In the context of cell division, which chromosomal behaviour takes place at the leptotene stage?

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Accepted Answer

1) Context: Leptotene is the first substage of prophase I of meiosis, immediately following interphase. It marks the onset of visible chromosome reorganization as the cell prepares for homologous interactions.

2) Chromatin reorganization: During leptotene the diffuse chromatin of interphase begins to coil and shorten, so that long, thin chromatin fibers become progressively thicker and more tightly packed. Microscopically this appears as the initial formation of slender, thread‑like chromosomes (often called leptotene threads).

3) Appearance of chromomeric pattern: As condensation proceeds, beads or nodules called chromomeres become distinguishable along the length of each thread. These chromomeric constrictions reflect progressive compaction of chromatin and are cytological evidence of early condensation.

4) Structural preparations for pairing: Condensation at leptotene facilitates subsequent assembly of axial elements (the precursors of the synaptonemal complex) along sister chromatids and sets the stage for homologous alignment in the next substages (zygotene and pachytene). Thus the primary observable event in leptotene is the beginning of chromosome condensation rather than completion of pairing or synapsis.

5) Conclusion: Because the defining, observable change in leptotene is the transition from diffuse chromatin to the first visible, condensed chromosome threads (with chromomeres and axial element formation starting), the statement “chromosomes begin to condense” correctly describes leptotene.

FINAL_ANSWER: D

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