Which one of the following precedes re-formation of the nuclear envelope during M phase of the cell cycle.

Options

A. (a) Transcription from chromosomes and re-assembly of the nuclear lamina.
B. (b) Decondensation from chromosomes, and re-assembly of the nuclear lamina.
C. (c) Formation of the contractile ring, and transcription from chromosomes
D. (d) Formation of the contractile ring, and formation of the phragmoplast

Answer

To answer the question "Which one of the following precedes re-formation of the nuclear envelope during M phase of the cell cycle?", we need to analyze the events that occur during the M phase, particularly focusing on the telophase stage when the nuclear envelope reforms. Here’s a step-by-step breakdown of the solution:

### Step 1: Understand the Cell Cycle Phases
The cell cycle consists of several phases: G1, S, G2, M (which includes mitosis and cytokinesis). Each phase has specific events:
- **G1 Phase**: Cell growth and preparation for DNA synthesis.
- **S Phase**: DNA replication occurs.
- **G2 Phase**: Further growth and preparation for mitosis.
- **M Phase**: Includes mitosis (prophase, metaphase, anaphase, telophase) and cytokinesis.

### Step 2: Identify the M Phase Events
During the M phase:
- **Prophase**: Chromosomes condense and become visible.
- **Metaphase**: Chromosomes align at the cell's equatorial plane.
- **Anaphase**: Sister chromatids are pulled apart to opposite poles.
- **Telophase**: Chromatids reach the poles, and the nuclear envelope begins to reform around each set of chromosomes.

### Step 3: Focus on Telophase
The question specifically asks about the event that occurs just before the reformation of the nuclear envelope. This occurs during telophase when:
- The chromosomes begin to decondense back into chromatin.
- The nuclear lamina reassembles to form the nuclear envelope.

### Step 4: Evaluate the Options
Now, let’s evaluate the options provided:
- **A**: Transcription from chromosomes and reassembly of the nuclear lamina. (Incorrect, as transcription occurs in G1 phase.)
- **B**: Decondensation from chromosomes and reassembly of the nuclear lamina. (Correct, as this occurs just before the nuclear envelope reforms.)
- **C**: Formation of contractile ring and transcription from chromosomes. (Incorrect, as these occur during cytokinesis and G1 phase.)
- **D**: Formation of contractile ring and formation of phragmoplast. (Incorrect, as these also occur during cytokinesis.)

### Conclusion
The correct answer is **B: Decondensation from chromosomes and reassembly of the nuclear lamina**, as this event occurs just before the reformation of the nuclear envelope during the M phase of the cell cycle.

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