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How do computers assemble short DNA reads into a complete genome sequence?

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Diagram of a DNA double helix showing its twisted ladder structure, with a close-up of the sugar-phosphate backbone, paired nitrogenous bases (adenine, thymine, cytosine, guanine), and labeled nucleotides and hydrogen bonds. — human genome The human genome is made up of approximately three billion base pairs of deoxyribonucleic acid (DNA). The bases of DNA are adenine (A), thymine (T), guanine (G), and cytosine (C).

To assemble a complete genome from short DNA reads, computers perform a process called sequence assembly, which relies on identifying overlapping segments of DNA.12

The process generally follows these steps:

Fragmentation: Genomic DNA is randomly broken into many short, manageable fragments.12
Sequencing: These fragments are sequenced to produce a large number of short DNA reads.12
Alignment and Assembly: Computer programs analyze these reads to find identical sequences that overlap. By identifying these overlaps, the software "stitches" the fragments together in the correct order to reconstruct the original, full-length genomic sequence.12

This task is computationally intensive, particularly for complex genomes that contain repetitive elements, which can create ambiguity during assembly. In de novo sequencing, where a genome is being mapped for the first time, researchers may use hierarchical shotgun sequencing—which organizes fragments into physical maps before assembly—or whole genome shotgun sequencing, which relies entirely on sophisticated algorithms to link the fragments. In contrast, re-sequencing projects align short reads directly to an existing reference genome. Once assembled, the data is deposited into databases for further analysis, such as gene identification and annotation, often within the field of bioinformatics.12