Question 1

What is the 5' cap and why is it added to pre-mRNA?

Accepted Answer

The 5' cap is a 7-methylguanosine, a modified guanine nucleotide attached to the first nucleotide of pre-mRNA via a triphosphate linkage. It protects the transcript from degradation by exonucleases and helps the ribosome attach during translation. The cap also helps cells distinguish mRNA from other RNA types.

Question 2

How does the poly-A tail protect mRNA and signal its readiness for translation?

Accepted Answer

Polyadenylate polymerase adds approximately 200 adenine nucleotides to the 3' end of pre-mRNA, forming the poly-A tail. This tail protects mRNA from degradation by exonucleases in the cytoplasm and signals to transport molecules that the transcript is ready to exit the nucleus for translation.

Question 3

What is a polyadenylation signal and how does it direct 3' end processing?

Accepted Answer

A polyadenylation signal is a specific sequence of nucleotides, typically A, A, U, A, A, A, located toward the 3' end of pre-mRNA. It recruits the 3' end processing machinery and directs an endoribonuclease to cut the transcript at the 3' cleavage site, allowing polyadenylate polymerase to add the poly-A tail.

Question 4

What is the difference between exons and introns in pre-mRNA?

Accepted Answer

Exons are RNA sequences that remain in the mature transcript and are expressed as proteins. Introns are portions removed during splicing by the spliceosome. Interestingly, a single RNA segment can function as an exon in one cell type and an intron in another, enabling alternative rna splicing to produce multiple proteins from one gene.

Question 5

When does the 5' cap get added during transcription?

Accepted Answer

The 5' cap is added early during transcription, once approximately 20-40 ribonucleotides have been joined together by RNA polymerase. A group of enzymes replaces the 5' phosphate with modified 7-methyl guanosine, which helps the cell recognize mRNA and prepares it for subsequent translation.

Question 6

How do the 5' cap and 3' poly-A tail work together to protect mRNA?

Accepted Answer

Both the 5' cap and 3' poly-A tail protect mRNA ends from degradation by exonucleases. The 5' cap marks the transcript as mRNA and facilitates ribosome binding, while the 3' tail signals nuclear export and protects the transcript in the cytoplasm, ensuring the mRNA remains stable for translation.

Question 7

What happens to pre-mRNA after the 5' cap and poly-A tail are added?

Accepted Answer

After both modifications are complete, the mature mRNA exits the nucleus for translation. The 5' cap and 3' poly-A tail signal to transport molecules that the transcript is ready to leave the nucleus. Outside the nucleus, the 5' cap helps the ribosome attach to initiate protein synthesis.

前体 mRNA 加工：前体 mRNA 末端的修饰

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