Question

Arrange the following events that occur during prophase-I of Meiosis - I
A) Formation of synaptonemal complex
B) Repulsion between Homologous chromosomes
C) Formation of Bivalents
D) Exchange of Genetic material between non-sister chromatids of Homologous chromosomes
E) Terminalization
The correct sequence is
Options :

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

Hint:

To arrange events in biological processes, especially complex ones like meiosis, recall the distinct phases and the key events occurring in each. For Prophase I, remember the acronyms (Leptotene, Zygotene, Pachytene, Diplotene, Diakinesis) and the characteristic event for each.

Answer 1

The Correct Option is A

Prophase I of Meiosis I is a complex and extended stage, further subdivided into several substages, each characterized by specific events. Let's arrange the given events in the correct chronological order based on these substages: Step 1: Understand the initial events (Zygotene)
During the Zygotene stage of Prophase I, homologous chromosomes begin to pair up in a process called synapsis. This pairing leads to the formation of structures known as bivalents (or tetrads, as each homologous chromosome consists of two sister chromatids). Concurrently, a protein structure called the synaptonemal complex forms between the paired homologous chromosomes, stabilizing their association. \begin{itemize} \item C) Formation of Bivalents: This is the visible outcome of homologous pairing in Zygotene. \item A) Formation of synaptonemal complex: This structure mediates the pairing and is formed during Zygotene. In the sequence, the formation of bivalents (the paired state) is often considered the overarching event, with the synaptonemal complex facilitating it. So, C typically precedes or occurs very early with A. \end{itemize} Thus, the sequence starts with C, A. Step 2: Subsequent event (Pachytene)
Following Zygotene is the Pachytene stage. At this point, the bivalents are fully formed, and the most significant event of meiosis, crossing over, occurs. Crossing over involves the exchange of genetic material between non-sister chromatids of homologous chromosomes, leading to genetic recombination. \begin{itemize} \item D) Exchange of Genetic material between non-sister chromatids of Homologous chromosomes: This refers to crossing over, which takes place in Pachytene. \end{itemize} So, D comes after C and A. Step 3: Events leading to separation (Diplotene)
After Pachytene, the cell enters the Diplotene stage. The synaptonemal complex begins to dissolve, and the homologous chromosomes start to repel each other. However, they remain attached at specific points where crossing over occurred, forming X-shaped structures called chiasmata. The initial separation is the repulsion. \begin{itemize} \item B) Repulsion between Homologous chromosomes: This occurs as the synaptonemal complex dissembles in Diplotene. \end{itemize} So, B comes after D. Step 4: Final event of Prophase I (Diakinesis)
The final substage of Prophase I is Diakinesis. During this stage, the chiasmata move towards the ends of the homologous chromosomes, a process known as terminalization. The chromosomes become fully condensed, and the nuclear envelope and nucleolus disappear. \begin{itemize} \item E) Terminalization: This is the final event marking the end of Prophase I, occurring in Diakinesis. \end{itemize} So, E comes last. Step 5: Final sequence formulation
Combining the events in chronological order of the Prophase I substages (Zygotene $\rightarrow$ Pachytene $\rightarrow$ Diplotene $\rightarrow$ Diakinesis): % Option (A) Formation of Bivalents (C) % Option (B) Formation of synaptonemal complex (A) % Option (C) Exchange of Genetic material (Crossing Over) (D) % Option (D) Repulsion between Homologous chromosomes (B) % Option (E) Terminalization (E) The correct sequence is C, A, D, B, E. Comparing this sequence with the given options:
% Option (1) C, A, D, B, E (Matches our derived sequence)
% Option (2) D, A, C, B, E (Incorrect order)
% Option (3) A, D, B, C, E (Incorrect order)
% Option (4) D, A, B, C, E (Incorrect order)
The final answer is $\boxed{\text{(1)}}$