Certain dyes selectively stain some regions of metaphase chromosomes more intensely than other regions, producing characteristic banding patterns that are specific for individual chromosomes. The regularity of chromosome bands provides useful visible landmarks along the length of each chromosome and can help to distinguish chromosomes of similar size and shape, as we will see later in this section.

Today the method of chromosome painting greatly simplifies the identification and differentiation of individual chromosomes within a karyotype, many of which have similar sizes and shapes. This technique, a variation of fluorescence in situ hybridization (FISH), makes use of probes specific for sites scattered along the length of each chromosome. The probes are labeled with several different fluorescent dyes with distinct excitation and emission wave lengths. Probes specific for each chromosome are labeled with a predetermined fraction of each of the dyes. After the probes are hybridized to chromosomes and the excess removed, the sample is observed with a fluorescence micro scope in which a detector determines the fraction of each dye present at each fluorescing position in the microscopic field. This information is conveyed to a computer, and a special program assigns a false-color image to each type of chromosome (Figure 1a). Computer graphics allows the two homologs of each chromosome to be placed next to each other and numbered according to their decreasing size. Such an image clearly displays the cell’s karyotype (Figure 1b).

EXPERIMENTAL Fig1. Human chromosomes are readily identified by chromosome painting. (a) Image of human chromosomes from a male cell in mitosis made by fluorescence in situ hybridization (FISH) using chromosome paint probes. (b) Alignment of these painted chromosomes by computer graphics to reveal the normal human male karyotype. [Courtesy of Dr. Michael R. Speicher.]

Chromosome painting is a powerful method for detecting an abnormal number of chromosomes, such as chromosome 21 trisomy in patients with Down syndrome, or chromosomal translocations that occur in rare individuals and in cancer cells (Figure2). The use of probes with different ratios of fluorescent dyes that hybridize to distinct positions along each normal human chromosome allows finer structural analysis of the chromosomes that can more readily reveal deletions or duplications of chromosomal regions. The chapter-opening figure illustrates the use of such multi color FISH in analysis of the karyotype of a normal human female.

EXPERIMENTAL Fig2. Chromosomal translocations can be analyzed using chromosome painting. Characteristic chromosomal translocations are associated with certain genetic disorders and specific types of cancers. For example, in nearly all patients with chronic myelogenous leukemia, the leukemic cells contain the Philadelphia chromosome, a shortened chromosome 22 [der (22)], and an abnormally long chromosome 9 [der (9)] (“der” stands for derivative). These forms result from a translocation between normal chromosomes 9 and 22. This translocation can be detected (a) by classical banding analysis or (b) by chromosome painting. [Part (b) courtesy of J. Rowley and R. Espinosa.]

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مواضيع ذات صلة

The TATA Box, Initiators, and CpG Islands Function as Promoters in Eukaryotic DNA

Chromosome Painting and DNA Sequencing Reveal the Evolution of Chromosomes

Chromosome Number, Size, and Shape at Metaphase Are Species-Specific

Origins and Types of Mutations

Regulation of Gene Expression

The Human Genome

The Polymerase Chain Reaction (PCR)

Additional Nonhistone Proteins Regulate Transcription and Replication

Nonhistone Proteins Organize Long Chromatin Loops

Modifications of Histone Tails Control Chromatin Condensation and Function

Chromatin Exists in Extended and Condensed Forms

Fetal DNA Sequencing and Fetal Genome Analysis