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chapter_10 - Clinical cytogenetics chromosomes Human...

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chromosomes Human chromosomes are classified according to their size and the position of their chromosome and banding pattern. Based on the centromere position, there types are discerned: square4 metacentric (centromere in the middle) square4 acrocentric (centromere on the end - these chromosomes have a very short p-arm) square4 submetacentric chromosome (centromere in between the metacentric and acrocentric position) The centromere divides the chromosome into a short (p) and long arm (q). Staining chromosomes allows obtaining a reproducible banding pattern. The most frequently used staining procedures include G- and R-banding. An international nomenclature was developed, assigning a specific number to the individual chromosome bands and resulting in the creation of ideograms: schematic representations of each individual chromosome. Depending on the metaphase stage, different levels of resolution can be obtained. The highest resolution is achieved in prometaphase banding, resulting in the detection of cytogenetic aberrations of ~1Mb. chromosome banding An absolute prerequisite for performing cytogenetic analysis is the availability of dividing cells. Cytogenetic analysis can be performed on different tissues: postnatal cytogenetics square4 blood square4 skin fibroblasts square4 malignant cells (bone marrow, biopsies, ...) prenatal diagnosis square4 chorionic villi (10-12w of gestation) square4 amniocytes (15-16w of gestation) square4 cord blood The karyotyping procedure and the different banding techniques are illustrated on the course slides. euploidy, numerical and structural chromosome aberrations Euploidy refers to the state in which chromosomes display a normal structure and number. An aberration in the number or structure of chromosomes is called aneuploidy. In numerical aneuploidy the chromosome number of diploid cells differs from 46, structural aneuploidy refers to the presence of a chromosome rearrangement. polyploidy Clinical cytogenetics
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Polyploidy indicates the presence of more than 2 sets of haploid chromosomes. Triploidy, in which the chromosome number is 69, most frequently results from the fertilization of a single female gamete by two sperm cells. Tetraploidy (96 chromosomes) frequently originates from the absence of a cellular division following a nuclear division. Polyploidy can be detected frequently in miscarriage tissue and in spontaneous abortions. autosomal aneuploidy Aneuploidy for individual chromosomes is most frequently the consequence of non-disjunction during meotic division (see slides). A trisomy refers to the presence of three chromosomes in diploid cells, whereas in a monosomy only one chromosome is present. Most aneuploidies are incompatible with life and result in a miscarriage, spontaneous abortion or mors in utero.
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