Discussion
What is the approximate percentage of repetitive DNA sequences in human DNA?
Answer: B
Correct answer: 20 to 30%
On the classical partition of the human genome by reassociation kinetics - the \( C_0 t \) experiments that first revealed the structure - DNA sorts into three fractions. About 10% is highly repetitive satellite DNA, short sequences present in hundreds of thousands of copies, concentrated at centromeres and telomeres. Roughly 20 to 30% is moderately repetitive: the interspersed elements such as Alu and LINE-1 sequences, together with the tandem repeats of the ribosomal RNA, transfer RNA and histone genes. The remaining 60 to 70% is unique or near-unique sequence, which is where the protein-coding genes lie. It is the moderately repetitive fraction the question is asking about, and the answer is 20 to 30%. Modern sequence-based estimates count considerably more repetition than the reassociation experiments could detect - when every degraded transposable element is included, at least half the genome is repetitive in origin, and some analyses put it at two thirds. The figure keyed here is the textbook one and reflects the older method.
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