Normal human fibroblasts, cancer cells (that originated in stem cells), and fibroblasts transduced with hTERT (hTERT cells) were passaged for 35 generations. Southern blot analysis was performed using DNA from above cells using radio-labelled probes for telomeric sequences. Which one of the following band patterns would be observed in the autoradiogram?

Correct option is A

The correct answer is (a) because telomere length is maintained differently in normal, hTERT-transduced, and cancer cells:

• In normal human fibroblasts, which lack active telomerase, telomeres progressively shorten with each cell division, leading to shorter telomeric DNA over generations (seen as 7–9 kb bands on a Southern blot).
• hTERT-transduced fibroblasts express telomerase (hTERT), which adds telomeric repeats and maintains telomere length, resulting in longer telomeric sequences (18–20 kb).
• Cancer cells, particularly those derived from stem cells, often have reactivated or constitutively active telomerase, allowing them to maintain or elongate telomeres, similar to hTERT cells. Hence, they also show 18–20 kb telomere bands.

Thus, in a Southern blot with telomeric probes, the observed bands would be shorter in fibroblasts and longer in hTERT and cancer cells.

Additional Information:

• (b) 18–20 kb bands in fibroblasts: Incorrect — fibroblasts without telomerase will show shorter telomeres.
• (c) 7–9 kb bands in hTERT cells: Incorrect — hTERT maintains telomere length; bands should be longer.
• (d) 7–9 kb bands in cancer cells: Incorrect — cancer cells maintain long telomeres due to telomerase activity.