All the DNA strands of a cell containing 4 chromosomes are labelled. After one division how many
chromosomes in the daughter cell will have labelled DNA?
(1) 1
(2) 2
(3) 4
(4) 8
How Many Chromosomes Will Have Labeled DNA After One Cell Division?
DNA replication and cell division are fundamental processes in biology. One intriguing experiment involves labeling DNA strands in a cell and tracking how the label is distributed after cell division. A common exam or research question is:
All the DNA strands of a cell containing 4 chromosomes are labeled. After one division, how many chromosomes in the daughter cell will have labeled DNA?
Let’s break this down with a clear explanation of the concepts involved.
✅ Correct Answer: (3) 4
Understanding the Basics: DNA Replication and Cell Division
Before we solve the question, it’s important to understand how DNA replication and mitotic division work:
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DNA Replication:
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Occurs during the S-phase of the cell cycle before mitosis.
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Each chromosome (double-stranded DNA) replicates to form two sister chromatids, each with one old (template) strand and one newly synthesized strand.
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This is known as semi-conservative replication, because each daughter DNA molecule contains one old and one new strand.
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Mitosis:
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During mitosis, the sister chromatids separate and are distributed into two daughter cells.
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Each daughter cell receives an identical set of chromosomes.
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Applying the Concept to the Question
Let’s walk through the scenario step-by-step:
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The original cell has 4 chromosomes, and all DNA strands are labeled.
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After semi-conservative replication, each chromosome will consist of one labeled (old) strand and one unlabeled (new) strand.
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During mitosis, each daughter cell will receive 4 chromosomes—one copy of each chromosome from the original cell.
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Since each of those chromosomes contains one labeled strand (from the parent) and one new strand, all chromosomes in the daughter cell will contain labeled DNA.
Visualizing the Process
Here’s a simplified version:
| Step | Description | Labeling Status |
|---|---|---|
| Original cell | 4 chromosomes (all labeled) | All DNA strands labeled |
| After replication | 4 replicated chromosomes (8 chromatids) | Each with 1 labeled + 1 new strand |
| After mitosis | 2 daughter cells, each with 4 chromosomes | Each chromosome contains 1 labeled strand |
➡️ Therefore, all 4 chromosomes in the daughter cell will contain labeled DNA.
Final Answer
(3) 4
Why This Is Important
This question highlights an important concept in molecular biology—semi-conservative replication and how labeled molecules can be used to track DNA through generations of cells. This principle is used in various experiments, including those involving radioactive labeling, fluorescent dyes, or isotope-labeled nucleotides to study DNA replication, gene expression, and cell lineage tracing.
Understanding how labeled DNA is distributed after replication helps researchers uncover mechanisms of cell division, genome stability, and even the development of cancer or genetic disorders.
Conclusion
After one round of cell division in a cell with 4 chromosomes where all DNA strands are labeled, each of the daughter cells will receive 4 chromosomes, each containing one labeled DNA strand. This reflects the semi-conservative nature of DNA replication. The correct answer is:
(3) 4
Mastering these fundamental concepts lays the foundation for deeper understanding in genetics, molecular biology, and biotechnology.
