Karyology
Related Terms
Amplification, banding, cancer, chromosomal abnormalities, chromosomes, cytogenic maps, deletion, disease, inversion, karyology, karyotype, karyotype analysis, microscope, trisomy.
Background
Cytogenetics is an area of biology that deals with the study of chromosomes, which are composed of DNA (deoxyribonucleic acid) and contain hundreds of genes. They are located in a compartment of the cell called the nucleus. Genes contain the instructions for making the proteins that serve various functions in the human body.
By studying chromosomes under a microscope, researchers can learn how many chromosomes a particular cell contains. For example, cells in the human body have two pairs of 23 chromosomes, for a total of 46 chromosomes per cell. In some diseases, an individual has an extra chromosome or is missing a chromosome. By examining a patient's chromosomes under a microscope, researchers may be able to observe which chromosome has an abnormal number of copies and may therefore play a role in disease development.
When stained with a dye and viewed under a microscope, chromosomes appear to have a banded pattern of light and dark regions. The dark regions are areas where the DNA is tightly compacted together, while the light regions are areas where the DNA is not as tightly compacted. These dark and light regions can help researchers distinguish between different chromosomes.
In some diseases, such as cancer, genetic mutations can cause part of a chromosome to become deleted, repeated, or reversed in orientation. By looking for abnormalities in the light and dark banded patterns, researchers may be able to observe these changes under the microscope and learn more about the genetic mutations that cause a particular disease.
Methods
To observe chromosomes under a microscope, researchers typically need to follow several steps:
Grow cells: First, a line of cells that a researcher is interested in studying are grown in the laboratory. Cell lines made from blood, amniotic fluid, or bone marrow are commonly used for cytogenetic analysis. After the cells have grown in number, a chemical, such as colcemid, is used to stop further growth. Colcemid stops the cells from growing by interfering with the function of microtubules, proteins needed by the cell for growth. Colcemid causes the cells to stop growing when they reach a stage of the cell cycle called metaphase. At this stage, the chromosomes are easily visible.
Prepare cells for observation: Once the metaphase cells have been obtained, the cells are exposed to a hypotonic solution, which expands the cells and spreads the chromosomes apart to make them easier to examine. The cells are killed (using a fixative, such as methanol and acetic acid) and then transferred to a microscope where the chromosomes can be observed. Some of the metaphase cells are dropped onto a microscope slide (a thin piece of glass) so that they break open and the chromosomes spread apart. They are then stained with a dye, such as Giemsa stain, which makes the light and dark bands of the chromosome easily visible.
Observe chromosomes: Once the cells have been prepared, researchers are able to observe the chromosomes under a microscope. Typically, a researcher will prepare and observe the chromosomes from about 20 cells from each cell line. Sometimes, a photograph will be taken of the chromosomes under the microscope. This may make analysis easier because it allows the researcher to cut the chromosomes out of the photograph and arrange them side by side. Researchers may analyze and compare several features of the chromosomes, including chromosome number, banding pattern, and size.
Research
Determine karyotype: A karyotype is the complete set of chromosomes in an individual after they have been photographed and arranged. The preparation and study of karyotypes is part of cytogenetics. Using cytogenetic methodology, researchers can count how many chromosomes are in a particular cell. When researchers study all of the chromosomes in a cell, it is referred to as a karyotype analysis. Because the chromosomes in a cell are often of different sizes and have different banding patterns, researchers may use these features to distinguish them from each other and to group similar chromosomes together.
Most, but not all, species have a standard karyotype. The normal human karyotype contains 22 pairs of autosomal chromosomes and one pair of sex chromosomes. The normal karyotype for women contains two X chromosomes and is denoted 46,XX. The normal karyotype for men has both an X and a Y chromosome and is denoted 46,XY. However, some individuals have other karyotypes with added or missing chromosomes, which in most cases, causes developmental abnormalities. Karyotypes can be used for many purposes: to study chromosomal abnormalities, cellular function, and relationships between different species; or to gather information about past evolutionary events, such as when a new species evolved.
In humans and many other mammals, autosomal chromosomes are present in two identical copies. These types of cells are called diploid. Some types of organisms have polyploidy cells, which contain more than two copies of a chromosome. Other types of cells such as sperm or eggs are haploid, which means they contain only one copy of each chromosome. The study of whole sets of chromosomes is sometimes referred to as karyology. The chromosomes are depicted by rearranging a microphotograph in a standard format known as a karyogram or idiogram. They are organized in pairs according to size and position of centromere (the region of DNA in the middle of a chromosome).
Researchers may perform a karyotype analysis on an organism if they are unsure how many chromosomes the organism has. For example, researchers recently performed a karyotype analysis to find out how many chromosomes exist in a primitive type of arachnid called palpigrades. They also used this type of analysis to determine the number of chromosomes in the North Atlantic right whale.
Study mutations with banding: In the late 1960s, Caspersson developed banding techniques that differentially stain chromosomes. This allows chromosomes of equal size to be distinguished and identifies the breakpoints and chromosomes involved in chromosome translocations. Translocations are mutations in which part of one chromosome breaks off and becomes attached to another, whereas breakpoints are the regions of the chromosome where the piece becomes detached. Deletions within one chromosome can now be more specifically named and understood. Deletion syndromes, such as DiGeorge syndrome and Prader-Willi syndrome, were discovered to be caused by deletions in chromosome material. Diagrams identifying chromosomes based on their banding patterns are known as cytogenetic maps.
Study disease: In some diseases an individual has an extra chromosome (a trisomy) or is missing one. By looking at a patient's chromosomes under a microscope, researchers may be able to observe which chromosome is missing or is present in an extra copy. Other types of chromosomal defects that may be observed with cytogenetic analysis include a translocation (part of one chromosome breaks off and becomes attached to another), a deletion (part of a chromosome is lost, or deleted), and an insertion (extra DNA is added to a chromosome).
For example, trisomy 18 is a genetic condition in which individuals have an extra copy of chromosome 18. Most individuals with trisomy 18 are unable to live past one year of age due to a wide range of physical and mental developmental defects, including heart defects, kidney defects, mental retardation, and feeding problems. Down syndrome, also referred to as trisomy 21, is a condition in which individuals have an extra copy of chromosome 21. Down syndrome is a genetic disorder characterized by distinct physical characteristics and varying degrees of cognitive dysfunction ranging from mild to severe. Other diseases are known to be caused by abnormal numbers of chromosomes. Trisomy 13 is associated with Patau's syndrome and trisomy 18 with Edward's syndrome.
Other numerical abnormalities include sex chromosome abnormalities. An individual with only one sex chromosome (the X), instead of a pair, has Turner syndrome. A male with an extra X chromosome, resulting in 47 total chromosomes, has Klinefelter's syndrome. Many other sex chromosome combinations are compatible with live birth, including XXX, XYY, and XXXX. The ability for mammals to tolerate aneuploidies in the sex chromosomes arises from the ability to inactivate them, which is required in normal females to compensate for having two copies of the chromosome. Aneuploidy is a condition in which a cell has an extra copy of a chromosome or is missing a copy of a chromosome. Not all genes on the X chromosome are inactivated, which is why there is a phenotypic effect seen in individuals with an extra or missing X.
Study cancer: Cytogenetics can be used to understand other types of diseases, such as cancers, in which mutations occur causing a portion of a chromosome to become deleted, repeated, or reversed in orientation. For example, a deletion of part of chromosome 13 has been identified in people with multiple myeloma (a cancer affecting the blood cells). In 1960 scientists discovered a small chromosome, dubbed the Philadelphia chromosome, which was shown to be the cause of chronic myelogenous leukemia. Thirteen years later this chromosome was demonstrated to be a translocation of chromosomes 9 and 22.
In some forms of cancer, especially blood cancers, cytogenetics can determine which chromosomal translocations are present in the cancerous cells, making the conditions easier to treat and diagnose.
Implications
Diagnosis: Cytogenetics can be used to diagnose certain human diseases, sometimes even before a baby is born. For example, diagnosis of trisomy 18 or Down syndrome (as well as other genetic diseases) may be performed on a developing fetus through amniocentesis, in which the amniotic fluid surrounding the fetus is sampled through a needle inserted through the mother's abdomen. Cells in the amniotic fluid can be used to perform a karyotype analysis and can be checked for an extra copy of chromosome 18 in the case of trisomy 18, or an extra copy of chromosome 21 in the case of Down syndrome.
Cancer prognosis: Cytogenetics may be used by doctors to make a prognosis for some types of cancer. Karyotype analysis of patients with a type of bone or soft tissue cancer called Ewing sarcoma family of tumors (ESFT) showed that some patients with certain chromosomal abnormalities had poorer chances for survival than other patients.
Understanding disease: Cytogenetics can be used to better understand some diseases. By understanding the specific rearrangements that chromosomes undergo in patients with disease, researchers can better understand the disease and can use this information to develop treatments to fight the disease.
Limitations
Cytogenetics may be used to detect chromosomal abnormalities, such as regions where a chromosome has been deleted. However, cytogenetics analysis may not be able to give a researcher more specific information, such as which genes were affected by a chromosomal abnormality. Additional experiments may be needed to determine the consequences of a certain chromosomal abnormality.
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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