Final answer:
The first level of DNA organization in a eukaryotic cell is maintained by histone proteins which form the core around which DNA winds, creating nucleosome structures. These nucleosomes resemble beads on a string and allow DNA to be compacted within the nucleus. The correct answer is option (a).
Step-by-step explanation:
The First Level of DNA Organization in Eukaryotic Cells
The initial organization of DNA within eukaryotic cells is established through its interaction with histone proteins. Histones are integral to the formation of structures known as nucleosome complexes, functioning as the primary molecular component in the first level of DNA packing and organization. These complexes act as spools around which the DNA winds, effectively condensing the DNA into a more compact form. This structural unit consisting of DNA wrapped around histones appears under an electron microscope as 'beads on a string', with each 'bead' representing a nucleosome and the 'string' is the DNA itself. Not only do histones help in organizing the DNA, but they also contribute to regulating the accessibility of specific DNA regions to various proteins. Thus, the appropriate answer to the question is:
a) Histones
The progression of DNA organization from simple to more complex structures is sequenced as DNA, nucleosome, chromatin, and chromosome. This reflects increases in the complexity of packaging, ultimately enabling the vast quantity of DNA to be stored within the confines of a cell nucleus. Histones are evolutionarily conserved proteins rich in basic amino acids forming an octamer core to which DNA tightly adheres. This is followed by further compaction through the action of another histone variant resulting in a structure known as the 30-nm fiber. In metaphase chromosomes, DNA is maximally condensed, reaching widths around 700 nm through associations with additional scaffolding proteins.