Question 1

What is DNA fragmentation and how does it occur during apoptosis?

Accepted Answer

DNA fragmentation is a hallmark of apoptosis where chromosomal DNA is cleaved into fragments with nicked ends. Caspases, enzymes in the cytosol, activate caspase-activated DNase (CAD) by removing its inhibitor. Activated CAD travels to the nucleus and cleaves chromosomal DNA, creating fragments that accumulate and signal cell death.

Question 2

How does the TUNEL assay detect apoptotic cells?

Accepted Answer

The TUNEL assay fluorescently labels nicked DNA ends in apoptotic cells. Terminal deoxynucleotidyl transferase (TdT) catalyzes the attachment of fluorescently labeled deoxyuridine triphosphate (dUTP) nucleotides to free 3' hydroxyl groups on fragmented DNA. This creates TUNEL positive cells visible under fluorescence microscopy.

Question 3

What are the main steps in performing a TUNEL assay on tissue sections?

Accepted Answer

The TUNEL assay protocol includes tissue fixation using 4% paraformaldehyde, cryosectioning into 10 µm slices, permeabilization with proteinase K, adding TdT enzyme and labeled dUTPs, incubating for 1-3 hours at 37°C, stopping the reaction with stop buffer, and visualizing with fluorescence microscopy.

Question 4

Why is tissue permeabilization necessary before adding TUNEL reagents?

Accepted Answer

Permeabilization allows TUNEL reagents, particularly the TdT enzyme, to penetrate the cell nucleus and access fragmented DNA. Proteinase K treatment for 5-15 minutes at 37°C creates pores in cell membranes, enabling the enzyme to reach its substrate inside the nucleus.

Question 5

How can researchers use TUNEL staining to study apoptosis in disease?

Accepted Answer

Researchers apply TUNEL staining to examine apoptosis in various disease contexts. For example, scientists use it to assess tumor responsiveness to drugs by treating human tumor samples with experimental compounds and measuring increased apoptosis. TUNEL staining quantifies cell death by counting the percentage of TUNEL positive cells in tissue sections.

Question 6

What role do caspases play in initiating DNA fragmentation?

Accepted Answer

Caspases are proteolytic enzymes present in the cytosol that execute apoptosis by cleaving cellular proteins. They activate caspase-activated DNase (CAD) by detaching it from its inhibitor ICAD. Once activated, CAD enters the nucleus and cleaves chromosomal DNA, producing the fragmented DNA that TUNEL assays detect.

Question 7

What does TUNEL positive staining indicate about cells in a tissue sample?

Accepted Answer

TUNEL positive cells are undergoing apoptosis and contain fragmented DNA with nicked ends labeled by fluorescent dUTP. These cells appear fluorescent under microscopy and indicate programmed cell death. Counting TUNEL positive cells provides a quantitative measure of apoptosis occurring in a tissue section.

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