To solve the question regarding the events that take place during post-transcriptional modification in eukaryotes, we will analyze the three main processes involved: splicing, capping, and tailing.
### Step-by-Step Solution:
1. **Understanding Post-Transcriptional Modification**:
- Post-transcriptional modification refers to the changes that occur to the primary RNA transcript (hnRNA) after transcription and before it is translated into protein. This process is crucial for the maturation of mRNA in eukaryotic cells.
2. **Event 1: Splicing**:
- Splicing is the first event that occurs during post-transcriptional modification. It involves the removal of non-coding sequences called introns from the primary transcript.
- The remaining coding sequences, known as exons, are then ligated together to form a continuous sequence that can be translated into a functional polypeptide.
3. **Event 2: Capping**:
- The second event is capping, which involves the addition of a modified guanine nucleotide, specifically 7-methylguanosine, to the 5' end of the mRNA.
- This capping is important for mRNA stability, export from the nucleus, and initiation of translation. The linkage formed is an unusual 5'-5' triphosphate linkage.
4. **Event 3: Tailing**:
- The third event is tailing, which refers to the addition of a poly(A) tail to the 3' end of the mRNA.
- This process involves the cleavage of the 3' end of the transcript followed by the addition of 80 to 250 adenine residues. This addition is done in a template-independent manner by the enzyme poly(A) polymerase, meaning it does not require a template strand for synthesis.
5. **Evaluating the Options**:
- Now, we will evaluate the provided options based on the events discussed:
- **Option 1**: "Exons are removed from primary transcript" - Incorrect, as exons are retained while introns are removed.
- **Option 2**: "7 methyl guanosine cap is added at 3' end of the RNA transcript" - Incorrect, as the cap is added to the 5' end.
- **Option 3**: "Addition of poly A segment at the 5' end of transcript" - Incorrect, as the poly(A) tail is added to the 3' end.
- **Option 4**: "In tailing, adenylated residues are added in a template-independent manner" - Correct, as this accurately describes the tailing process.
6. **Final Answer**:
- The correct answer is **Option 4**: "In tailing, adenylated residues are added in a template-independent manner."
