Problem 53 - Objective Biology for NEET

Question

Before cell division, the entire DNA content of the cell gets doubled during interphase.This doubling takes place (a) Throughout the interphase (b) At the beginning of the interphase (c) At the end of the interphase (d) Somewhere during the middle of the interphase

Step-by-Step Solution

Answer

The doubling of the cell's DNA content takes place 'somewhere during the middle of the interphase'. So, the correct answer should be option (d).

Step 1: Understand Cell Cycle

Interphase is the longest phase in the cell cycle. It is the time when a cell grows, duplicates its organelles, and prepares for cell division by duplicating its DNA during a phase called the S (Synthesis) phase.

Step 2: Define DNA Replication Moment

One of the distinct parts of the interphase is the S phase, where the replication or duplication of DNA occurs. This is when the DNA content of the cell gets doubled. The DNA does not continuously double throughout interphase, nor it doubles at just the beginning nor the end. It is an event that occurs during a specific mid-point part of the interphase – the S phase.

Step 3: Comparing Options with Concepts

Option (d) suggests that DNA replication occurs 'somewhere during the middle of the interphase,' which accords with the understanding of the S phase taking place in the midst of the interphase. Hence, option (d) is the best fit for the answer.

Key Concepts

Interphase

Interphase is a crucial part of the cell cycle. During this phase, the cell is not dividing but is preparing for the next division. It is the longest phase in the cell cycle and can be compared to a period of rest and preparation. This is when a cell grows, duplicates its organelles, and ensures it has everything it needs for division. The interphase is subdivided into three major stages:

• G1 (Gap 1) phase: The cell grows and carries out normal metabolic roles. It also begins preparing to replicate its DNA.
• S (Synthesis) phase: DNA replication occurs during this stage, ensuring that each daughter cell will receive a complete copy of the genetic material.
• G2 (Gap 2) phase: Further growth occurs, and the cell prepares for the final steps of cell division. It checks for DNA replication errors and repairs any damage.

In summary, interphase is about growth and preparation. It gets the cell ready to go into the cell division process, making sure everything is in place for a successful division.

DNA Replication

DNA replication is an essential biological process occurring during the S phase of interphase. It is the mechanism by which a cell makes a complete copy of its DNA, ensuring that both daughter cells inherit identical genetic information. This complex process involves several key steps and components that work together to achieve precise duplication.

• Unwinding the DNA: The DNA double helix gets unwound by an enzyme called helicase, breaking the hydrogen bonds between the base pairs and separating the two strands.
• Building new DNA strands: Each original strand serves as a template for a new complementary strand. The enzyme DNA polymerase attaches new nucleotides to form these strands.
• Proofreading and repair: DNA polymerase also proofreads each new strand and corrects any mismatched pairs to ensure accuracy, contributing to the overall fidelity of the DNA replication process.

The result of DNA replication is two identical DNA molecules, each with one old and one new strand, known as semi-conservative replication. This ensures that genetic information is preserved and passed on to future cell generations.

S Phase

The S phase represents the "Synthesis" phase in the interphase of the cell cycle. It is the period when DNA synthesis, or replication, takes place. Unlike other phases in interphase, S phase is specifically focused on duplicating the cell’s DNA content.

During the S phase, each chromosome in the cell replicates, resulting in two sister chromatids for each chromosome. This is critical because these sister chromatids will later be separated into two cells during cell division, ensuring that each daughter cell has an identical set of chromosomes.

• Timing: The S phase occurs after the G1 phase and before the G2 phase. It is a carefully controlled process that happens at a specific point in interphase, rather than continuously throughout.
• Role in cell cycle: The S phase is vital for maintaining genetic stability. It ensures the cell has sufficient copies of DNA, ready for an accurate division.
• Coordination with other phases: The successful completion of the S phase dictates the progression to the G2 phase, wherein further preparatory processes for cell division occur.

Conclusively, the S phase is essential for the accurate transmission of genetic information, thereby playing a critical role in cellular growth and reproduction. It makes sure that DNA replication is complete and error-free, setting the stage for successful cell division.