Figure 2. Mechanisms of mtDNA replication.

(A) Replication priming in the OriH region. Transcription from LSP by POLRMT creates a hybrid R-loop anchored at CSB2, which is processed by RNASEH1 to generate a 3′ RNA end that can be utilised by POLγ for DNA synthesis. (B) Replication priming at OriL. When exposed in ssDNA form, the OriL sequence forms a stem-loop structure. POLRMT initiates RNA synthesis from a poly(T) stretch in the loop of this structure, generating a primer for DNA synthesis by POLγ. (C–E) Models of mtDNA replication. (C) The strand displacement model (SDM). Leading (heavy) strand replication is initiated at OriH, and the displaced lagging strand template is coated with MTSSB (yellow). Lagging strand replication is initiated by POLRMT, which forms a primer at a stem-loop structure at the OriL site, allowing continuous L-strand synthesis. (D) Strand coupled replication. Initiation is associated with a broad zone downstream of the NCR, Ori-z. Replication of the H-strand is continuous, while L-strand replication requires the formation of discontinuous Okazaki fragments. (E) Ribonucleotide incorporation throughout the lagging strand (RITOLS). Replication is initiated in the NCR, with OriL being the major site of lagging-strand DNA synthesis. The displaced lagging-strand template is coated with RNA transcripts (bootlaces) rather than MTSSB. (F) Hemicatenated mtDNA replication products are separated by TOP3A.