Lesson 5 of 6 / DNA and gene expression
Prepare a mature mRNA
What changes between transcription and translation?
In this lesson: Describe formation of mature mRNA from eukaryotic pre-mRNA.
About 5 min
The key ideaPre-mRNA is processed by intron removal, exon joining, addition of a 5' cap and addition of a 3' poly-A tail before mature mRNA is used in translation.
Explore the idea
Process RNA without deleting DNA
The transcript includes retained and removed regions. Predict which labels should remain after splicing.
Explanation
A eukaryotic pre-mRNA includes both exon and intron sequences transcribed from the gene. During splicing, introns are removed and exons are joined in order. Exon describes a region retained in the mature RNA; not every retained nucleotide necessarily codes for an amino acid.
A modified nucleotide cap is added at the 5' end, and a poly-A tail is added at the 3' end after processing of that end. These features contribute to RNA stability, export and effective use by the translation machinery. The tail is a run of adenine-containing RNA nucleotides, not a polypeptide.
The mature mRNA can leave the nucleus through nuclear pores and associate with ribosomes. Processing changes the RNA product without deleting introns from the DNA chromosome. The gene remains available for further transcription.
When comparing transcript lengths, identify which sequence is being measured. Removal of introns can shorten the RNA, while end modifications add material. A shorter mature transcript is therefore not by itself evidence of a mutation.
Step by step
- 1
Remove introns
Splicing removes selected internal RNA sequences.
- 2
Join and protect
Exons join and the 5' cap and 3' poly-A tail contribute to a usable message.
- 3
Export the message
The mature RNA reaches ribosomes; the DNA stays in the nucleus.
Worked example
Track an intron
A pre-mRNA contains exon 1, intron A and exon 2. After splicing, what has happened to the DNA intron?
One way to explain it
The RNA intron has been removed from that transcript and the RNA exons joined. The corresponding intron sequence remains in the DNA; splicing is RNA processing, not chromosomal deletion.
Why this answer works
- Identify the molecule being edited.
- Keep transcript processing separate from DNA mutation.
Is this true? "The poly-A tail is a chain of amino acids."
It is a sequence of adenine-containing RNA nucleotides at the 3' end of the RNA.