Unit 3: mRNA Processing in Eukaryotes Flashcards
MCB 181R University of Arizona
In eukaryotes, where and how is the primary transcript (pre-mRNA)processed into mature mRNA?
In the nucleus, before being exported to the cytoplasm.
What are the three main processing steps that occur to the primary transcript in eukaryotes?
- 5’ methyl cap
- mRNA splicing (introns & exons)
- 3’ Poly - A tail
When is the 5’ methyl cap added to the mRNA molecule?
As soon as the 5’ end of the mRNA emerges from the RNA polymerase.
How is the 5’ methyl cap attached to the mRNA?
An enzyme adds it via a covalent bond.
What are the two primary purpose of the 5’ methyl cap?
- Stabilizes the mRNA against degradation.
- Allows the ribosomes to “recongnize, bind to and translate the mRNA.”
In eukaryotes, where on the mRNA does a ribosome bind?
The 5’ cap.
After transcription is complete in eukaryotes, what is added to the 3’ end of the mRNA? what is this called?
A “tail” of about 250 adenine (A) base ribonucleotides. This is called the poly (A) tail.
What are the two primary purpose of the poly (A) tail?
1.Stabilizes the mRNA against degradation.
2. Allows the mRNA to be exported from the nucleus to the cytoplasm through a nuclear pore complex.
What are introns and exons un eukaryotic genes?
- Introns (intervening sequences): Long stretches that are transcribed from the gene but then spliced out of the RNA during processing in the nucleus.
- Exons: Sequences left in the mRNA after splicing. They are expressed.
What carries out RNA splicing? What is it composed of?
A complex protein and RNA called the spliceosome.
What is the main function of the spliceosome?
It catalyzes the cutting and joining of the RNA at sequence- specific sites, removing introns and joining exons.
What is the alternative splicing?
When primary transcripts from the same gene can be spliced in different ways, removing one or more exons along with the introns.
What is the result of alternative splicing?
A single gene can produce two or more different mRNa molecules, leading to the production of different proteins.
In the illustration of eukaryotic gene how are exons typically of different proteins.
In bold.
What are the key components of a mature eukaryotic mRNA molecule shown in the diagram (from 5’ to 3’)?
- 5’ cap
- 5’ untranslated region (UTR)
-Coding region - 3’ untranslated region
- Poly (A) tail
When is the 5’ cap added to the mRNA molecule relative to transcription?
It is added as soon as the 5’ end of the RNA emerges from the RNA polymerase, while transcription is still in progress.
When is the poly (A) tail added to the mRNA molecule relative to transcription and nuclear export?
It is after transcription is compete but before the mRNA leaves the nucleus.
According to the diagram, when are introns spliced out of the RNA molecule?
Introns are spliced out as transcription is taking place.
What is the region of the mature mRNA that contains the codons that will be translated into a protein?
The coding region.
What are the 5’ and 3’ untranslated regions (UTRs)?
Regions at the ends of the mRNA that are transcribed but not translated into protein. They can have regulatory roles.
