Number of chromatids at metaphase is:

BHU NO-2019

Pathology & Genetics

Easy

Appeared in: BHU NO-2019

Explanation

Before any cell division (mitosis or meiosis), the cell's DNA replicates during the S phase of interphase.
This replication results in each chromosome being composed of two identical sister chromatids, which are joined together by a centromere.
This two-chromatid structure is maintained when the chromosomes align at the equatorial plate during the metaphase stage of both mitosis and meiosis (I and II).
Therefore, a single chromosome at metaphase always consists of two chromatids.

Option B: A chromosome is composed of a single chromatid only after the separation of sister chromatids, which occurs during anaphase, not metaphase.
Option C: This option is confusing. While a bivalent (a pair of homologous chromosomes) in metaphase I of meiosis has four chromatids, an individual chromosome still only has two. This statement is also incorrect for meiosis II, where individual chromosomes with two chromatids align.
Option D: A chromosome in mitotic metaphase has two chromatids, not one. DNA replication precedes mitosis, doubling the chromatids per chromosome.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Understanding of chromosome structure during different phases of cell division (mitosis and meiosis) as background academic context rather than a clinical decision trigger.
Understanding cell division is fundamental to grasping concepts like growth, tissue repair, and reproduction.
Errors in mitosis and meiosis, such as non-disjunction (failure of chromatids or chromosomes to separate), lead to genetic disorders. For example, non-disjunction in meiosis can cause conditions like Down syndrome (Trisomy 21).
Many chemotherapy drugs, such as vinca alkaloids (e.g., vincristine) and taxanes (e.g., paclitaxel), target the mitotic process to stop cancer cells from dividing.

How to Approach the Question

First, identify the key terms in the question: 'chromatids', 'metaphase', 'mitosis', and 'meiosis'.
Recall the cell cycle, specifically what happens before cell division begins. Remember that DNA replication (S phase) occurs during interphase, which precedes both mitosis and meiosis.
Visualize a chromosome after DNA replication. It looks like an 'X' shape, composed of two identical sister chromatids joined at a centromere.
Consider the metaphase stage. The key event is the alignment of chromosomes at the cell's equator (the metaphase plate).
Apply this to each process: In mitosis and meiosis II, individual chromosomes (each with 2 chromatids) align. In meiosis I, homologous pairs of chromosomes align, but each chromosome within that pair still has 2 chromatids.
Conclude that in all metaphase scenarios, a single chromosome is composed of two chromatids. Evaluate the options based on this conclusion.

Concept Tested & Keywords

Concept Tested: Understanding of chromosome structure during different phases of cell division (mitosis and meiosis).
Stem keywords: Chromatids, Metaphase, Mitosis, Meiosis
Lead-in keywords: Number of

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