Q1. Source: CSIR NET Nov 2020; Session I
If an early-stage wild-type Drosophila embryo is injected with bicoid mRNA at the posterior pole, then:
a. Head structures develop at posterior pole, while the same is inhibited at anterior pole.
b. Head structures form at both poles.
c. Head structures are not formed at posterior pole due to presence of posterior morphogens.
d. Duplications of usual posterior structures occur.
Concept
Bicoid is a maternal-effect gene that acts as a master regulator of anterior development in Drosophila. Under normal conditions, bicoid mRNA is deposited at the anterior end of the egg by the mother. After fertilization, it is translated into Bicoid protein, which forms a concentration gradient and specifies head and thoracic structures.
If bicoid mRNA is artificially introduced at the posterior pole, a second Bicoid gradient forms there. As a result, anterior structures develop at both ends of the embryo.
Statement analysis
a. The anterior pole already contains endogenous bicoid mRNA. Injecting additional bicoid at the posterior does not inhibit head formation at the anterior end.
b. Correct. Head structures develop at both poles due to the formation of two Bicoid gradients.
c. Posterior morphogens do not prevent bicoid from specifying anterior structures. However, remember that posterior determinants such as Nanos inhibit maternal hunchback mRNA translation.
Exam Tip: If a similar question is asked with hunchback mRNA instead of bicoid mRNA, think carefully about the role of Nanos.
d. Bicoid specifies anterior structures, not posterior structures. Therefore, duplication of posterior structures will not occur.
Correct Answer: b. Head structures form at both poles.
Q2. Source: CSIR NET Nov 2020; Session II
Which one of the following statements regarding double fertilization in plants is correct?
a. The same sperm cell fuses with both egg cell and central cell.
b. Two sperm cells fuse with the egg cell.
c. One sperm cell fuses with the egg cell and the second with the central cell.
d. Two sperm cells fuse with the central cell.
Concept
Double fertilization is a unique feature of flowering plants. Two sperm cells are delivered into the embryo sac:
One sperm cell fuses with the egg cell to form the diploid zygote. The second sperm cell fuses with the central cell (containing two polar nuclei) to form the triploid endosperm, which nourishes the developing embryo.
Correct Answer: c. One sperm cell fuses with the egg cell and the second with the central cell.
Exam insights
One sperm = Embryo
One sperm = Endosperm
That is the essence of double fertilization.