G-banding

Related Terms

Cancer, chromosome, chromosome banding, cytogenetic techniques, DNA, DNA amplification, G-banding, Giemsa staining, library, metaphase arrest, mutation, probe, rearrangement.

Background

Cytogenetics is the study of chromosomes, which contain the genetic information of a cell. Chromosomes are composed mostly of DNA and are located in a compartment of the cell called the nucleus. Human cells contain 23 pairs of chromosomes for a total of 46 chromosomes per cell.
To study a specific region of a chromosome in greater detail, researchers may use a cytogenetic technique called chromosome microdissection. When cells are about to divide, the chromosomes become tightly wound around proteins. When stained with a dye and viewed with a microscope, the chromosomes appear as distinct X-shaped units. This staining procedure, called Giemsa staining or G-banding, gives the chromosomes a banded pattern of light regions and dark regions. The dark regions are areas where the DNA is tightly compacted together, and the light regions are areas where the DNA is not as tightly compacted.
In chromosome microdissection, a fine needle is used to physically remove a small section of a particular chromosome. This isolated piece of the chromosome can then be studied by researchers in greater detail.
Chromosomal abnormalities may cause abnormal G-banding. Therefore, chromosome microdissection may be used to study a variety of diseases involving chromosomal rearrangements, including cancers. In some inherited diseases or cancers, genetic mutations occur that may cause a portion of a chromosome to become deleted, repeated, or reversed in orientation. Using chromosome microdissection, researchers may isolate this abnormal region of the chromosome and study it in more detail to learn what genes it contains. This may help researchers to understand the causes of a particular disease.

Methods

Metaphase arrest: Before the chromosomes can be dissected, the cells need to be frozen, or "arrested," when the chromosomes are tightly wound and visible. A chemical called colcemid causes the cells to stop growing right before they divide. This part of the cell cycle, which is called the metaphase stage, occurs when the chromosomes are tightly wound and easily visible with Giemsa staining.
Giemsa staining: Once the cells are frozen in metaphase, they are transferred to a microscope where the chromosomes can be observed. Some of the metaphase cells are dropped onto a thin glass microscope slide so that they break open and the chromosomes spread apart. They are then stained with Giemsa stain, which makes the light and dark G-bands visible under the microscope.
Microdissection: While observing the stained chromosomes under the microscope, a researcher uses a fine sharp glass needle to physically remove part of the chromosome. This needle may be attached to a mechanical device that the researcher controls. The needle can be used to scrape the portion of the chromosome that is of interest to break it away from the rest of the chromosome. This small chromosomal fragment can then be put in a small tube and stored for further study. Generally, chromosome microdissection is used to obtain fragments that are 10 million base pairs or larger in size (base pairs are individual chemical units of DNA).
DNA amplification: Typically, a researcher needs to physically isolate about four to eight copies of a specific chromosomal region in order to have enough material to further study it. After the chromosome microdissection is performed, however, the chromosomal region may be further amplified. A technique called polymerase chain reaction may be used to increase the number of copies of specific DNA segments in the isolated region, so that thousands of copies are present.

Research

Probe generation: Chromosome microdissection may be used to make probes to study a chromosome. After a region from the chromosome is microdissected, it can be labeled with a detectable marker, such as radiation or a fluorescent dye. This labeled DNA can then be used to check other cells or DNA samples for the presence or absence of a specific DNA sequence.
Chromosome microdissection has been used to study problems in lung development. Using probes generated through microdissection, researchers have identified a chromosomal deletion that causes defective lung development.
Chromosome microdissection has also been used to study certain cancers in which regions of DNA are duplicated. Probes generated through microdissection can detect highly duplicated regions and have been used to study some types of lung, skin, and testicular cancer. Genes that are present in these duplicated regions may be involved in causing the disease.
Probes made from the DNA of one organism may be used to study the DNA of other closely related organisms, and may therefore be used to study evolution. Based on whether the probe can detect the corresponding DNA sequence in the other organisms and depending on where the sequence is located, researchers can learn more about how closely related the organisms are to each other.
Library generation: If multiple regions from a chromosome are isolated using chromosome microdissection, these regions can be used to build a library of that chromosome. A library is a collection of small DNA fragments that represent a larger region of DNA and may even contain an entire chromosome. A library could be used to find specific genes in the region or to determine DNA sequences within that region.

Implications

Rearrangements in chromosomes may cause specific genes to be amplified or deleted, and the genes that have increased or decreased in number may play a role in causing disease. Chromosome microdissection can be used to study specific rearrangements that chromosomes undergo in patients, which allows researchers to better understand a disease.
By identifying these genes, scientists can better understand how a disease is caused and may be able to use this information to develop therapies to fight the disease. This approach is especially useful for the study of cancer, because many types of cancer are characterized by mutated and rearranged chromosomes. For example, chromosome microdissection has been used to identify specific genes that become amplified in liver cancer and that increase may contribute to the disease.

Future research

Not Applicable

Author information

This information has been edited and peer-reviewed by contributors to the Natural Standard Research Collaboration (www.naturalstandard.com).

Bibliography

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