Problem 100 - Objective Biology for NEET

Question

A cell has 23 pairs of chromosomes just after the completion of mitotic telophase. The number of chromatids at the preceding metaphase was (a) 23 (b) 46 (c) 69 (d) 92

Step-by-Step Solution

Answer

The number of chromatids at the preceding metaphase was 92.

Step 1: Understanding Mitosis and Chromosome Number

Mitosis is a process where a single cell divides into two identical daughter cells (cell division). Just after the completion of a mitotic telophase, each cell has 23 pairs of chromosomes: so, there are 46 chromosomes.

Step 2: Chromatids at Metaphase

Each chromosome at the metaphase of mitosis consists of two sister chromatids. These chromatids are identical copies of DNA produced during the S phase. So, the number of chromatids at metaphase is twice the number of chromosomes. Therefore, it would be 46 chromosomes * 2 = 92 chromatids.

Step 3: Choose the Correct Answer

Therefore, based on the calculation in step 2, the correct answer is: (d) 92.

Key Concepts

Understanding Cell Division

Cell division is a fundamental biological process where a parent cell divides to produce two or more daughter cells. It's vital for growth, development, and tissue repair in multicellular organisms. In this context, when we refer to mitosis, we are discussing a type of cell division that results in daughter cells with the same chromosome number as the parent cell.

This means that if a parent cell has 23 pairs of chromosomes, after mitosis, each daughter cell will also have 23 pairs, for a total of 46 chromosomes. Mitosis is a precise process involving several stages to ensure that each daughter cell receives an exact copy of the parent cell's DNA.

What is a Chromatid?

Before diving into the phases of mitosis, it's important to understand what a chromatid is. A chromatid is one of the two identical halves of a replicated chromosome. During the S phase of the cell cycle, DNA replication occurs, resulting in each chromosome being composed of two identical chromatids connected by a central region called the centromere.

When we discuss chromatids in the context of mitosis, it's critical to note that although there are two chromatids, they're considered one chromosome. It's not until anaphase of mitosis that the sister chromatids separate and are recognized as individual chromosomes.

The Role of Metaphase in Mitosis

Metaphase is a crucial stage in mitosis, often described as one of the most visually identifiable stages under a microscope. During metaphase, chromosomes align at the center of the cell, called the metaphase plate. This precise arrangement ensures that each sister chromatid is properly attached to a spindle fiber that extends from opposite spindle poles.

Understanding the number of chromatids during metaphase is vital for grasping the mechanics of mitosis. If we consider a cell with 23 pairs of chromosomes, at metaphase, there would be 46 chromosomes lined up at the metaphase plate. However, since these chromosomes each consist of two sister chromatids at this point, the total number of chromatids would be 46 pairs, or 92 chromatids. It's this doubling that is key to ensuring that each daughter cell receives the correct number of chromosomes during cell division.