Problem 81 - Objective Biology for NEET

Question

Before undergoing meiosis, the amount of DNA of a cell (a) Halves (b) Doubles (c) Remains the same (d) Quadruples

Step-by-Step Solution

Answer

The correct answer is (b) Doubles.

Step 1: Understand Meiosis

Meiosis is a type of cell division that reduces the number of chromosomes in the parent cell by half and creates four gamete cells. It is essential for sexual reproduction in humans and most other organisms. Before a cell begins the process of meiosis, it undergoes a process known as DNA replication.

Step 2: Identify the Process Before Meiosis

In the 'Interphase' period before a cell begins meiosis (or indeed mitosis), the cell replicates its DNA during a phase known as the 'S phase'. Therefore, each chromosome is duplicated, and each duplicate is linked to the original via a structure called a centromere, effectively creating sister chromatids.

Step 3: Determine the Change in DNA content

Through the process of DNA replication during Interphase, the amount of DNA in a cell prepares for meiosis by duplicating itself. Hence, the amount of DNA in the cell doesn't half, stay the same or quadruple, but rather, it doubles.

Key Concepts

DNA replication

DNA replication is a vital process that occurs before a cell divides, ensuring each new cell has a complete set of genetic instructions. During DNA replication, the cell's DNA molecule "unzips" into two separate strands. Each of these strands acts as a template for a new, complementary strand. - The enzyme DNA polymerase plays a crucial role here, as it adds nucleotides— the building blocks of DNA— to the growing DNA strand. - This process helps to produce two identical DNA molecules, each with one old strand and one newly synthesized strand. Once replication is complete, the cell is ready for division, whether it goes through mitosis, where two identical daughter cells are formed, or in this case, meiosis, which prepares the cell for sexual reproduction.

Interphase

Interphase is a period in a cell's life cycle during which it is not actively dividing. Instead, the cell is busy preparing for division. This phase is crucial because it allows the cell to grow and duplicate its DNA before division occurs. During interphase, the cell goes through three main stages:

• G1 phase: A period of cell growth and normal function.
• S phase: The DNA synthesis phase, where DNA replication occurs.
• G2 phase: A further period of growth and preparation for division.

Each stage is essential in ensuring the cell divides correctly, maintaining the integrity of the organism’s genome.

S phase

The 'S phase' is a sub-phase of interphase, specifically dedicated to DNA synthesis. It's a critical stage where DNA replication actually takes place: - During S phase, the entire genome of the cell is replicated. - This means that every chromosome is duplicated, resulting in two identical sister chromatids for each chromosome. - These sister chromatids remain attached at a point called the centromere until they are later separated during cell division. The doubling of the DNA during the S phase is essential, especially in meiosis, because it ensures that when the cell divides, each new cell receives a complete set of DNA.