The big idea: When exons are spliced, they can be joined in different combinations. So one gene can code for several different polypeptides.
The points to remember
- A gene's pre-mRNA contains several exons.
- During splicing, some exons can be left out along with the introns.
- Different combinations of exons give different mature mRNAs.
- Each mRNA is translated into a different polypeptide: variants of a protein from one gene.
Remember it as: Same exons, different mixes, different proteins.
Real example: calcitonin. In the thyroid gland one human gene is spliced to make calcitonin, a hormone that helps control blood calcium. In some nerve cells the same gene is spliced differently to make a different peptide, CGRP, which widens blood vessels.
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Why it matters: Alternative splicing lets a cell make more kinds of protein than it has genes.
Stays the same
- The gene (DNA) itself
- The pre-mRNA made from it
Can differ
- Which exons are kept
- The mature mRNA
- The polypeptide made
Not a mutation: Alternative splicing does not change the DNA. The gene is the same; only the exons joined together differ.
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How it is asked: A short Explain of how one gene gives several polypeptides, or a 1A item on which exon combinations are possible.
IB-style questionExplain[2 marks]
Explain how one gene can code for more than one polypeptide.
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See the answer broken into clear points, with guidance on how each point earns marks.