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8.4.10The Control of Gene Expression (A Level only)

Variable Number Tandem Repeats — AQA A Level Biology 7402 Revision Notes & Topical Questions

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📚 AQA A Level Biology 7402 Hub / Topical Revision
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8.4.10 Variable Number Tandem Repeats — AQA A Level Biology 7402 notes: Describe what VNTRs are and explain why the number of repeats differs between individuals.Only a small fraction of the human genome, about 1.5 per cent, codes for proteins.Much of the rest is non-coding DNA, …

  • number of times
  • variable number tandem repeat (VNTR)
  • minisatellite
  • microsatellites
  • short tandem repeats (STRs)
  • either side

Section 1 of 9Non-coding DNA

Only a small fraction of the human genome, about 1.5 per cent, codes for proteins.

Much of the rest is non-coding DNA, including introns, regulatory regions and repetitive sequences.
Number of times
Some of the repetitive sequences are found in the same place (locus) on a chromosome in everyone,
  • but the number of times they repeat varies between people.
This variation is the basis for identifying individuals from their DNA.

Section 2 of 9What is a VNTR?

A variable number tandem repeat (VNTR) is a short sequence of bases, typically 10 to 100 bases long, repeated one after another (in tandem) several times at a particular locus.

Minisatellite
A minisatellite has a repeated unit of around 10 to 60 bases; microsatellites, also called short tandem repeats (STRs), have units of only 2 to 6 bases and are the type used in modern forensic work.
Number of repeats
The repeated unit is the same in all people, but at each locus the number of repeats differs.
Each version of the locus with a different number of repeats is an allele, even though no protein is coded.

Section 3 of 9Why do they differ?

Each person inherits one chromosome of each pair from each parent, so each has two alleles per VNTR locus, which may have different repeat numbers.
Mutation
Repeats arise by mutation: during DNA replication the polymerase can slip, or during meiosis unequal crossing over can add or delete repeat units.
Because VNTRs are in non-coding DNA, the changes do not affect protein structure and are usually not removed by natural selection,
  • so many different alleles build up in a population.

The number of repeats is therefore inherited: a child has one allele at each locus from each parent, so the child's bands match bands from the mother and father.

Section 4 of 9Measuring VNTRs

VNTR lengths are found by cutting the DNA with restriction enzymes that cut either side of the repeat, or by using PCR with primers that bind either side of the repeat.

1
The fragments are separated by gel electrophoresis.
2
A person with more repeats has a longer fragment that moves a shorter distance.
3
Different individuals show different patterns of bands.
4
Because several loci are examined, the chance of two unrelated people having the same pattern becomes extremely small.
5
This is the basis of genetic fingerprinting.

Section 5 of 9Worked context

If a locus has a repeat of 20 bases, an allele with 8 repeats contains 160 bases of repeat plus flanking DNA, and an allele with 12 repeats contains 240 bases.

Worked context
The longer fragment travels the shorter distance through the gel.
A heterozygous person shows two bands at that locus, a homozygous person shows one.
If the flanking DNA amplified by the primers is 50 bases in total, the product sizes for the two alleles are 210 and 290 bases,
  • and the difference between them, 80 bases, equals four repeat units.

Section 6 of 9Why VNTRs and not genes? and distinguishing related terms

Coding genes are very similar between individuals, as changes would alter proteins and many would be harmful. Non-coding repeats tolerate change, and so they are far more variable and therefore more useful for telling people apart. Their variation also provides a way of studying how closely related populations are, and of tracing inheritance in families.

Take care with the vocabulary.

Locus
A locus is a position; an allele is one version at that position; a repeat unit is the short sequence that is copied;
  • and the repeat number is how many copies lie in a row.
Genetic fingerprinting compares the repeat numbers at several loci, and the pattern across loci is much more discriminating than any one locus.
If the frequency of one allele at locus A is 0.1 and of one at locus B is 0.05,
  • and the loci are on different chromosomes and so inherited independently, the chance of a person having both is the product, 0.1 × 0.05 = 0.005,
  • which is 1 in 200.
  • Adding more loci multiplies the probabilities, so that a profile of ten loci can give odds of one in many millions.
This multiplication rule is valid only for independent loci,
  • and relatives share alleles so the rule overestimates how rare a profile is if a close relative is a suspect.

Section 7 of 9Exam-style questions

  1. Explain why VNTRs show variation between individuals but are not subject to natural selection. [3]3 marks

  2. A child has 7 and 11 repeats at a VNTR locus. The mother has 7 and 9. Which repeat number must the father have contributed? Explain. [2]2 marks

Section 8 of 9Key terms

TermMeaning
VNTRVariable number tandem repeat: repeated base sequence whose number varies between people
MinisatelliteTandem repeat of about 10 to 60 bases
Microsatellite (STR)Tandem repeat of 2 to 6 bases
Non-coding DNADNA that does not code for a polypeptide
LocusPosition of a gene or sequence on a chromosome
Unequal crossing overMisaligned crossing over that changes repeat numbers

Section 9 of 9Exam technique

1
Saying VNTRs code for proteinsthey are in non-coding DNA.
2
Writing that the repeat sequence differs between people: it is the number of repeats that varies.
3
Forgetting that shorter fragments travel further in the gel.
4
Thinking each person has only one band per locus: heterozygotes have two.

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