# DBT JRF 2020 — Question 18

Exam: DBT JRF 2020 · Section: B · Topic: Cell Biology · Difficulty: Medium

## Question

What is Endoreduplication?

## Options

- **A.** recurrent DNA replication without subsequent mitosis and cytokinesis
- **B.** mobilization of DNA into ER and replication of DNA in the ER
- **C.** splitting up of endoplasmic reticulum (ER) to form rough and smooth ERs
- **D.** replication of DNA in the nuclei and endocytosis of one copy to another organelle

## Answer

Option A

## Explanation

HAT (Hypoxanthine-Aminopterin-Thymidine) selection in hybridoma production: Aminopterin is a folate antagonist that blocks dihydrofolate reductase (DHFR), inhibiting the de novo nucleotide synthesis pathway ( $\text{PRPP} \rightarrow$ purines; $\text{dUMP} \rightarrow \text{TMP}$ ). Cells must then rely entirely on the salvage pathway (HGPRT for purines; thymidine kinase for pyrimidines) using the supplied hypoxanthine and thymidine. Unfused myeloma cells lack HGPRT (a selection trait of the parental myeloma cell line) and CANNOT use the salvage pathway → they die. Unfused spleen B cells have HGPRT but are mortal → die after a few divisions. Only hybridomas (fused myeloma + B cell) have both immortality and HGPRT → survive in HAT.

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