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4.1.7Genetics, Variation and Interdependence

Non-Coding DNA — AQA A Level Biology 7402 Revision Notes & Topical Questions

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4.1.7 Non-Coding DNA — AQA A Level Biology 7402 notes: Describe the types of non-coding DNA in eukaryotes and explain why they matter.In eukaryotes only a small proportion of the DNA in the nucleus codes for polypeptides.

  • Multiple repeats between genes
  • Regulatory sequences
  • Genes for functional RNA
  • Telomeres and centromeres
  • alternative splicing
  • genetic fingerprinting

Section 1 of 9Much of eukaryotic DNA does not code for polypeptides

In eukaryotes only a small proportion of the DNA in the nucleus codes for polypeptides. In humans, about 1.5 per cent of the genome codes for proteins. The remainder is non-coding DNA. Prokaryotes by comparison have little non-coding DNA, with most of their genome made up of coding sequences.

Section 2 of 9Types of non-coding DNA

1Introns
Introns sequences within genes that are transcribed but are removed from the pre-mRNA by splicing before translation.
  • Exons are joined together to form the mature mRNA.
2Multiple repeats between genes
Multiple repeats between genes short sequences of bases that are repeated many times, one after another, in the regions between genes.
  • These are not transcribed into proteins.
3Regulatory sequences
Regulatory sequences including promoters and enhancers that control when genes are expressed; these are not translated, but have a function.
4Genes for functional RNA
Genes for functional RNA such as rRNA and tRNA, which are transcribed but not translated.
5Telomeres and centromeres
Telomeres and centromeres repetitive sequences with structural roles.

Section 3 of 9Introns and splicing

When a eukaryotic gene is transcribed, the primary transcript includes both exons and introns.

Enzymes (assisted by small nuclear ribonucleoproteins) cut out the introns and join the exons,
  • producing the mature mRNA which leaves the nucleus through a nuclear pore.
Because a gene can be spliced in different ways, using different combinations of exons, one gene may give rise to several different mRNAs and so to several different polypeptides.
Alternative splicing
This alternative splicing increases the number of proteins that the genome can code for,
  • and helps to explain why humans, with only around 20,000 genes, make over 100,000 proteins.

Section 4 of 9What is the significance of non-coding DNA?

Non-coding DNA was once called 'junk', but we now know that some of it has functions, and some may not.

Possible roles include regulating gene expression, protecting chromosome ends (telomeres), providing the attachment site for spindle fibres (centromeres),
  • and allowing the genome to evolve by providing raw material for new genes.
Genetic fingerprinting
The repeating sequences are the basis of genetic fingerprinting: the number of repeats differs between individuals,
  • so a comparison of the lengths of the repeats in a sample can identify individuals or relatives.
    • (using PCR and electrophoresis)

Section 5 of 9Links and comparison

FeatureProkaryotesEukaryotes
IntronsAbsent (generally)Present
Repeats between genesVery fewMany
Proportion of coding DNAHighLow

This topic links to transcription and mRNA processing, to the idea of the genome, to DNA profiling and to mutations: a mutation in a non-coding region may have no effect on the phenotype, but it can matter if it is in a regulatory region or at a splice site.

Section 6 of 9Non-coding DNA, evolution and disease

Non-coding DNA is important in the study of evolution and in medicine.

Non-coding DNA, evolution and disease
Because mutations in non-coding sections often have no effect on fitness, they accumulate at a steady rate,
  • and the number of differences between the non-coding sequences of two species can be used as a molecular clock to estimate when they shared a common ancestor.
Mutations in regulatory regions may alter when and where a gene is switched on,
  • and are thought to explain many of the differences in form between closely related species.

Some diseases are caused by expansion of repeated sequences.

Non-coding DNA, evolution and disease
In Huntington disease, a three-base sequence (CAG) is repeated many more times than normal in the gene,
  • producing a protein with a long stretch of glutamine that is toxic to neurones.
The more repeats, the earlier the onset.
Mutations at the boundaries between introns and exons can also stop correct splicing,
  • giving a faulty mRNA and a non-functional protein, even though the coding exons are intact.
Non-coding DNA, evolution and disease
When you are asked to explain why a mutation in non-coding DNA might still have an effect, think of promoters, enhancers, splice sites and telomere shortening.
Make a table listing each kind of non-coding DNA, where it is found, whether it is transcribed, and one possible function.
  • Then decide whether the word 'junk' is a fair description of non-coding DNA and give two pieces of evidence to support your view.
Remember also that the proportion of coding DNA differs greatly between species, so genome size is a poor guide to the number of genes.

Section 7 of 9Exam-style questions

  1. Explain the difference between an intron and an exon. [2]2 marks

  2. Explain how a single gene can code for more than one polypeptide in a eukaryotic cell. [3]3 marks

  3. Describe how non-coding repeats are used in genetic fingerprinting. [3]3 marks

Section 8 of 9Key terms

TermMeaning
Non-coding DNADNA that does not code for amino acids of a polypeptide.
IntronA non-coding section of a gene removed from the pre-mRNA.
ExonA coding section of a gene that is expressed.
SplicingRemoval of introns and joining of exons in pre-mRNA.
Alternative splicingJoining exons in different combinations to make different mRNAs.
TelomereRepeated sequence at the end of a chromosome.
Genetic fingerprintA pattern of bands showing the number of repeats in an individual's DNA.

Section 9 of 9Exam technique

1
Saying that non-coding DNA is useless. Some has regulatory or structural functions.
2
Saying that introns are not transcribed. They are transcribed and removed afterwards.
3
Claiming that prokaryotes have many introns.
4
Stating that all repeats are genes.
5
Thinking that splicing happens in the cytoplasm. It happens in the nucleus.

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