The big idea: Only a small part of the DNA in a eukaryotic cell codes for polypeptides. The rest is non-coding, but much of it still has a job.
The points to remember
- Regulators of gene expression, such as promoters, are binding sites that control whether a gene is transcribed.
- Introns are transcribed but cut out of the mRNA before translation.
- Telomeres are repeated sequences at the ends of chromosomes that protect the genes.
- Genes for rRNA and tRNA are transcribed, but the RNA is used as it is, never translated.
Remember it as: Regulators, introns, telomeres, rRNA and tRNA genes: transcribed or not, never translated.
Real example: the human genome. Less than 2% of human DNA codes for polypeptides. Human telomeres are thousands of repeats of the short sequence TTAGGG.
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Five kinds, five jobs: Each kind of non-coding DNA has a role, even though none codes for a polypeptide.
| Non-coding DNA | Transcribed? | Role |
|---|---|---|
| Regulators (e.g. promoter) | No | Binding sites that switch genes on or off |
| Introns | Yes, then removed | Cut out of mRNA before it leaves the nucleus |
| Telomeres | No | Protect the ends of chromosomes when DNA is copied |
| rRNA genes | Yes | rRNA forms part of ribosomes |
| tRNA genes | Yes | tRNA carries amino acids to ribosomes |
Exons are coding: Exons code for the polypeptide. Do not give 'introns and exons' as an example of non-coding DNA.
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How it is asked: A 1A item picking a pair of non-coding examples, or a short Describe of what non-coding DNA does.
IB-style questionDescribe[3 marks]
Describe three examples of non-coding sequences in eukaryotic DNA.
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