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Wize University Biology Textbook > Transcription and Translation

RNA Maturation in Eukaryotes

Eukaryotic TranscriptionPrevious Section
Translation into ProteinsNext Section
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RNA Maturation in Eukaryotes

After transcription of eukaryotic pre-mRNA, three modifications occur before translation:
  • 5’ capping
  • Polyadenylation
  • Splicing

5’ capping

A modification to the 5’ end of the pre-mRNA, which consists of a methylated guanosine (7- methylguanosine). This cap allows for efficient translation and prolongs the stability of the mRNA.


Polyadenylation

A modification to the 3’ end of the pre-mRNA, which consists of several hundred “A” nucleotides to give a poly-A tail. Like the 5’ cap, the poly-A tail helps prevent RNA degradation and helps export pre-mRNA to cytoplasm.

Splicing

Eukaryotic genes contain introns and exons. The resulting mRNA is spliced to join together exons and remove introns. (Hint: think “exons are expressed; introns are in between.”)



Photo by CNX OpenStax / CC BY


Wize Concept
After these modifications, the pre-mRNA is now fully matured and can be called mRNA.

Splicing occurs in the nucleus and is conducted by proteins called spliceosomes. This has to be a very precise process since leaving nucleotides in can cause frameshift mutations that generate nonfunctional proteins.


Photo by OpenStax / CC BY
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Example: mRNA Transcription

Transcribe the following template strand of DNA. Identify where the cap and poly-A tail appear on the mRNA.

DNA:
3'- ATACCTCGACTAG-5'

The resulting mRNA would be:
5'-UAUGGAGCUGAUC- 3'

The cap would be linked to the 5' end of the sequence and the poly-A tail would added to the 3' end of the sequence.

Practice: Gene Splicing

The diagram represents regions around a gene encoding a mRNA. The translation initiation codon (AUG) is present in exon 1 of the mRNA. The stop codon (UGA) is present in exon 4.

What region of the primary transcript is removed from the mature mRNA?

Practice: Transcription In Eukaryotes


The diagram represents an mRNA with exons (shaded regions) and introns (white regions).

a) In order to create a protein with the regions I, II, IV, what splicing pattern is required?
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mRNA maturation

  • Adding the 5’ cap and poly-A-tail during/after transcription
  • Splicing out introns of pre-mRNA
  • spRNA is bound by proteins to form snRNPS (small nuclear riboproteins)
  • snRNPs recognize intron-exon boundaries, loop the intron into a lariat shape
  • snRNPs cut out the intron and ligate the exons together
  • 5’ splice site (5’-CAG/GUAAGU-3’) base pairs with the U1 snRNA
  • 3’splice site (5’-CAG-3’) is cut after the U2 snRNA base pairs with it’s recognition sequence (5’-UACUAC-3’) in the intron

PAGE BREAK

What is alternative splicing?

  • Alternative splicing = different mature mRNAs produced from a single pre-mRNA transcript/gene
  • Often happens when proteins are produced in different cell types
  • The same sequence but different protein products

Extra Practice
tRNA processing
Explain the steps of pre-mRNA processing to produce mature mRNA.
Where in a gene would a 2-bp deletion mutation have the least effect on gene expression?
Which of the following are understood by the eukaryotic cell as RNA in the nucleus? - Practice Question
Which of the following are understood by the eukaryotic cell as RNA in the nucleus?
Start codon
Intron-exon boundaries
The diagram represents regions around a gene encoding a mRNA. The translation initiation codon (AUG) is present in exon 1 of the mRNA. The stop codon (UGA) is present in exon 4.
Where is the guanylated cap added to the mature mRNA?
The diagram represents regions around a gene encoding a mRNA. The translation initiation codon (AUG) is present in exon 1 of the mRNA. The stop codon (UGA) is present in exon 4.
Where is the poly(A) tail added to the mature mRNA?
Post-transcriptional modification of mRNA include:
i. Removal of exons
ii. Addition of a poly-U tail
Which of the following statements are false?
i. Introns are the coding regions in DNA
ii. Exons are the non-coding regions in DNA
Which of the following are understood by the eukaryotic cell as RNA in the nucleus? - Practice Question
Which of the following are understood by the eukaryotic cell as RNA in the nucleus?
Is it possible for a gene to be coded for within an intron of another gene?
Which of the following are neither transcribed nor translated?
TATA box
Enhancer
If a single base pair was inserted into DNA coding for the second exon of a eukaryotic mRNA transcript with three exons and two introns, which of the following statements is most likely true about the resulting protein?
Which of the following statements are true regarding open reading frames of a gene?
They do not code for proteins
They represent a complete gene
The following diagram represents a primary mRNA transcript where the black regions represent exons and the white regions represent introns. If the transcript was alternatively spliced at splice sites 2 and 3; and at site 6, the resulting mature mRNA transcript would contain which exons?
Which of the following best describes RNA splicing?
Where in a gene would a 2-bp deletion mutation have the least effect on gene expression?
tRNA processing
Explain the steps of pre-mRNA processing to produce mature mRNA.