 phase include:

• Protein synthesis to increase the cell's size.
• Production of mRNA, which is essential for DNA replication.
• Duplication of organelles to prepare for cell division.

This phase acts as a checkpoint ensuring that the cell is ready and equipped with the necessary nutrients and energy to enter the S phase. Any damage to DNA or lack of nutrients can halt the cell's progression into the next phase, ensuring errors are minimized.

S Phase

The S phase, or synthesis phase, is characterized by the replication of the cell's DNA. This is a crucial phase where each chromosome is duplicated to ensure that both daughter cells will have an identical set of genes after cell division.
Here’s what happens during the S phase:

• All the DNA is carefully replicated.
• Each chromosome is duplicated, resulting in two sister chromatids.
• Energy is consumed significantly to fuel these processes.

This phase is essential for maintaining genetic consistency between generations of cells. By the end of the S phase, the cell has twice as much DNA as it did at the end of the G1 phase.

G2 Phase

The G2 phase is the second gap phase and is primarily a preparatory phase for mitosis. During this phase, the cell continues to grow and prepares all necessary components needed for cell division.
Key processes of the G2 phase include:

• Additional protein synthesis and organelle production to support cell division.
• Repair of DNA damage that might have occurred during DNA replication.
• Further cellular growth as a last preparation step.

Towards the end of the G2 phase, the cell checks for DNA damage and ensures all chromosomes have been replicated correctly, serving as another critical checkpoint in the cell cycle before mitosis begins.

Mitosis (M Phase)

The M phase, known as mitosis, is the stage where the cell divides its duplicated DNA and cytoplasm to create two identical daughter cells. Mitosis ensures that each new cell receives an exact copy of the parent cell's DNA.
Mitosis is further divided into several stages:

• Prophase: Chromosomes condense and become visible, and the nuclear envelope dissolves.
• Metaphase: Chromosomes align in the center of the cell.
• Anaphase: Sister chromatids are pulled apart to opposite ends of the cell.
• Telophase: Chromatids arrive at poles, nuclear membrane reforms around each set of chromosomes.

After mitosis, cytokinesis occurs, splitting the cell's cytoplasm and concluding the cell cycle. This process ensures equal distribution of cellular components and is vital for growth, repair, and reproduction.