- In eukaryotes the average length of a new messenger RNA (mRNA) transcript is about 8000 nucleotides. It takes only 1200 nucleotides, however, to code for the average-sized protein of 400 amino acids. The mRNA transcript and the DNA template used to synthesize it have long segments that are not translated. In fact, most of these noncoding sequences occur between coding segments, meaning that the sequence that codes for a polypeptide is not continuous.

- In the first step of posttranscriptional modification, a 5’ cap and a poly (A) tail are added to a newly transcribed mRNA molecule. The cap and tail protect the transcript from hydrolysis, signal ribosome attachment, and help export the molecule from the nucleus.

second step of posttranscriptional modification

- involves RNA splicing. The mRNA transcript contains long sequences that do not code for proteins. The sequences are called intervening sequences, or introns. The sequences that code for proteins are called exons. Before the transcript is translated into a protein, the introns are removed. The illustration shows that short nucleotide sequences at the ends of introns provide a signal to identify the splice sites

small nuclear ribonucleoproteins, or snRNPs (pronounced snurps)

- recognize the splice sites. snRNPs consist of RNA and protein molecules. In mRNA splicing, RNA present in snRNPs functions as a catalyst, a role generally taken on by proteins of the cell. The idea of a catalytic role for snRNP arose from the discovery of ribozymes, RNA molecules that function as enzymes.

- Several snRNPs join with additional proteins to form a large complex called a spliceosome. The illustration shows a spliceosome splicing out the intron and joining the adjacent exons together. When the introns are removed and the exons are joined together, the modified mRNA molecule will be exported from the nucleus into the cytosol

- removes introns from the transcriptional unit

The following statements concerning mRNA processing are true except:

- Protein catalysts splice out introns and join exons together.

6.5.4 Posttranscriptional Modification/RNA Splicing Flashcards by Irina Soloshenko

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6.5.4 Posttranscriptional Modification/RNA Splicing Flashcards

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