51. After fertilization, cells in vertebrate embryos divide at regular intervals giving rise
to a mass of cells termed the blastula. As cells divide, they undergo cleavage,
where the enormous volume of the egg cytoplasm is slowly divided into
numerous small cells, such that the total volume of the embryo remains the
same. How embryo maintains its overall volume during this process?
a) Cells in early vertebrate embryos do not have G1 and G2 phases and
hence do not increase in volume.
b) Cells in early vertebrate embryos do not have the S phase and hence do not
increase in volume.
c) Cells have all cell cycle phases, but they halve protein production in every
cell cycle.
d) Cells normally increase in volume as any dividing cell but degrade the half
the material to ensure embryo volume is maintained.
The correct answer is a) Cells in early vertebrate embryos do not have G1 and G2 phases and hence do not increase in volume.
Cleavage Process Overview
Cleavage involves rapid mitotic divisions of the zygote into blastomeres, partitioning the large egg cytoplasm without overall embryo growth. The total cytoplasmic volume remains constant as cells divide within the original zygote boundaries, ensuring no net increase in embryo size.
Option Analysis
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a) Correct: Early cleavage cycles lack G1 and G2 phases, featuring only M (mitosis) and S (DNA synthesis) phases, preventing cell growth and volume increase. Blastomeres halve in size per division while conserving total volume.
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b) Incorrect: S phase is essential for DNA replication during these rapid cycles; its absence would halt division, not maintain volume.
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c) Incorrect: Early embryos do not have full cell cycles with all phases; they skip growth phases entirely, rather than halving protein production.
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d) Incorrect: Cells do not grow and then degrade material; cleavage avoids growth altogether through abbreviated cycles.
Biological Importance
This volume conservation optimizes maternal resources, enables high cell numbers for patterning, and prepares for gastrulation without expansion. Smaller blastomeres improve nucleo-cytoplasmic ratios for gene activation.
