New high-throughput sequencing technologies have spurred explosive growth in the use of sequencing to discover mutations and structural variants in the human genome and in the number of projects to sequence and assemble new genomes.
Highly efficient algorithms have been developed to align next-generation sequences to genomes, and these algorithms use a variety of strategies to place repetitive reads.
Ambiguous mapping of sequences that are derived from repetitive regions makes it difficult to identify true polymorphisms and to reconstruct transcripts.
Short read lengths combined with mapping ambiguities lead to false reports of single-nucleotide polymorphisms, inserts, deletions and other sequence variants.
When assembling a genome de novo, repetitive sequences can lead to erroneous rearrangements, deletions, collapsed repeats and other assembly errors.
Long-range linking information from paired-end reads can overcome some of the difficulties in short-read assembly.
Repetitive DNA sequences are abundant in a broad range of species, from bacteria to mammals, and they cover nearly half of the human genome. Repeats have always presented technical challenges for sequence alignment and assembly programs. Next-generation sequencing projects, with their short read lengths and high data volumes, have made these challenges more difficult. From a computational perspective, repeats create ambiguities in alignment and assembly, which, in turn, can produce biases and errors when interpreting results. Simply ignoring repeats is not an option, as this creates problems of its own and may mean that important biological phenomena are missed. We discuss the computational problems surrounding repeats and describe strategies used by current bioinformatics systems to solve them.
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We thank K. Hansen for useful comments on an earlier draft.
The authors declare no competing financial interests.
- Next-generation sequencing
(NGS). Any of several technologies that sequence very large numbers of DNA fragments in parallel, producing millions or billions of short reads in a single run of an automated sequencer. By contrast, traditional Sanger sequencing only produces a few hundred reads per run.
- Interspersed repeats
Identical or nearly identical DNA sequences that are separated by hundreds, thousands or even millions of nucleotides in the source genome. Repeats can be spread out through the genome by mechanisms such as transposition.
- Tandem repeats
DNA repeats (≥2bp in length) that are adjacent to each other and can involve as few as two copies or many thousands of copies. Centromeres and telomeres are largely comprised of tandem repeats.
- Short interspersed nuclear elements
(SINEs). Repetitive DNA elements that are typically 100–300 bp in length and spread throughout the genome (such as Alu repeats).
- Long interspersed nuclear elements
(LINEs). Repetitive DNA elements that are typically >300 bp in length and spread throughout the genome (such as L1 repeats).
A DNA sequence fragment (a 'read') that aligns to multiple positions in the reference genome and, consequently, creates ambiguity as to which location was the true source of the read.
- Paired-end reads
Reads that are sequenced from both ends of the same DNA fragment. These can be produced by a variety of sequencing protocols, and paired-end preparation is specific to a given sequencing technology. Some recent sequencing vendors use the terms 'paired end' and 'mate pair' to refer to different protocols, but these terms are generally synonymous.
- De Bruijn graph
A directed graph data structure representing overlaps between sequences. In the context of genome assembly, DNA sequence reads are broken up into fixed-length subsequences of length k, which are represented as nodes in the graph. Directed edges are created between nodes i and j if the last k–1 nucleotides of i match the first k–1 nucleotides of j. Reads become paths in the graph, and contigs are assembled by following longer paths.
Contiguous stretches of DNA that are constructed by an assembler from the raw reads produced by a sequencing machine.
- DNA fragment
In the sequencing process, millions of small fragments are randomly generated from a DNA sample. In paired-end sequencing, both ends of each fragment are sequenced, and the fragment length becomes the 'library' size.
A widely used statistic for assessing the contiguity of a genome assembly. The N50 value is computed by sorting all contigs in an assembly from largest to smallest, then cumulatively adding contig sizes starting with the largest and reporting the size of the contig that makes the total greater than or equal to 50% of the genome size. The N50 value is also used for scaffolds.
A scaffold is a collection of contigs that are linked together by paired end information with gaps separating the contigs.
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Treangen, T., Salzberg, S. Repetitive DNA and next-generation sequencing: computational challenges and solutions. Nat Rev Genet 13, 36–46 (2012). https://doi.org/10.1038/nrg3117
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