Beads on string like structures of A are seen in B, which further condense to form chromosomes in C stage of cell division.
A B C
(a) Chromonema Chromatin Metaphase
(b) Chromatin Chromatid Metaphase
(c) Chromonema Chromosome Anaphase
(d) Chromonema Chromatid Anaphase.
(a) : The "beads-on-a-string" structure is seen in electron microscope of isolated metaphase chromosomes. The chromonema form the gene bearing portions of the chromosome. Basically chromonema is made up of nucleosome chains. Nucleosome chain gives a beads on string appearance under electron microscope. Nucleosome is the fundamental packaging unit in eukaryotic chromosomes.