Q12 - Genome Size, Genetic Code and Alien DNA

Theoretical 1 Real exam question - full text reproduced under IBO's CC BY-NC-SA 4.0 license

Nucleotide & Nucleic Acid Chemistry Translation & the Genetic Code

Humans have about 20,000 protein-coding genes. Assume the average (mean) human protein weighs 50 kDa. The average amino acid weighs 110 Da. Use this information to estimate the total length, in base-pairs, of coding sequence in the human genome.

In chimpanzees, < 1% of the genome is protein-coding. Centromeres are about 0.2–7 Mbp, depending on chromosome. Telomeres are about 5–15 Kbp at birth.

An alien plant has 100 million base-pairs of protein-coding sequence in its genome. The alien uses 20 different amino acids (like us) but has 3 different DNA base pairs (A/T, G/C and X/Y).

The chemical structure of an X/Y base pair is shown below.

Structural diagram of the X/Y base pair showing three hydrogen bonds between the two bases, similar to a G/C pair. Chemical structure of the alien X/Y base pair, showing three hydrogen bonds.

Q12.1. Estimate the total length, in base-pairs, of coding sequence in the human genome. Give your answer to the nearest million base-pairs.

Q12.2. What kind of sequences comprise the majority of the chimpanzee genome?

Q12.3. Calculate the maximum total length of protein sequence (in amino acids) this alien could encode. Assume no overlapping coding regions. Give your answer to the nearest million amino acids.

Q12.4. Which DNA sequence has the highest melting temperature?

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Question reproduced from IBO 2023, Theoretical Paper 1, licensed under CC BY-NC-SA 4.0 - attributed to the International Biology Olympiad. Open the full exam PDF