Cytogen - Chromosome Terminology chromosome number – generally a constant for a species haploid – the number of chromosomes found in a gamete(n

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Unformatted text preview: Chromosome Terminology chromosome number – generally a constant for a species haploid – the number of chromosomes found in a gamete (n) diploid – the number of chromosomes found in a zygote (2n) chromosome size – relative length is one way to classify chromosomes within a species chromosome homologues – members of a chromosome pair, found in the diploid state telomere – the termination of the linear chromosome, marked by long, repetitive sequences centromere – a constriction along a chromosome with specific repetitive sequences, which organize its kinetochore. kinetochore – a protein structure which attaches a chromosome to the spindle fibers during cellular division chromosome morphology – another way to classify chromosomes within a species using specific stains during periods of cellular division metacentric – describing chromosomes with centrally located centromeres acrocentric – describing chromosomes with centromeres towards one end telocentric – describing chromosomes with terminally located Cytogenetics Cytogenetics is the study of cells at the chromosomal level. Chomosomes have historically been studied by microscopic analysis during mitotic metaphase. Banding of the chromatin is revealed using the Giemsa staining complex. Images of the chromosomes are sorted in a chart to form what is known as a karyotype. Polyploidies Polyploidy is the state of a nucleus having three or more sets of the genome. In organisms that can tolerate polyploidy, even-numbered ploidies are fertile, and odd-numbered are not. Tetraploid yeasts, fish, and amphibia have all been observed in nature and may represent increased potential for evolution. Endopolyploidy is polyploidy of a specific tissue within a diploid organism. The human liver, for example, will contain tetraploid and octoploid cells. Polyploidy in plants is common. They are usually more robust and larger than their diploid counterparts. Triploid and pentaploid crops produce seedless varieties of fruit and can be cultivated vegetatively. An alloploid individual contains the genomes of two or more related species. Autopolyploidy refers to an increase in the number of chromosome sets within a species. Human Triploidy Individuals with three copies of each chromosome are triploid. Triploidy accounts for 17% of all spontaneous abortions and 3% of stillbirths. Causes: Two spermatozoa fertilize one oocyte. A haploid spermatozoon fertilizes an oocyte that undergoes a complete nondisjunction of chromosomes. Classes of Aneuploidy abnormality description monosomy one missing chromosome trisomy one extra chromosome deletion portion of a chromosome missing duplication portion of a chromosome present twice inversion portion of a chromosome reversed in directionof two chromosomes exchanged portions translocation Autosomal monosomies and trisomies are deleterious due to imbalance in relative gene dosages. Duplications are generally less harmful than deletions. Inversions and translocations are often associated with position effects. Partial Meiotic Nondisjunctions Nondisjunction in meiosis I Anaphase I Anaphase II Nondisjunction in meiosis II Gametes Abnormal gametes Abnormal gametes Normal gametes Mosaic Aneuploidy When an aneuploid gamete is fertilized by a normal gamete, an aneuploid zygote is produced. Partial nondisjunctions can also occur during mitosis, resulting in a somatic mutation and mosaicism. A mosaic can also be produced from a trisomic zygote if an extra chromosome is lost in mitosis during development, correcting the aneuploidy for that cell’s progeny. Mosaic Down Syndrome, for example, has far more variability in symptoms than typical Down Syndrome, depending on what portions of the body are affected. Autosomal Monosomies and Trisomies Most autosomal monosomies and trisomies are inviable. Trisomies of some of the smaller autosomes can survive birth but will manifest syndromic disorders. Trisomy 13 (Patau Syndrome), ~1/20,000 births, less than 20% live past first month. Symptoms include mental retardation, microcephaly, seizures, and cyclopia. Trisomy 18 (Edward Syndrome), ~1/8,000 births, less than 50% live past first month. Symptoms include mental deficiencies, micrognathia, heart and kidney abnormalities. Trisomy 21 (Down Syndrome), ~1/800 births, over 85% live past first year. Symptoms include retardation of mental development, social skills, and physical growth, facial abnormalities such as flattened nose and epicanthal folds. Alterations in Sex Chromosome Number Sex chromosomal aberrations are better tolerated due to the small size of the Y and a natural mechanism for correcting relative gene dosage for the X – Lyonization or X-chromosome inactivation. Triple-X syndrome: Occurring in 1:2000 live births, these females may be asymptomatic but some may have tall stature, learning disabilities, delayed motor coordination, and infertility. Turner syndrome (XO): Occurring in 1:2000 live births, these females may show short stature, webbed neck, and infertility. Klinefelter syndrome (XXY): Occurring in 1:2000 live births, these males may be asymptomatic but some may have tall stature, feminized body shapes, and infertility. Jacob syndrome (XYY): Occurring in 1:2000 live births, these males are usually asymptomatic but some may Other Chromosome Aberrations Misaligned or unequal crossover events lead to deletions and duplications. Crossover events between non-homologous chromosomes leads to translocation. Crossover events within a chromatid results in inversions. Evolution by Gene Duplication Genes can be duplicated by a number of mechanisms: genome duplication (tetraploidy) chromosome duplication (tetrasomy) mobile element expansion (retrotransposition) unequal crossing over Unequal crossover results in tandem gene duplication. Over the course of many generations, replicated genes accumulate mutations and evolve into paralogues. Paralogues are genes within a species sharing a common ancestral gene, ...
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This note was uploaded on 04/15/2008 for the course CELL 211 taught by Professor Dotson during the Fall '07 term at Tulane.

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