Free response · Unit 6 Beta

One gene, several proteins by alternative splicing

Conceptual Analysis · 4 points

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The question

Name: ______________________   Date: ____________   One gene, several proteins by alternative splicing (4 points)

Model

Two mRNAs from one gene

A human gene has five exons. Muscle cells and nerve cells both transcribe the gene, but the mature mRNAs differ, as shown. The two mRNAs code for related but different proteins.

A gene with five exons and two ways of splicing itTop: a gene with five exons, numbered 1 to 5, separated by four introns. Middle: the mRNA made in muscle cells contains exons 1, 2, 3 and 5. Bottom: the mRNA made in nerve cells contains exons 1, 2, 4 and 5.Gene (DNA)exon 1exon 2exon 3exon 4exon 5mRNA in muscle cellsexon 1exon 2exon 3exon 5mRNA in nerve cellsexon 1exon 2exon 4exon 5

  1. (a) Describe what happens to the introns of this gene's pre-mRNA, and where in the cell it happens. [1 point]

  2. (b) Explain how the same gene produces different proteins in muscle cells and nerve cells. [1 point]

  3. (c) A change in the DNA destroys the splicing signal at the start of exon 4, so exon 4 can no longer be included in any mRNA. Predict the effect on the protein made in each cell type. [1 point]

  4. (d) Justify the claim that the difference between the muscle and nerve proteins does not require any difference in the cells' DNA. [1 point]