Question

In the following statements, identify the stages of oogenesis in the order in which they occur:

• [(A)] During fertilisation, formation of egg and second polar body.
• [(B)] Meiosis II commences but stops at metaphase II.
• [(C)] Meiosis I commences but stops at prophase I.
• [(D)] Formation of primary oocyte from oogonia.
• [(E)] Meiosis I completes and results in formation of secondary oocyte and first polar body.

• A. (D), (C), (B), (E), (A)
• B. (C), (E), (D), (A), (B)
• C. (D), (C), (E), (B), (A)
• D. (D), (C), (A), (E), (B)

Hint:

Oogenesis shows two meiotic arrests:

• Prophase I arrest during fetal stage
• Metaphase II arrest at ovulation

Answer 1

The Correct Option is C

Concept: Oogenesis is the process of formation of female gametes or ova inside the ovaries. Unlike spermatogenesis, oogenesis begins during fetal development and gets completed only after fertilization. The process involves mitotic multiplication, growth phase, meiotic divisions, and formation of polar bodies. An important feature of oogenesis is that meiosis remains arrested at two different stages:

• Prophase I arrest
• Metaphase II arrest

Step 1: Formation of primary oocyte from oogonia.
During fetal life, primordial germ cells divide repeatedly by mitosis to form oogonia. Some oogonia enlarge in size and differentiate into primary oocytes. Therefore, formation of primary oocyte from oogonia occurs first. Hence, event (D) is the first step.

Step 2: Initiation and arrest of Meiosis I.
The primary oocyte starts Meiosis I during fetal development. However, this meiotic division does not complete immediately. The primary oocyte becomes arrested at Prophase I stage, specifically in diplotene stage, and may remain arrested for many years until puberty. Therefore, event (C) occurs after formation of primary oocyte.

Step 3: Completion of Meiosis I.
After puberty, during every menstrual cycle, one primary oocyte resumes Meiosis I. Completion of Meiosis I produces:

• One large secondary oocyte
• One small first polar body

This unequal cytokinesis ensures that most cytoplasm remains within secondary oocyte. Hence, event (E) occurs next.

Step 4: Initiation and arrest of Meiosis II.
The secondary oocyte immediately begins Meiosis II after its formation. However, Meiosis II again gets arrested, this time at Metaphase II stage. The secondary oocyte is ovulated at this arrested stage. Thus, event (B) occurs after completion of Meiosis I.

Step 5: Completion of Meiosis II during fertilization.
Meiosis II completes only if fertilization occurs. Entry of sperm into secondary oocyte stimulates completion of Meiosis II, resulting in:

• Formation of mature ovum
• Formation of second polar body

Hence, event (A) occurs last.

Step 6: Write the final correct sequence.
Therefore, the correct order of events is: \[ (D)\rightarrow(C)\rightarrow(E)\rightarrow(B)\rightarrow(A) \] Hence, the correct option is: \[ \boxed{(C)} \]