Left: Schematic structure of the diatom cell (center) and diatom cell cycle. Diatom cells are shown in cross section. The gray area represents the protoplast, the green line depicts the plasma membrane. For simplicity, intracellular organelles are not shown.

Right: Cells arranged in the circle show different stages of the cell cycle: (1) Shortly before cell division the cell wall contains the maximum number of girdle bands; (2) immediately after cytokinesis new biosilica (red ) is formed in each sibling cell inside a valve silica deposition vesicle (SDV, yellow); (3) expansion of the valve SDVs as more and more silica is deposited; (4) at the final stage of valve SDV development, each SDV contains a fully developed valve; (5) the newly formed valves are deposited in the cleavage furrow on the surface of each protoplast by SDV exocytosis; (6) the sibling cells have separated; (7+8) expansion of the protoplast in interphase requires the synthesis of new silica (red ) inside girdle band SDVs ( yellow); each girdle band is synthesized in a separate SDV, and after SDV exocytosis is added to the newly formed valve (hypovalve); (9) after synthesis of the final hypovalve girdle band (pleural band) cell expansion stops, and DNA replication is initiated.