10    ◾    Bioinformatics

called highly accurate long sequencing reads (HiFi reads). The ­sequencing is ­carried out in

parallel on thousands of nanophotonic wells, generating long reads. PacBio can produce

long reads with 99% accuracy (>Q20) and uniform coverage.

1.2.3.2  Oxford Nanopore Technology

The Oxford Nanopore Technology (ONT) was announced in 2012, but it was made com-

mercially available in 2015 as MinION technology. ONT uses a flow cell to allow mas-

sive parallel sequencing. Their flow cell is made of an electrical resistant membrane that

contains thousands of tiny pores, each with a diameter of one nanometer. The sequencing

principle is based on applying a steady current to the nanopores. The magnitude of current

depends on the size and shape of the nanopore. During sequencing, a ssDNA is allowed to

pass through the nanopore like a needle and thread so that each nucleotide passes at a time.

When a nucleotide passes the opening of the pore, the voltage magnitude is changed. We

can use known DNA sequence to capture the unique electrical change for each nucleotide

and then allow the DNA molecule that we need to sequence to go through the nanopore

opening. The order of the nucleotides on the DNA sequence is determined by matching

the patterns. ONT has a higher error rate because the DNA fragments tend to pass faster

through the nanopores that may generate errors.

FIGURE 1.5  Pacific Bioscience (PacBio) sequencing.