Chromosomes and Chromosome Diseases

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Chromosomes and Chromosome Diseases
2015-11-26 08:02:44

Chromosomes and Chromosome Diseases
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  1. What is a Barr body
    Inactive X chromosome in a female somatic cell, or the inactive Z in a male. It is not condensed, and therefore can be detected in a swab without culturing the cells
  2. Lyonization
    All extra X chromosomes in a female is inactivated and packed into heterochomatin causing mosaic traits
  3. In Interphase chromosomes are not
  4. Chromosome parts are named how
    • The ends are the telomeres
  5. Chromosomes are for karyotyping are obtained from a cell in mitosis because
    They are condensed in metaphase
  6. Steps to obtaining chromosomes for karyotyping
    • 1. Obtain a blood sample
    • 2. Grow the cells in a growth medium with PhytoHemAgglutinin (lymphocytes will start division)
    • 3. Add colchicine to block division in metaphase
    • 4. Few hours later, harvest cells, add fixative and hypotone liquid
    • 5. Let one drop fall onto a slide (When the drop hits, the cell membrane bursts, and the chromosomes will spread a short distance)
    • 6. Banding pattern is obtained through one of several chemical/enzymatic treatments
    • 7. Analyzing the result normally starts with ordering the chromosomes
  7. What are some sources for obtaining cells for karyotyping in adults
    • – Blood (leukocytes): culture 2-3 days
    • – Bone marrow biopsy: no culture
    • – Fibroblast cells: culture 2-3 weeks
  8. What are some sources for obtaining cells for karyotyping in Prenatals
    Amniotic cells: culture 2-3 weeks (can be performed around week 16

    Chorionic villus sample: culture not necessary, but separation from maternally derived tissue is necessary (This can be done around week 11)
  9. Karyotyping
    Ordered display of all chromosomes from one cell
  10. How are karyotyped cells ordered
    According to size and position of centromere
  11. Acrocentric chromosome are
    A chromosome with its centromere located quite near one end of the chromosome. This is where the RNA genes are located (five pairs)
  12. Metacentric
    A chromosome having its centromere in the middle
  13. Chromosome is said to be submetacentric
    If arms' lengths are unequal
  14. The earlier in the cell cycle you harvest chromosomes
    The less condensed they are
  15. G-banding is a technique for visible karyotyping and contains chromosomes that have
    • Fewer transcriptionally active genes
    • More condensed chromatin structure
    • Higher in A-T basepairs
  16. Heterochromatin is a
    Chromosomal region that is not transcriptionally active
  17. Constitutive (useful) heterochromatin is mostly around
    Centromeres, & near telomeres (<6% of total DNA)
  18. Heterochromatin is condensed when
    even in interphase!
  19. What kind of genes do Heterochromatin often have
    • Repeat areas
    • Genes not needed in a specific cell type
  20. Polyploidy:
    3 (rarely 4) copies of all chromosomes; not viable; 3 copies = triploid

    One of most common reasons for early pregnancy loss
  21. Trisomy
    Excess of one chromosome
  22. What is the most common trisomy but not compatible with life
    Trisomy 16, common cause of miscarriages
  23. In trisomy which pairs are compatible with life
    Only trisomy of 13, 18, 21, or X,Y
  24. Monosomy
    • Lack of one chromosome
    • Only monosomy X is compatible with life
  25. In trisomy 21 how many actually survive to birth
    80% of conceptions die in utero
  26. Monosomy is only viable with X chromosome, what is the chance of it surviving
    More than 99% of these die in utero
  27. What is an inversion
    The chromosomes were flipped
  28. When counting bands on the chromosome, which direction do you count
    From the centromere out
  29. Karyotype nomenclature
  30. The Types of Structural Abnormalities in chromosomes are
    • • Deletion (del)
    • • Inversion (inv)
    • • Duplication (dup)
    • • Insertion (ins)
    • • Ring (r) (Lost telomeres at both end)
    • • Translocation, reciprocal (t)
    • • Translocation, Robertsonian (der) or (rob)
    • • Isochromosome (iso)
  31. Isochromosome Formation
  32. Ring Chromosome Formation
  33. Translocations have two types
  34. Results of Reciprocal Translocations
    • The unbalanced version affects multiple genes
    • Common: Mental retardation and heart defects
  35. Downs Syndrome
    Diagnostic signs include mental retardation and specific shapes of the head and often hand and feet

    Complications: 40 % have heart malformations

  36. Causes of Downs Syndrome
    Most often an error in segregating the chromosomes in the first maternal meiosis
  37. Mosaicism is caused by
    Non-disjunction in mitosis, therefore it happens after the zygote formation
  38. Ageing of Down Syndrome Patient often exhibits
    • Early senility
    • Alzheimer’s disease
  39. What is thought to be the reason for Alzheimer’s disease in trisomy 21
    The APP gene is on chromosome 21 and presumably overexpressed when three copies are present
  40. What are the only chromosome numbers in trisomy that can be compatible with life
    13, 18, and 21 or X,Y
  41. Multiple cases in a family of Downs syndrome is Likely due to
    Robertsonian translocation in the family
  42. Master control (SRY) is
    Sex-determining gene on the Y chromosome
  43. Presence of SRY (master control gene) will in most cases lead to
    A male gonad; without SRY, the gonad stays female
  44. True Hermaphrodites are caused by what two possibilities
    Prenatal exposure to progesterone or androgen

    Inherited deficiency of 11- or 21-hydroxylase in adrenal cortex
  45. Virilized
  46. Adrenogenital Syndrome
    • Hyperplasia of adrenal cortex
    • Early detection is critical in females
    • In males, penile enlargement rarely detected Premature puberty in toddlers
    • Treatment with cortisol, surgical reconstruction when needed
  47. Androgen Insensitivity Syndrome
    46, XY Externally and psychosexually female; no uterus and fallopian duct, no Wolffian ducts, non-descended testes in abdomen
  48. 5α-reductase type 2 deficiency (Male Intersex (B))
    • Partial feminization at birth (variable), testes often non-descended.
    • Many of these raised as females until puberty
  49. Indications for Karyotyping should be used in what cases
    • Suspicion of recognized syndrome
    • Unexplained mental retardation
    • Unexplained congenital malformations
    • Pregnancy loss/infertility
    • Abnormality in sexual development
    • Malignancies
  50. Pleitropy
    One gene that influences many phenotypes negative or positively
  51. Locus Heterogeneity
    Many different genes can cause the same genetic issue
  52. Allelic Heterogeneity
    Different mutations at the same locus cause the same disorder
  53. Balanced reciprical translocation
    An equal exchange of chromosome arms between two sets of chromosomes. (harmless) but can cause miscarriages in women with it in their genome
  54. The Acrocentric chromosomes are
    13, 14, 15, 21, and 22
  55. Robertsonian translocation is
    Translocations involving the acrocentric chromosomes