Arrange the stages of oogenesis in order of their occurrence: A. Formation of ovum & second polar body B. Meiosis II begins but is held at Metaphase II C. Meiosis I started but is held at Prophase I D. Formation of primary oocyte from oogonia E. Completion of Meiosis-I and formation of secondary oocyte and first polar body Choose the correct answer from the options given below:
D, C, E, B, A
Understanding Oogenesis Stages: Formation of the Ovum
Oogenesis is the process by which the female gametes, called ova or egg cells, are produced. This complex process involves several distinct stages, including mitotic and meiotic divisions. Understanding the sequence of these stages is crucial for comprehending female reproductive biology.
Arranging the Stages of Oogenesis in Order
The question asks us to arrange the given stages of oogenesis in their correct chronological order. Let's examine each stage provided:
- A. Formation of ovum & second polar body
- B. Meiosis II begins but is held at Metaphase II
- C. Meiosis I started but is held at Prophase I
- D. Formation of primary oocyte from oogonia
- E. Completion of Meiosis-I and formation of secondary oocyte and first polar body
Detailed Explanation of Each Oogenesis Stage
Let's break down the events described in each stage:
- Stage D: This is one of the earliest stages. Oogenesis begins with diploid germ cells called oogonia. These oogonia proliferate by mitosis and then differentiate into primary oocytes. This differentiation happens before birth.
- Stage C: The primary oocyte, formed from oogonia, enters Meiosis I. However, this meiotic division is arrested at the prophase I stage (specifically in a substage called dictyotene) during fetal development. It remains in this arrested state until puberty.
- Stage E: Starting from puberty, under hormonal influence, some primary oocytes resume Meiosis I. This division is unequal, resulting in a large haploid cell called the secondary oocyte and a much smaller haploid cell called the first polar body. Meiosis I is completed at this point.
- Stage B: The secondary oocyte immediately begins Meiosis II. However, this division is arrested again, this time at the metaphase II stage. This arrest occurs before ovulation. The secondary oocyte, still arrested at metaphase II, is ovulated.
- Stage A: Meiosis II is only completed if the secondary oocyte is fertilized by a sperm. Upon fertilization, the secondary oocyte quickly finishes Meiosis II, producing a large haploid ovum (the mature female gamete) and a second polar body. The first polar body may also divide to form two polar bodies.
Chronological Sequence of Oogenesis Stages
Based on the biological process of oogenesis, the stages occur in the following sequence:
- D. Formation of primary oocyte from oogonia: Oogonia differentiate into primary oocytes.
- C. Meiosis I started but is held at Prophase I: Primary oocytes begin Meiosis I but arrest.
- E. Completion of Meiosis-I and formation of secondary oocyte and first polar body: Meiosis I completes, forming secondary oocyte and first polar body.
- B. Meiosis II begins but is held at Metaphase II: Secondary oocyte starts Meiosis II but arrests.
- A. Formation of ovum & second polar body: Meiosis II completes upon fertilization, forming ovum and second polar body.
Therefore, the correct chronological order of the given stages is D, C, E, B, A.
Summary Table: Oogenesis Stages Arrangement
| Order | Stage Label | Description |
|---|---|---|
| 1 | D | Oogonia differentiate into primary oocytes. |
| 2 | C | Primary oocyte begins Meiosis I and arrests at Prophase I. |
| 3 | E | Primary oocyte completes Meiosis I, forming secondary oocyte and first polar body. |
| 4 | B | Secondary oocyte begins Meiosis II and arrests at Metaphase II. |
| 5 | A | Secondary oocyte completes Meiosis II upon fertilization, forming ovum and second polar body. |
Revision Table: Key Oogenesis Events
| Cell Type | Ploidy/State | Key Event/Process | Arrest Point |
|---|---|---|---|
| Oogonia | Diploid (2n) | Mitosis, differentiate into primary oocytes | N/A |
| Primary Oocyte | Diploid (2n) | Starts Meiosis I | Prophase I |
| Secondary Oocyte | Haploid (n) | Starts Meiosis II, receives most cytoplasm | Metaphase II |
| First Polar Body | Haploid (n) | Small, non-functional, may divide | N/A |
| Ovum | Haploid (n) | Mature female gamete | N/A (Fertilized) |
| Second Polar Body | Haploid (n) | Small, non-functional | N/A |
Additional Information on Oogenesis: Polar Bodies and Meiosis
Oogenesis is characterized by unequal cytoplasmic division during meiosis. This ensures that the ovum receives the maximum amount of cytoplasm, which contains essential nutrients and organelles needed for early embryonic development.
Polar Bodies: The polar bodies are essentially sacs of chromosomes with very little cytoplasm. They are non-viable and typically degenerate. The formation of polar bodies is a way to discard excess chromosomes while conserving cytoplasm in the developing ovum.
Meiotic Arrests: The two arrest points in oogenesis (Prophase I and Metaphase II) are critical. The arrest at Prophase I means that primary oocytes are present from birth and mature over many years. The arrest at Metaphase II ensures that Meiosis II is only completed upon fertilization, linking the final maturation of the egg to the presence of sperm.
Understanding these stages and their timing provides insight into the regulation of the female reproductive cycle and potential points where errors could occur.
Important Questions from Human Reproduction
Which of the following hormones is secreted by the ovary in the later phase of pregnancy?
Choose the correct sequence of events for the process of oogenesis:
A. The tertiary follicle changes into Graafian follicle
B. Formation of oogonia in the fetal ovary
C. Completion of first meiotic division by primary oocyte within the tertiary follicle
D. Release of secondary oocyte from the ovary
Choose the correct answer from the options given below:
Spermiogenesis is the process of formation of:
Signals of parturition originate from which of the following?
Human Placental Lactogen
Parturition hormone
Fully developed foetus
Placenta
In plants, continued self-pollination results in
