What are those structures that appear as 'beads-on-string' in the chromosomes, when viewed under electron microscope?

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Nucleosomes

Chromatin

Genes

All of the above

answer is A.

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Detailed Solution

Concept- In eukaryotes, the packing of the DNA helix is more complicated than in prokaryotes. A class of positively charged proteins is known as histones. The number of amino acid residues with charged side chains influences these charges. The amino acid residues arginines and lysines are abundant in histones. The side chain of these amino acid residues is positively charged. Histones are organized into eight-molecule units known as histone octamers. The negatively charged DNA is wrapped around a positively charged histone octamer to create a structure known as a nucleosome.A typical nucleosome has 200bp of DNA helix. The nucleosome is the repeating unit of a complex called chromatin in the nucleus. It is a stained thread-like entity seen in the nucleus. The nucleosome in chromatin looked as a bead-on-string structure when studied under electron microscopes. The bead-on-string is packed into the chromatin, forming chromatin fibers. Furthermore, during the metaphase stage of cell division, these fibers consolidated and coiled to create chromosomes. At a higher level, chromatin packaging requires an additional set of a protein known as non-histone chromosomal protein.
The fundamental component of DNA's structure is a nucleosome. It is present in eukaryotes and absent in prokaryotes. When nucleosomes are viewed under the electron microscope, the beads-on-strings structure appears upon chromatin. These are Nucleosomes. The fundamental unit of heredity is the gene.
Hence, option 1 is the correct answer.

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