8876 / 2027

Lesson 5 of 6 / DNA and gene expression

Prepare a mature mRNA

What changes between transcription and translation?

I can describe formation of mature mRNA from eukaryotic pre-mRNA.

  • I can describe transcription, pre-mRNA processing and translation.Syllabus

    Syllabus 8876, Core 2(c). Describe transcription, pre-mRNA processing and translation.

About 5 min

Make a guess. You are not marked.

A newly made eukaryotic RNA contains introns. Are they translated into protein?

  1. They make a separate small protein.
  2. They add extra amino acids.
  3. They are spliced out first.
Show the answer

They are spliced out first.

Splicing removes introns and joins exons. A cap and poly-A tail are also added before translation.

The key idea

Pre-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

Pre-mRNAE1IE2IE3E = exon; I = intron

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. 1

    Remove introns

    Splicing removes selected internal RNA sequences.

  2. 2

    Join and protect

    Exons join and the 5' cap and 3' poly-A tail contribute to a usable message.

  3. 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.

Watch out for this

A student says: "The poly-A tail is a chain of amino acids." What is wrong with this?

Show the answer

It is a sequence of adenine-containing RNA nucleotides at the 3' end of the RNA.

Check your understanding

Which sequence represents the spliced product of exon 1 - intron - exon 2?

  1. Only the intron remains.
  2. Exon 1, intron and exon 2 remain joined in the mature mRNA.
  3. Exon 1 joined to exon 2.
Show the answer

Exon 1 joined to exon 2.

Retained exon sequences are joined in order.

Quick recall

Card 1 of 3. Answer in your head, then check.

What changes between transcription and translation?

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