Eukaryotic Gene Expression Problem Set

Problem 1: Post transcriptional modification to the 3'-end of eukaryotic mRNAs

Help to answer the question

What is added to the 3'-end of many eukaryotic mRNAs after transcription?


Transcription of eukaryotic protein-coding genes
Transcription of eukaryotic protein-coding genes results in an mRNA precursor molecule that is extensively processed in the cell nucleus.

When eukaryotic nuclear genes for proteins are transcribed by RNA polymerase, the product is a precursor RNA (pre-mRNA) that is further processed in the nucleus before transport to the cytoplasm for translation. The important processing steps are the following:
1. Capping. A modified G-nucleotide, termed a "cap," is added to the 5'-end of most mRNA. The cap is retained in mRNA and functions in ribosome binding and mRNA stability.
2. 3'-Polyadenylation. A tail of A-nucleotides, generally 100-200 long, is added to the 3'-end of most eukaryotic pre-mRNAs. The poly A tail, which is not coded in the DNA, is also retained in the mRNA exported to the cytoplasm.
3. Intron Splicing. The pre-mRNA transcripts often contain introns, which are noncoding sequences that interrupt the coding regions known as exons. Introns are removed and exons are joined together by the spliceosome in the nucleus.

The Biology Project
University of Arizona
Monday, October 27, 1997
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