Why are my insert size distribution plots displaying a spiky, sawtooth pattern?

Question:

Why are my insert size distribution plots displaying a spiky, sawtooth pattern?

Answer:

Rolling circle amplification (RCA) has oscillating efficiency when different-sized templates are present. This produces a periodic, sawtooth-like pattern in the final insert size distribution. This is a biophysical property of rolling circle amplification and does not affect analysis.

Symptoms

• Periodic peaks and dips in the insert size distribution occurring at approximately 10 bp intervals
• Sequencing metrics otherwise appear normal

Figure 1. PhiX spike-in insert size distribution demonstrating the characteristic periodic, sawtooth pattern associated with RCA.