karyotyping

A Karyotype Definition A photographic arrangement of a complete set of chromosomes of a cell or organism 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 X Y A karyotype is a photographic arrangement of homologous pairs of a complete set of chromosomes. The illustration on the left shows an example of a karyotype (note that real karyotypes can be seen on the web practice in step 2). Notice that there are 22 pairs that decrease in size as the numerical identification get bigger. X and Y chromosomes are also displayed and therefore the sex status can be determined. (1)

A karyotype is the number and appearance of chromosomes in the nucleus of a eukaryotic cell. The term is also used for the complete set of chromosomes in a species, or an individual organism

History of karyotype Chromosomes were first observed in plant cells by Carl Wilhelm von Nägeli in 1842. in animal (salamander) cells was described by Walther Flemming, the discoverer of mitosis, in 1882.

karyotyping Karyotypes describe the chromosome count of an organism, and what these chromosomes look like under a light microscope. Attention is paid to their length, the position of the centromeres, banding pattern, any differences between the sex chromosomes, any other physical characteristics

Human karyotype The normal human karyotypes contain 22 pairs of autosomal chromosomes and one pair of sex chromosomes (allosomes). Normal karyotypes for  females contain two X chromosomes and are denoted 46,XX;  males have both an X and a Y chromosome denoted 46,XY. Any variation from the standard karyotype may lead to developmental abnormalities. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 X Y

Karyotyping Karyotyping is a laboratory procedure that allows a physician to examine a patient’s set of chromosomes. “Karyotype” also refers to the actual collection of chromosomes being examined. Examining these chromosomes through karyotyping allows your physician to determine whether there are any abnormalities or structural problems.

How the Test Is Performed sampling The first step in karyotyping is to take a sample of your cells. The sample cells can come from a number of different tissues, including bone marrow, blood, amniotic fluid, or placenta. After the sample has been taken, it is placed in a laboratory dish that allows the cells to grow.

How the Test Is Performed These stained cells are examined under a microscope for potential abnormalities, including: extra chromosomes missing chromosomes missing portions of a chromosome extra portions of a chromosome portions that have broken off of one chromosome and reattached to another

How the Test Is Performed The laboratory technician can see the chromosomes’ shape, size, and number. This information is instrumental in determining whether or not any genetic abnormalities are present.

What the Test Results Mean A normal test result will show 46 chromosomes, 44 of which are autosomes (which are unrelated to determining gender) and two of which are sex chromosomes. These sex chromosomes determine one’s gender: females have two X chromosomes, while males have one X chromosome and one Y chromosome.

What the Test Results Mean Abnormalities that appear in a test sample could be the result of any number of genetic syndromes or conditions. Sometimes an abnormality will occur in the lab sample that is not reflected in the patient’s actual body. In order to confirm that an abnormality is present in the patient, the karyotype test can be repeated.

A. Chromosome Staining & Structure Metaphase chromosome spread Staining techniques G-banding R-banding Q-banding

A. Chromosome Staining & Structure Centromere Position Metacentric Submetacentric Acrocentric; P & q arms Telocentric Centromere & telomere structures

Staining Banding patterns can be visually identified on chromosomes after staining. Traditional Types G-Banding – Giemsa stain Q-Banding – Fluorescent stain R-Banding – Reverse Giemsa stain New Type Fluorescence In Situ Hybridization techniques Staining procedures for karyotypes produce patterns of bands that are unique for each chromosome. Comparison of these band can help to match homologous pairs in a karyotype. Types of staining include G-banding which involves Giemsa stain, Q-banding, which involves fluorescent staining and R-banding which involves a reverse Giemsa stain, in which the complements of G-banding can be visualized. (4) It is important to note that new molecular techniques such as Fluorescence In Situ Hybridization (FISH) are now replacing some of the more traditional staining methods because of the limitations of the light microscope. These new techniques enable visualization of small duplications, deletions, or rearrangements that can no be seen with traditional cytogenetics. (8)

Procedure of staining G-Banding : Chromosomes are treated with an enzyme (trysin) to digest some chromosomal proteins. Chromosomes are then exposed to Giemsa stain, which consists of a mixture of dyes and results in darkly stained G bands which are visible under a microscope.

Procedure of staining Q-Banding : Chromosomes are treated with quinacrine mustard and patterns are observed by placing the sample under a special type of ultraviolet light microscope. The chromosomes will show bright fluorescent bands.  quinacrine mustard a nitrogen mustard derived from mepacrine and used as a stain for chromosomes Y chromosomes

Procedure of staining Heterochromatin: 1. It is darkly stained region of the chromatin (chromosome). 2. It is compactly coiled regions and with more DNA. 3. It is genetically inert as can not transcribe mRNA due to tight coiling. Euchromatin: 1. It is lightly stained region. 2. It is loosely coiled region and with less DNA. 3. It is genetically active. 4. It is early replicative. R-Banding : Chromosomes are treated with acridine orange and observed with a light microscope. The result is a darkly stained centromere region of the chromosome.

B. Chromosomal Abnormalities Trisomy Kleinfelter syndrome (47, XXY) Trisomy Down syndrome (47, +21)