Fluorescence in situ hybridization

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Fluorescence in situ hybridization


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USMLE® Step 1 questions

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Fluorescence in situ hybridization

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USMLE® Step 1 style questions USMLE

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Using fluorescent DNA probes, an investigator performs a fluorescence in situ hybridization (FISH) study. The study demonstrates a lack of fluorescence at the 5p region of chromosome 5 compared to fluorescence at the same locus on the second copy of that chromosome, as shown below. Based on the results of this FISH study, which of the following is a possible diagnosis?
Reproduced from Journal of Medical Genetics  

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Fluorescence in situ hybridization p. 53


fluorescence in situ hybridization p. 53


Fluorescence in situ hybridization is a cytogenetic technique that uses fluorescent DNA segments, called “probes”, to bind to a known DNA sequence.

It’s used to localize particular DNA sequences, or lack thereof, on a chromosome in order to detect chromosomal abnormalities, or mutations; like deletion, duplication, or translocation of a DNA segment; which may be the underlying cause of a genetic disease.

Ok, now, our DNA is like a library, found in the nucleus of our cells, that carries our genetic information.

On the molecular level, DNA is made up of two strands of nucleotides that are coiled around one another to form a double helix.

There are four different nucleotides: adenine, or A, thymine, or T, cytosine, or C and guanine, or G. A binds with T, and C binds with G; nucleotides on opposite strands form hydrogen bonds to keep the two strands together.

To fit inside the nucleus, DNA wraps around proteins which further condense to form chromatin fibers.

These chromatin fibers are loosely or tightly packed depending on the phase of the cell’s cycle.

The cell cycle represents a series of events that somatic cells, that is, all cells besides the gametes, go through from the moment they’re formed until the moment they divide into two identical daughter cells.

And it has two phases: interphase, or cell growth in preparation for division, and mitosis, or cellular division.

During early interphase, chromatin fibers float in a loose state inside the nucleus, like DNA-rich noodles.

Each of the chromatin noodles represents a single DNA molecule.

Now, during later interphase, when the cell prepares for mitosis, or cellular division, the DNA noodles replicate and chromatin condenses to form chromosomes.

Remember that human somatic cells have 23 pairs of chromosomes, so 46 chromosomes in total.

And right before mitosis, each chromosome carries two identical DNA molecules, called chromatids.

Chromatids join together in the center in a region called the centromere, which in turn divides both chromatids into a short “p” arm, and a long “q” arm.

Finally, during mitosis, the chromosomes condense, so they can be observed in more detail - and they’re at their most condensed during a phase of mitosis called metaphase, when they neatly align in the middle of the cell, like 46 little X shapes - where each side of the X represents a chromatid.


Fluorescence in situ hybridization (FISH) is a molecular cytogenetic technique that uses fluorescent probes to detect and localize the presence of specific DNA sequences on chromosomes. This technique can be used to detect chromosomal abnormalities, such as translocations, duplications, insertions, and deletions. FISH can also be used to identify and map the location of gene sequences on chromosomes.


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