OBJECTIVE LIFE SCIENCE
DEVELOPMENTAL BIOLOGY
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Question
[CLICK ON ANY CHOICE TO KNOW THE RIGHT ANSWER]
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uneven distribution of the maternal/cytoplasmic determinants (generally the anterior side will be where there are more maternal/cytoplasmic determinants.
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the position of the egg within the uterus will determine which side will be the anterior side (faces up in the uterus) and which will be the posterior side (faces down in the uterus).
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the sperm entry point and the rotation of the cortex will expose the gray crescent which will become the future dorsal side.
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once the anterior-posterior and ventral-dorsal axes are established, the left-right axes are determined automatically.
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Detailed explanation-1: -In birds, gravity is critical in determining the anterior-posterior axis, while pH differences appear crucial for distinguishing dorsal from ventral. The left-right axis is formed by the expression of nodal on the left side of the embryo, which signals pitx2 expression on the left side of developing organs.
Detailed explanation-2: -Dorsal-ventral polarity is regulated by the entry of the Dorsal protein into the nucleus. Dorsal-ventral polarity is initiated by the nucleus being positioned in the dorsal-anterior of the oocyte and transcribing the gurken message.
Detailed explanation-3: -The anterior/posterior axis is specified by the sperm at the posterior side. At the two cell stage, the anterior cell is the AB cell while the posterior cell is the P1 cell. The dorsal/ventral axis of the animal is set by a random position of cells during the four cell stage of the embryo.