24. During eukaryotic cell division, the amount of DNA doubles

(A) between prophase and anaphase of mitosis

(B) between prophase I and prophase II of meiosis

(D) during the M phase of the cell cycle

DNA Doubling During the Eukaryotic Cell Cycle

Introduction

The eukaryotic cell cycle is a precisely regulated sequence of events that ensures accurate duplication and distribution of genetic material from one generation of cells to the next. Every dividing cell must replicate its DNA exactly once before mitosis or meiosis so that each daughter cell receives a complete set of chromosomes. Failure to duplicate DNA correctly can lead to mutations, chromosomal abnormalities, genomic instability, developmental defects, and cancer.

The cell cycle consists of four major phases: G₁ (Gap 1), S (Synthesis), G₂ (Gap 2), and M (Mitosis). Among these phases, DNA replication occurs exclusively during the S phase, which lies between G₁ and G₂. As DNA replication proceeds, the amount of DNA in the nucleus doubles, although the chromosome number remains unchanged because each chromosome now consists of two identical sister chromatids.

Correct Answer

Correct Option: (C) Between the G₁ and G₂ phases of the cell cycle

Detailed Explanation

DNA replication occurs exclusively during the S (Synthesis) phase of interphase. The S phase is positioned between the G₁ and G₂ phases, making Option (C) the correct answer. During this stage, DNA polymerases synthesize an identical copy of every chromosome, producing two sister chromatids joined together at the centromere.

At the beginning of the G₁ phase, a diploid cell contains a normal amount of DNA, represented as 2C. During the S phase, DNA replication doubles the DNA content to 4C, while the chromosome number remains 2n because sister chromatids are still attached. The G₂ phase follows DNA replication and serves primarily for DNA repair, synthesis of mitotic proteins, and preparation for mitosis. No additional DNA replication occurs during G₂ or M phase.

Therefore, DNA doubling occurs during the interval between G₁ and G₂, specifically in the S phase.

Explanation of Each Option

Option (A): Between Prophase and Anaphase of Mitosis

This statement is incorrect. Mitosis is responsible for separating already duplicated chromosomes into two daughter cells. Between prophase and anaphase, chromosomes condense, align at the metaphase plate, and sister chromatids separate. DNA replication has already been completed before mitosis begins, so no increase in DNA content occurs during this interval.

Option (B): Between Prophase I and Prophase II of Meiosis

This statement is incorrect. DNA replication occurs only once before meiosis I. There is no S phase between meiosis I and meiosis II. After prophase I and completion of meiosis I, cells proceed toward meiosis II without another round of DNA synthesis. Therefore, DNA content does not double between prophase I and prophase II.

Option (C): Between the G₁ and G₂ Phases of the Cell Cycle

This statement is correct. The S phase lies between G₁ and G₂ and is the only phase during which DNA replication occurs. Every chromosome is duplicated to form two sister chromatids, doubling the total DNA content while maintaining the same chromosome number.

Option (D): During the M Phase of the Cell Cycle

This statement is incorrect. The M phase includes mitosis and cytokinesis, during which duplicated chromosomes are distributed equally between two daughter cells. DNA synthesis does not occur during mitosis. Instead, the DNA content per daughter cell decreases as chromosomes are partitioned into separate nuclei.

Why Option (C) is Correct

The S phase is located between G₁ and G₂ and is exclusively dedicated to DNA synthesis. During this phase, every chromosome replicates, doubling the total DNA content from 2C to 4C. Since no DNA replication occurs during mitosis or meiosis II, Option (C) correctly identifies the interval during which DNA doubles.

Why the Other Options are Incorrect

Why Option (A) is Incorrect

Prophase through anaphase represents chromosome segregation, not DNA synthesis. DNA replication is completed before mitosis begins.

Why Option (B) is Incorrect

There is no DNA replication between meiosis I and meiosis II. Meiosis II begins directly after meiosis I without an intervening S phase.

Why Option (D) is Incorrect

The M phase distributes duplicated DNA into daughter cells rather than increasing the amount of DNA present within the cell.

Comparison of All Options

Option	Statement	Correct or Incorrect	Reason
A	Between prophase and anaphase of mitosis	Incorrect	Chromosome segregation occurs, not DNA replication.
B	Between prophase I and prophase II of meiosis	Incorrect	No S phase occurs between meiosis I and meiosis II.
C	Between G₁ and G₂ phases	Correct	S phase occurs between G₁ and G₂ where DNA replication takes place.
D	During M phase	Incorrect	Mitosis separates chromosomes instead of replicating DNA.

DNA Content During the Cell Cycle

Phase	Chromosome Number	DNA Content	Major Event
G₁	2n	2C	Cell growth and preparation for DNA replication
S	2n	2C → 4C	DNA replication and sister chromatid formation
G₂	2n	4C	Preparation for mitosis and DNA repair
M	2n	4C → 2C per daughter cell	Chromosome segregation and cytokinesis

Comparison Between DNA Replication and Chromosome Segregation

Feature	DNA Replication	Chromosome Segregation
Occurs During	S Phase	M Phase
Main Purpose	Duplicate DNA molecules	Separate sister chromatids
DNA Polymerase Required	Yes	No
Spindle Apparatus Required	No	Yes
DNA Content	Doubles	Distributed equally to daughter cells

Biological Significance of DNA Doubling

DNA replication during the S phase ensures that every daughter cell receives a complete and identical genome after cell division. High-fidelity DNA polymerases, proofreading enzymes, and checkpoint proteins minimize replication errors and maintain genomic stability. Accurate DNA duplication is essential for growth, tissue repair, embryonic development, and inheritance of genetic information across generations.

Final Answer