Genome assembly in the telomere-to-telomere era

Genome sequences largely determine the biology and encode the history of an organism, and de novo assembly — the process of reconstructing the genome sequence of an organism from sequencing reads — has been a central problem in bioinformatics for four decades. Until recently, genomes were typically assembled into fragments of a few megabases at best, but now technological advances in long-read sequencing enable the near-complete assembly of each chromosome — also known as telomere-to-telomere assembly — for many organisms. Here, we review recent progress on assembly algorithms and protocols, with a focus on how to derive near-telomere-to-telomere assemblies. We also discuss the additional developments that will be required to resolve remaining assembly gaps and to assemble non-diploid genomes. In this Review, Li and Durbin discuss how to generate telomere-to-telomere assemblies for large haploid or diploid genomes using currently available data types and algorithms, and outline remaining challenges in resolving highly repetitive sequences and polyploid genomes.