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Q28 - How Long Until a Random Sequence Repeats in the Genome?

Theoretical A Real exam question - full text reproduced under IBO's CC BY-NC-SA 4.0 license

The human genome is composed of about 3×10⁹ base pairs. Assume that the distribution of base pairs is random. How long should a random sequence be to appear once in the human genome, on average?

Q28.1. The human genome is about 3×10⁹ base pairs, and base composition is assumed random (4 possible bases per position). How long (in bp) should a random sequence be to appear, on average, once in the genome?

Question reproduced from IBO 2019, Theoretical Exam A, licensed under CC BY-NC-SA 4.0 - attributed to the International Biology Olympiad. Open the full exam PDF · Official answer key