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Aneuploidy in Human Oocytes and Preimplantation Embryos
Published in Carlos Simón, Carmen Rubio, Handbook of Genetic Diagnostic Technologies in Reproductive Medicine, 2022
During meiosis I, the sister kinetochores are co-oriented such that they are pulled to the same side of the spindle. At the same time, cohesion between sister chromatids along chromosome arms is released, allowing the homologous chromosome to separate. In contrast, cohesion near the centromeres is protected until meiosis II, when it is released to allow sister chromatids to separate (Figure 8.6).
The Fight Against Cancer
Published in Nathan Keighley, Miraculous Medicines and the Chemistry of Drug Design, 2020
Chromosomes containing mutated genes will become more prevalent if cell division and replication of the defective DNA is allowed to continue unimpeded, when the necessary control mechanisms fail, and this vastly increases the risk of cancer; as defective cells multiply in number, they can grow uncontrollably and defective genes can be inherited in gametes, if damage occurs during meiosis. Humans have 22 pairs of homologous chromosomes (X and Y) and a pair of sex chromosomes; a total of 46 chromosomes, known as the diploid number. The diploid number varies between species, for example dogs have 78. These are always even numbers because half comes from each parent. The chromosomes are dispersed throughout the nucleus and each comprise a single molecule of DNA that is supercoiled, held in place by protein molecules called histones. The chromosomes are only distinctly visible under the microscope during cell division.
Molecular Approaches Towards the Isolation of Pediatric Cancer Predisposition Genes
Published in John T. Kemshead, Pediatric Tumors: Immunological and Molecular Markers, 2020
When total genomic DNA is “cut” with a particular enzyme, a heterogeneous population of characteristic restriction fragments is produced. Each fragment has a fixed length, the restriction fragment length (RFL). Agarose gels can be used to separate these fragments according to size. Using standard techniques, developed by Southern5 and Rigby et. al.,6 radioactive DNA probes can be used to hybridize with, i.e., “recognize”, homologous sequences within the heterogenous population. This homology is manifested as a band (or set of bands)on an autoradiograph at one or more specific locations determined by the size of the fragment being recognized (Figure 1). If there is no sequence variation between individuals in the region of the chromosome recognized by the probe, a consistent band pattern is produced at the same position on the gel. However, any variation in DNA sequence that affects a given restriction site, such that an existing one is removed or a new one is created, will generate variations in the band pattern observed. Since the bands produced are polymorphic, the variation is referred to as a restriction fragment length polymorphism (RFLP). This principle is illustrated in Figure 2. If an individual is heterozygous for the polymorphic variants, then both of the individual homologous chromosomes can be identified (see Figure 2). Differences in DNA sequence at the same locus are allelic and the bands produced on the autoradiograph are conveniently referred to as alleles.
Heterochromatin extension: a possible cytogenetic fate of primary amenorrhea along with normal karyotype
Published in Journal of Obstetrics and Gynaecology, 2022
Bishal Kumar Dey, Shanoli Ghosh, Ajanta Halder, Somajita Chakraborty, Sanchita Roy
The region of heterochromatin also acts as a key part in chromosome structure, histone modification and gene regulation. There is evidence from where we come to know that there may be displacement of heterochromatin from one chromosome to another. Perhaps, this displacement is helping in the extension of a particular chromosome at the heterochromatin portion of the long arm (Bannister and Kouzarides 2011). The mechanisms of spindle fibres, chromosome movement, meiosis crossover and change of sister chromatids are considered to be the integral region as heterochromatin for a chromosome. At the time of meiosis, there may be a change in area of synapses of homologous chromosomes in the polymorphic heterochromatin region. The heterochromatin in chromosomal polymorphism can also regulate gene expression by reversible transformation between heterochromatin (non-coding DNA sequences) and euchromatin (expressed DNA sequences) thus justifying certain clinical expression like short stature or PA. It was also postulated that defective histone protein methylation due to presence of heteromorphic variants may play a more crucial role in ovarian failure. Association of heterochromatin polymorphism with ovarian dysgenesis may be a reason for the occurrence of PA. For that, we need to study on a greater number of patients on the basis of their nucleosome’s functionality and heteromorphic polymorphism by sequencing.
Genetic variations as molecular diagnostic factors for idiopathic male infertility: current knowledge and future perspectives
Published in Expert Review of Molecular Diagnostics, 2021
Mohammad Karimian, Leila Parvaresh, Mohaddeseh Behjati
Balanced chromosomal translocations involve the breakage of two chromosomes and abnormal repair of chromosomal fragments resulting in the transfer of genetic material from one chromosome to another without loss of any genetic material. In vast majority of cases, carriers of balanced translocations are phenotypically normal, unless one of the breakpoints at the site of translocation disrupts an important gene. Chromosomal translocation, while phenotypically normal, may experience fertility loss, miscarriage, or birth defects. Normal meiotic segregation of these translocations in gametes can lead to duplication or deletion of chromosomal regions involved in translocation [171]. Like chromosomal translocations, inversions can lead to infertility, miscarriage, and birth defects. During meiosis, chromosomes are forced to form specialized structures, so that homologous chromosomes can be paired. Chromosomal inversions can affect these specialized structures. Research on the production of unbalanced gametes in balanced inversion carriers has been done to a much lesser extent than translocations. However, a handful of studies have reported an unbalanced sperm range of 1–54% [172–174].
Analysis of an NGS retinopathy panel detects chromosome 1 uniparental isodisomy in a patient with RPE65-related leber congenital amaurosis
Published in Ophthalmic Genetics, 2021
Fabiana Louise Motta, Rafael Filippelli-Silva, Joao Paulo Kitajima, Denise A. Batista, Elizabeth S. Wohler, Nara L. Sobreira, Renan Paulo Martin, Juliana Maria Ferraz Sallum
Uniparental disomy (UPD) is characterized by the inheritance of a pair of homologous chromosomes from only one parent. This genetic event may entirely or partially affect the chromosome (complete or segmental uniparental disomy, respectively) and may result in identical copies of a chromosome (isodisomy) or different copies of the same chromosome (heterodisomy) from one parent (5,6). Some mechanisms may lead to UPD, such as monosomy rescue, postfertilization errors, trisomy rescue, or gamete complementation (5). Despite being a non-Mendelian event, uniparental disomy can lead to a recessive monogenic disease by the formation of a homozygous allele derived from a heterozygous parent (7). The identification of UPD is extremely important because it impacts family genetic counseling and the risk of disease recurrence.