Questions: Arrange the steps involved in fluorescence in situ hybridization (FISH) in the correct order, placing the first step at the top. Treat cells with agents that cause them to swell and fix them onto a slide. Denature chromosomal DNA. Hybridize chromosomal DNA to single-stranded DNA probes containing biotin. Add fluorescently labeled avidin. View with a fluorescence microscope.

The correct order of the steps involved in fluorescence in situ hybridization (FISH) is as follows:

1. Treat cells with agents that cause them to swell and fix them onto a slide.
2. Denature chromosomal DNA.
3. Hybridize chromosomal DNA to single-stranded DNA probes containing biotin.
4. Add fluorescently labeled avidin.
5. View with a fluorescence microscope.

Explanation:

1. Treat cells with agents that cause them to swell and fix them onto a slide: This step prepares the cells for the FISH procedure by making them more accessible for subsequent treatments.
2. Denature chromosomal DNA: Denaturation involves heating the DNA to separate the double-stranded DNA into single strands, which is necessary for the hybridization process.
3. Hybridize chromosomal DNA to single-stranded DNA probes containing biotin: The single-stranded DNA probes, which are complementary to the target sequences, are allowed to hybridize (bind) to the denatured chromosomal DNA.
4. Add fluorescently labeled avidin: Avidin binds to biotin with high affinity. The fluorescent label on avidin allows for the visualization of the hybridized probes.
5. View with a fluorescence microscope: The final step involves using a fluorescence microscope to detect and visualize the fluorescent signals, indicating the presence and location of the target DNA sequences.

This sequence ensures that each step logically follows the previous one, leading to successful visualization of the target DNA sequences.