Roxanita
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Q.- The organization of human chromosomes has seven levels. Which of the following is characterized by 75 kb DNA loops attached to non-histone acidic proteins?
a) Base Pairs
b) Chromatid fibers
c) Double helix
d) Scaffold
e) Nucleosomes
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| suvi21
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b??
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| mash
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scaffold
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| Roxanita
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Answer: d
The organization of DNA in human chromosomes has six levels, which are:
Base pairs - form the paired sequences of the complimentary DNA strands.
Double helix - constitutes the spatial organization of DNA as a result of the interaction of the strands between and among themselves.
Nucleosomes - This most fundamental unit of DNA packaging consists of a central core of eight basic histone proteins around which a stretch of 146 base pairs (bp) of double stranded DNA are coiled. They have a diameter of 10nm.
Chromatin fibers - Nucleosomes are linked with each other by short lengths of spacer DNA. The elementary fiber of linked nucleosomes is in turn coiled into chromatin fibers. They have a diameter of 30nm.
Scaffold - During cellular division, chromosomes become even more condensed, so that at metaphase they have 10-5 of the expected linear length of DNA. Chromatin fibers are attached in loops of about 75kb to a central scaffold of nonhistone acidic proteins, such as topoisomerase II. The loops are attached to the scaffold by AT-rich DNA regions (scaffold attachment regions = SARs).
Chromatid fibers - The loop-scaffold complex is then further compacted to generate the chromatid fibers. They have a diameter of 600nm, and form the chromatids or chromosomes.
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| suvi21
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thnx 8)
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