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Monday, May 27, 2024

Telomere length in humans

Telomeres are repetitive DNA sequences at the ends of chromosomes. In humans, the repeat sequence is TTAGGG. The purpose of telomeres is to protect the ends of the chromosomes from shortening after DNA replication [Telomeres].

Telomeres are just one of many functional DNA elements in the human genome. The average length of human telomeres was long thought to be about 10 kb and since there are 24 distinct chromosomes in the human genome this amounts to about 480 kb of telomere sequence or about 0.015% of the human genome. With the advent of new sequencng technology it is now possible to generate long reads of DNA sequence and this has led to a somewhat shorter estimate of telomere length (Karimian et al., 2024). The figure from thier paper shows that the average length of telomeres gets shorter with age but the starting length in newborns (cord DNA) is about 8 kb instead of 10 kb. The authors explain why their sequencing technique is likely to give more accurate results than the earlier estimates.

This doesn't make much difference to the previous estimate but I thought I'd post an update since I overestimated the contribution of telomeres in my book and I made a calculation error in a previous post [Telomeres].

If we use 8 kb as the average length, then that means a total of 8 × 2 × 24 = 384 kb or 0.012% of the standard human genome, which includes 22 autosomes and both sex chromsomes.

Image Credit: The image shows human chromosomes (blue) labelled with a telomere probe (yellow), from Christopher Counter at Duke University.

Karimian, K., Groot, A., Huso, V., Kahidi, R., Tan, K.-T., Sholes, S., Keener, R., McDyer, J.F., Alder, J.K., Li, H., Rechtsteiner, A. and Greider, C. (2024) Human telomere length is chromosome end–specific and conserved across individuals. Science 384:533-539. [doi: 10.1126/science.ado0431]

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