8 Questions
What molecule is added to the 5' end of mRNA during processing?
7-methylguanosine cap
What is the function of the 5' cap on mRNA?
To protect mRNA from 5' exonuclease degradation
Which enzyme synthesizes the poly(A) tail?
Polyadenylate polymerase
What sequence does RNA polymerase II read to know where to cleave for the poly(A) tail?
AAUAAA
What happens to introns during RNA processing?
They are excised from the RNA transcripts
When does the capping of mRNA occur?
Early in transcription
What are exons?
Coding and non-coding sequences that remain in mature mRNA
Which of the following statements is true about eukaryotic gene transcripts?
They undergo three major modifications before transport to the cytoplasm.
Study Notes
RNA Processing in Eukaryotes
- Almost all primary transcripts undergo processing in eukaryotic cells.
5'-End Capping
- 5'-end capping protects mRNA from 5' exonuclease degradation.
- The cap is 7-methylguanosine linked to the 5' end of mRNA.
- Capping occurs early in transcription and is formed by condensation of GTP with the 5' end of mRNA.
- Guanine is then methylated.
Interrupted Genes: Exons and Introns
- Most eukaryotic genes contain non-coding introns that interrupt coding exons.
- Introns are excised from RNA transcripts prior to transport to the cytoplasm.
Poly(A) Tail Formation
- Pol II synthesizes RNA up to and beyond the highly conserved sequence AAUAAA.
- An endonuclease cleavage signal sequence is bound by an enzyme complex.
- The RNA is cleaved 10-30 nucleotides downstream of the sequence AAUAAA.
- Polyadenylate polymerase synthesizes a poly(A) tail.
Introns
- Introns are non-coding sequences located between coding sequences.
- Introns are removed from pre-mRNA and are not present in mature mRNA.
- Exons are composed of sequences that remain in mature mRNA after splicing.
- Introns are variable in size and may be very large.
Key Facts
- Three different RNA polymerases are present in eukaryotes, each transcribing a distinct set of genes.
- Eukaryotic gene transcripts undergo three major modifications: 5'-end capping, intron excision, and poly(A) tail formation.
- All modifications occur in the nucleus prior to transport to the cytoplasm.
RNA processing in eukaryotic cells involves modifying the 5' and 3' ends of mRNA. Learn about capping with methylguanosine, splicing out introns, and adding a poly-A tail.
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