Chromosomic chart

The chromosomic chart (or caryogramme) is the standard arrangement of the whole of the Chromosome S of a cell. The chromosomes are photographed and laid out according to a general format: by pair and classified by size. One carries out chromosomic charts with an aim of detecting chromosomal aberrations (like the Trisomie 21) or of identifying certain aspects of the Génome of the individual, like the Sexe (XX or XY).

Realization of a chromosomic chart

After a levy on the individual, his cells (often of the Lymphocyte S obtained by a blood sample) are put in culture In-vitro . The culture is then put in the presence of Colchicine which disturbs the spindles mitotic and blocks the cells in Métaphase of the Mitose. The cells in mitosis are then collected, one makes them inflate by an incubation in a medium Hypotonique; one then puts them in the presence of a fixer, and one spreads out the cellular suspension fixed over a blade of glass, for the observation out of microscope, it is what one calls a chromosomal spreading out. This preparation is then coloured. The most traditional coloring is coloring with the Giemsa which involves, after the application of a treatment adapted, the appearance of dark and clear bands alternate on the chromosomes: the “G-banding”. The topography of the bands is characteristic of a chromosome and makes it possible to identify it (note: the two chromosomes of the same pair have the same topography of bands). There also exists of other methods of coloring which reveal of different types of bands (Q-Banding, R-Banding, etc). Some of these methods call upon fluorescent dyes . Not to confuse with the technique FISH which use specific probes of some sequences of Nucléotide S and not of the dyes which are fixed on all the chromosomes.

Hybridization in situ with fluorescent probes ( FISH: Fluorescent In situ Hybridization )

See also: fluorescent Hybridization in-situ

Sequences of DNA complementary to the sequence of a gene or part of the genome are prepared in vitro and coupled to fluorochromes. This probe as well as the chromosomes are heated, which causes to dissociate the complementary bits. They are then put in presence and one lets them cool. In this manner, the probe (fluorescent) can be paired with its homologous sequence on the chromosome. A hybrid DNA is thus obtained. This makes it possible to locate using a Fluorescence microscope, the position of a gene or a genomic sequence on the chromosomic chart and thus to highlight, for example, an abnormal position due to a chromosomal recombination.

spectral Chromosomic chart ( technical SKY: Spectral Karyotype ).

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In this novel method, various probes, specific each one of only one whole chromosome, and marked with variable proportions of five fluorochromes different are hybridées with the chromosomes. This gives a spectral Signature characteristic to each pair of chromosomes (i.e. colors different because of the variable proportions from fluorochrome on each chromosome). The chromosomes can then be automatically identified in microscopy of fluorescence coupled with a interferometer (spectral Imagerie). It is thus possible to identify chromosomal recombinations very easily.

Criteria to make a chromosomic chart

To know at which time one decides to make a chromosomic chart, there exist criteria:
  • the patient has a backwardness
  • miscarriage with repetition
  • antécédant family of anomaly concerning the chromosomes
  • congenital malformation
  • difficulty of determining the primary sex
  • aménohrée (to 17 or 18 years)
  • maternal age high
  • cancerous pathology of the cells of blood

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