Several marine organism release their gametes into the environment, where sperm attraction and subsequent events lead to successful fertilization. With reference to sea urchins, which one of the following statements is NOT true?
Ca2+ prevents docking of cortical granules of the egg to the cell membrane.
Sea urchin fertilization is a well-studied process involving several key events that lead to the fusion of sperm and egg and the initiation of embryonic development. Understanding these steps is crucial to identifying the incorrect statement among the options provided.
Sea Urchin Fertilization Processes Explained
Let's analyze each statement in the context of known sea urchin fertilization mechanisms:
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Statement 1:
Addition of resact into a drop of seawater containing sperms specifically attracts sperms of A. punctulata.This statement refers to chemotaxis, the process where sperm are guided towards the egg by chemical signals released from the egg's jelly coat. Resact is a well-known chemoattractant peptide found in the egg jelly of the sea urchin species Arbacia punctulata. It specifically binds to receptors on the sperm and attracts them. Therefore, this statement is true.
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Statement 2:
IP3 is formed initially at the site of sperm entry and releases sequestered Ca2+Sperm entry into the egg triggers a signaling cascade, often involving the enzyme Phospholipase C (PLC), which produces Inositol Trisphosphate (IP3). IP3 binds to receptors on the endoplasmic reticulum, causing the release of stored calcium ions (Ca2+) into the cytoplasm. This Ca2+ wave is essential for initiating the cortical reaction and development. This process typically starts near the point of sperm entry and spreads across the egg. Therefore, this statement is true.
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Statement 3:
Ca2+ prevents docking of cortical granules of the egg to the cell membrane.The cortical reaction is a critical event that prevents polyspermy (fertilization by multiple sperm). It involves the fusion of cortical granules, which are small vesicles located just beneath the egg's plasma membrane, with the plasma membrane. This fusion releases the granule contents into the space between the egg membrane and the vitelline envelope. This entire process of cortical granule fusion (docking and fusion) is triggered by the increase in intracellular Ca2+ concentration, which starts as a wave from the sperm entry site. Therefore, Ca2+ promotes, rather than prevents, the docking and fusion of cortical granules. This statement is NOT true.
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Statement 4:
Inhibitors that specifically block PLCγ can be circumvented by microinjecting IP3 into the egg.As mentioned earlier, PLC (including the γ isoform in some species) produces IP3, which then causes Ca2+ release. If PLCγ is inhibited, the cell cannot produce IP3 in response to sperm entry, thus preventing the Ca2+ release and subsequent cortical reaction. However, if IP3 is directly microinjected into the egg cytoplasm, it can bypass the need for PLCγ activity and directly trigger Ca2+ release from the endoplasmic reticulum. This would successfully initiate the downstream events like the cortical reaction. Therefore, this statement is true.
Identifying the Incorrect Statement
Based on the analysis, the statement that is NOT true regarding sea urchin fertilization is the one claiming that Ca2+ prevents the docking of cortical granules. In reality, the rise in intracellular Ca2+ concentration is the trigger for the fusion of cortical granules with the egg membrane during the cortical reaction.
Important Questions from Morphogenesis and organogenesis in animals
A "morphogen" can determine the fate of a cell by its concentration. Given below are some statements on the experiment performed to study the gradient-dependent effect of the morphogen, activin on cell fate by placing activin (4 nm)-secreting beads on unspecified cells from an early Xenopus embryo:
A. Beads without activin did not elicit expression of either Xbra or goosecoid genes.
B. Cells nearest to the beads getting highest concentration of activin induced goosecoid gene whose product is a transcription factor, specifies the frog's dorsal-most structures.
C. Cells nearest to the beads getting highest concentration of activin induced Xbra gene whose product is a transcription factor, specifies the frog's dorsal-most structures.
D. Cells farthest from the beads getting negligible activin activate Xbra gene and become blood vessels and heart.
E. Cells farthest from the beads getting negligible activin, activated neither Xbra nor goosecoid and the 'default' gene expression instructed the cells to become blood vessels and heart.
Which of the above observations and conclusions drawn are correct?
The individuals considered in this question are having two haploid sets of autosomes and no Y- chromosome. The X∶A ratio of the individuals, the type of organisms chosen, their primary sex and number of Barr bodies expected in their cells are shown in the table below:
X∶A ratio Organism Primary Sex Number of Barr bodies i. 0.5
ii. 2
A. Human
B. Drosophila
I. Male
II. Female
III. Metafemale
a. Zero
b. One
c. Two
d. Three
Select the option below with all correct matches:
The following statements regarding the generation of dorsal/ventral axis in Drosophila was made:
A. Gurken protein moves along with the oocyte nucleus and signals follicle cells to adopt the ventral fate.
B. Maternal deficiencies of either the gurken or torpedo gene cause ventralization of the embryo.
C. Gurken is active only in the oocyte and Torpedo is active only in the somatic follicle cells.
D. The Pipe protein is made in the dorsal follicle cells.
E. The highest concentration of Dorsal is in the dorsal cell nuclei, which becomes the mesoderm.
Which one of the following combination of the above statements is true?
During wing development in chick, if Apical Ectodermal Ridge (AER) is removed, the limb development ceases, on the other hand placing leg mesenchyme directly beneath the wing AER, distal hindlimb structures develop at the end of the wing, and if limb mesenchyme is replaced by non-limb mesenchyme beneath AER, the AER regresses. This may demonstrate that:
A. the limb mesenchyme cells induce and sustain AER.
B. the mesenchyme cells specify the type: wing or limb.
C. the AER is responsible for specifying the type: wing or limb.
D. the AER is responsible for sustained outgrowth and development of the limb.
E. the AER does not specify the type: wing or limb.
Which combination of above statements is demonstrated by the experiment?
Human polysyndactyly (joining of extra digits) syndrome results from a homozygous mutation at
