AQA · A-Level · Biology
Course version: 7402 · Version from Sept 2016
Within the gene, these exons are separated by one or more non-coding…
Full specification wording
DNA, genes and chromosomes: Within the gene, these exons are separated by one or more non-coding sequences, called introns.
Introns are non-coding base sequences lying between the exons of a eukaryotic gene.
Checked against the AQA 7402 specification (opens in a new tab) on .
- Point 11 of 11
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Introns are non-coding base sequences lying between the exons of a eukaryotic gene. They are transcribed into pre-mRNA along with exons, then removed by splicing in the nucleus. Because they never reach a ribosome, their bases are not translated into amino acids. A mutation within an intron usually has no effect on the polypeptide, unless it alters a splice site. For genes containing introns, the gene is longer than the mature mRNA transcribed from it. This length difference is useful: complementary DNA (cDNA), made by reverse transcriptase from mature mRNA, contains only exons. Comparing cDNA to the original gene shows where the introns lay.
What earns the marks
- Introns are non-coding sequences within a gene, lying between the exons.
- Introns are transcribed into pre-messenger RNA and then removed by splicing.
- Splicing occurs in the nucleus, before the messenger RNA reaches a ribosome.
- For genes with introns, the DNA sequence is longer than the mature messenger RNA transcribed from it.
- Complementary DNA made from mature messenger RNA contains no introns.
Common mistakes and how to put them right
Saying introns are cut out of the DNA itself.
Put it right: They are spliced out of the pre-messenger RNA.
Describing introns as having no function.
Put it right: Mutations at splice sites can alter which sections are removed.
Assuming all eukaryotic genes have introns.
Put it right: Some, like histone genes, are intronless.
Calculating the number of amino acids from the whole length of a eukaryotic gene.
Put it right: Use only the exon bases.
Exam tips
- When calculating amino acid number, use only the exon bases and divide by three.
- State explicitly that splicing happens in the nucleus.
Try the lesson
The start of the DNA, genes and chromosomes: Within the gene, these exons are separated by one or more non-coding sequences, called introns. lesson, made from checked teaching notes for this specification point.
Have a go first
What name is given to the non-coding sequences that separate exons within a gene?
Listen
Introns are non-coding base sequences lying between the exons of a eukaryotic gene. They are transcribed into pre-mRNA along with exons, then removed by splicing in the nucleus. Because they never reach a ribosome, their bases are not translated into amino acids. A mutation within an intron usually has no effect on the polypeptide, unless it alters a splice site. For genes containing introns, the gene is longer than the mature mRNA transcribed from it. This length difference is useful: complementary DNA (cDNA), made by reverse transcriptase from mature mRNA, contains only exons. Comparing cDNA to the original gene shows where the introns lay. A strong answer earns marks for these points. Introns are non-coding sequences within a gene, lying between the exons. Introns are transcribed into pre-messenger RNA and then removed by splicing. Splicing occurs in the nucleus, before the messenger RNA reaches a ribosome. For genes with introns, the DNA sequence is longer than the mature messenger RNA transcribed from it. A common slip: Saying introns are cut out of the DNA itself. Instead, they are spliced out of the pre-messenger RNA. Exam tip: When calculating amino acid number, use only the exon bases and divide by three.
See it
Key words
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introns
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pre-messenger RNA
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splicing
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See every definition
- introns
- Non-coding sequences within a gene, lying between the exons.
- pre-messenger RNA
- The molecule into which introns are transcribed along with exons, before they are removed by splicing.
- splicing
- The removal of introns, occurring in the nucleus before the messenger RNA reaches a ribosome.
Label the diagram
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1Box 1
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3Box 3
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