14.6
pre-mRNA Processing
In eukaryotic cells, transcripts made by RNA polymerase are modified and processed before exiting the nucleus. Unprocessed RNA is called precursor mRN…
In eukaryotic cells, newly-transcribed mRNA is called precursor mRNA, or pre-mRNA. Each pre-mRNA receives two important modifications. One at the five-prime end of the molecule, called the cap, and another at the three-prime end of the molecule, called a tail.
The five-prime cap is composed of a single 7-methylguanosine, a modified guanine nucleotide, that is attached to the first nucleotide of the pre-mRNA by a triphosphate linkage. A specific sequence of nucleotides toward the end of the pre-mRNA transcript, usually A, A, U, A, A, A, called a polyadenylation signal, recruits an RNA binding protein, and directs an enzyme, an endonuclease, to cut the transcript at the three-prime end of the signal sequence. A different enzyme, polyadenylate polymerase, then adds a long string of adenine nucleotides, as many as 200, to the three-prime end of the transcript.
The five-prime cap and the three-prime poly-A tail protect the ends of the transcript from degradation. The three-prime tail also signals to transport molecules that the mRNA transcript is ready to leave the nucleus. Outside of the nucleus, the five-prime cap helps the ribosome attach to the transcript so it can begin translation.
The last major change to an RNA transcript is the removal of non-coding sequences called introns. A complex of proteins and RNA, called the spliceosome, searches for markers at the ends of introns, cuts the introns out of the transcript, and attaches together the remaining exons, which are sequences that code for proteins.
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Q1: What is the 5' cap added to pre-mRNA and why is it important?
The 5' cap is a 7-methylguanosine, a modified guanine nucleotide attached to the first nucleotide of pre-mRNA by a triphosphate linkage. It protects the transcript end from degradation and helps the ribosome attach during translation. The cap also helps cells distinguish mRNA from other RNA types.
Q2: How does the 3' poly-A tail form and what does it do?
After a polyadenylation signal recruits an endonuclease to cut the pre-mRNA, polyadenylate polymerase adds approximately 30-200 adenine nucleotides to the 3' end. This poly-A tail protects mRNA from degradation in the cytoplasm and signals transport molecules that the transcript is ready to leave the nucleus.
Q3: What role does the spliceosome play in pre-mRNA processing?
The spliceosome is a complex of proteins and RNA that searches for markers at intron ends, cuts introns out of the transcript, and joins the remaining exons together. This process removes non-coding sequences while preserving coding sequences, producing mature mRNA ready for translation.
Q4: What is the difference between exons and introns in pre-mRNA?
Exons are expressed sequences that remain in the mature mRNA and code for proteins. Introns are non-coding sequences removed during splicing. Interestingly, a single RNA segment can function as an exon in one cell type and an intron in another, depending on alternative splicing patterns.
Q5: When does the 5' cap get added during transcription?
The 5' cap is added early during transcription, once approximately 20-40 ribonucleotides have been joined together by RNA polymerase. This early modification occurs while the transcript is still being synthesized, distinguishing the mRNA from other cellular RNA types and protecting it from degradation.
Q6: How does alternative splicing allow one gene to produce multiple proteins?
A single cell can contain multiple variants of a gene transcript through alternative splicing, where different combinations of exons are joined together. This enables the production of multiple distinct proteins from a single gene, increasing protein diversity without requiring additional genes.
Q7: Why are pre-mRNA modifications necessary before translation?
Pre-mRNA modifications prepare the transcript for translation and protect it from degradation. The 5' cap and 3' poly-A tail shield the ends from enzymatic breakdown, while splicing removes non-coding sequences. These modifications ensure only mature, functional mRNA exits the nucleus for translation and expression.