Complete Human Genome Sequenced by the Telomere-to-Telomere Consortium

2022, scientists announced the first truly complete sequence of a human genome, marking the end of gaps that had remained since the original Human Genome Project draft published in 2001. The achievement was reported by the Telomere-to-Telomere (T2T) Consortium in the journal Science, providing a full, gapless sequence spanning all 22 autosomes and the X chromosome. The original Human Genome Project, completed in April 2003 after beginning in 1990, produced a reference genome that covered approximately 92% of the human genome. However, around 8% remained unresolved, largely consisting of highly repetitive DNA regions such as centromeres, telomeres, and segmental duplications. These regions were technically difficult to sequence with earlier technologies because short-read sequencing methods could not reliably assemble long, repetitive stretches of DNA. The 2022 accomplishment was made possible by advances in long-read sequencing technologies, including Oxford Nanopore and PacBio HiFi sequencing. Researchers assembled approximately 3.055 billion base pairs without gaps, adding nearly 200 million previously missing base pairs to the reference genome. The new assembly also identified about 99 predicted protein-coding genes that had not been fully characterized before, many located in repetitive regions. The genome used for the complete assembly came from a hydatidiform mole cell line known as CHM13, which contains a nearly homozygous set of chromosomes. This reduced genetic variation simplified the assembly process and allowed researchers to generate a continuous sequence from one telomere to the other on each chromosome. The publication provided, for the first time, a complete reference sequence including all centromeric regions and the short arms of acrocentric chromosomes. Researchers noted that the expanded reference would improve studies of genetic variation, structural variants, and disease-associated mutations, particularly in previously inaccessible regions of the genome. On 09/05/2022, the T2T Consortium further announced the completion of the Y chromosome sequence, filling one of the last remaining major gaps in the human reference genome. Together, these milestones established the first fully complete human genome assembly more than two decades after the initial draft.