CHROMATIN STRUCTURE & FUNCTION
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16 Terms
What is chromatin composed of?
DNA, histone proteins, and non-histone proteins.
How does chromatin appear during interphase and mitosis?
Interphase: filamentous, invisible under light microscope
Mitosis: condenses into visible chromosomes
What is heterochromatin?
Inactive DNA regions
Tightly packed
Stains dark with Giemsa
Associated with structural functions
What is euchromatin?
Active DNA regions (transcribed)
Loosely packed
Stains light with Giemsa
Associated with RNA polymerase activity
What is facultative heterochromatin?
DNA that was active (euchromatin) but became inactivated temporarily depending on cell function or development.
What is constitutive heterochromatin?
Permanently inactive DNA regions, present in all cells and throughout life.
What is sex chromatin?
Heterochromatic bodies representing sex chromosomes, visible in interphase nuclei.
What is a Barr body?
A condensed, inactivated X chromosome seen in females as a flat/convex mass at the nuclear periphery.
How large is a Barr body?
About 0.7 × 1.2 microns; doubles in size from G1 to G2 due to DNA replication.
Where are Barr bodies typically found?
In oral mucosa epithelium, nervous tissue, and polymorphonuclear neutrophils (PMNs).
What genes regulate X inactivation?
XIST: produces RNA that coats and inactivates the X chromosome
TSIX: expressed on the active X, blocks XIST
What is the formula for number of Barr bodies?
B = (X – 1) × P / 2
Where X = number of X chromosomes, P = ploidy (P = 2 in humans)
What is the diagnostic value of sex chromatin?
Rapid sex determination (XX or XY)
Identifies sex chromosome aneuploidies (e.g. XXY, X0)
What is the Y corpuscle (F body)?
A fluorescently stained Y chromosome visible with quinacrine or atebrine dyes.
In which cells can the Y corpuscle be observed?
Buccal mucosa
Lymphocytes
Fibroblasts
Sperm
Amniotic cells
What is the diagnostic value of sex chromatin?
Rapid sex determination (XX or XY)
Identifies sex chromosome aneuploidies (e.g. XXY, X0)
Note 
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