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| 12.1 Portrait of a Chromosome 1. Cytogenetics is the study of chromosome abnormalities and associated effects on health or other traits. 2. Excess or deficient genetic material can cause syndromes or end prenatal development. 3. Chromosomes consist of highly coiled DNA, RNA, histones, and non-histone proteins. Telomeres and Centromeres are Essential 1. Telomeres consist of repeat sequences and protect chromosome tips. 2. A centromere is a constricted site where spindle fibers attach during cell division. 3. Centromeres are regions of repeated DNA bound to centromere-associated proteins. Karyotypes are Chromosomal Charts 1. Karyotypes are charts that display chromosomes in size order. 2. Chromosomes are numbered from largest to smallest, 1 through 22, plus X and Y. 3. Chromosomes are distinguished by size, centromere location, differential staining, and DNA probes. 4. Five human chromosomes (13, 14, 15, 21, and 22) have satellites, which carry repeats of genes encoding ribosomal components. 5. Translocations result in exchanges of material between two chromosomes. 6. Some cancers arise from translocations. 12.2 Visualizing ChromosomesObtaining Cells for Chromosome Study 1. Any cell with a nucleus can be used to obtain chromosomes to prepare a karyotype. 2. Fetal karyotypes are constructed from cells obtained by amniocentesis, chorionic villus sampling, and by fetal cell sorting from maternal blood. 3. Fetal karyotypes are prepared for patients with advanced maternal age, repeated pregnancy losses, and increased risk of a chromosomal anomaly as indicated by a maternal serum marker test or family history.Preparing Cells for Chromosome Observation 1. To obtain chromosomes for karyotyping, cells are halted in metaphase, broken open on a glass slide, and the chromosomes spread over the surface. 2. Chromosomes are stained (or exposed to fluorescent DNA probes), identified, and arranged in order of size and centromere location. 3. Chromosomal shorthand summarizes the total number of chromosomes, types of sex chromosomes, and type of aberration present. 4. Ideograms are maps of the distinguishing features of individual chromosomes. 5. Fluorescent in situ hybridization (FISH) combines chromosome specific probes and fluorescent dyes to "paint" chromosomes (or label specific gene loci). 12.3 Abnormal Chromosome NumberPolyploidy 1. A euploid cell has a normal chromosome number (46). 2. Polyploid cells have extra chromosome sets. 3. Polyploids result from the fertilization of an oocyte by two sperm or after fertilization involving a diploid gamete. 4. Abnormal chromosome number is a common cause of spontaneous abortion. Aneuploidy 1. An aneuploid cell lacks a chromosome or has an extra one and can result from meiotic nondisjunction. 2. Individuals with trisomies are more likely to survive than those with monosomies. Sex chromosome aneuploidy is less severe than autosomal aneuploidy. 3. The most common autosomal aneuploids are trisomies 13, 18 and 21. Sex chromosome aneuploids include XO (Turner syndrome), XYY (Klinefelter syndrome), triplo-X females, and XYY males. 12.4 Abnormal Chromosome StructureDeletions and Duplications 1. Deletions and duplications can result when translocations or inversions disrupt pairing of chromosomes in meiosis. Translocations 1. In a Robertsonian translocation, a piece of one chromosome breaks off and attaches to another chromosome. 2. In a reciprocal translocation, two nonhomologous chromosomes exchange parts. 3. A translocation that deletes, duplicates, or disrupts a gene can harm health. 4. Translocation carriers may have a normal phenotype may have affected children. Inversions 1. Inversions result when part of a chromosome flips, and may affect health. Isochromosomes and Ring Chromosomes 1. An isochromosome has two identical arms and therefore introduces duplications and deletions. 2. Isochromosomes arise in meiosis when the centromere splits in the wrong plane. 3. Ring chromosomes arise when telomeres are lost, leaving sticky ends that close and form rings. 4. Ring chromosomes can produce symptoms when they result in additional genetic material. 12.5 Uniparental Disomy-Two Genetic Contributions from One Parent 1. Uniparental disomy (UPD) results when two chromosomes or chromosome parts are inherited from the same parent. UPD therefore doubles part of one parent's contribution. 2. UPD can arise from a trisomy and subsequent chromosome loss, or from two nondisjunction events. 3. UPD can cause disease if it creates a homozygous recessive condition associated with an illness. 4. UPD can also cause disease by disrupting genomic imprinting. |