a-tubulin and b-tubulin comprising 445–450 amino acids, which are tightly held together by noncovalent bonds.

TLS Online TPP Program

#Id: 3186

#Unit 3. Fundamental Processes

Mitochondria use different origin sequences to initiate replication of each DNA strand

Replication of the H strand is initiated in a D loop.

Replication of the L strand is initiated when its origin is exposed by the movement of the first replication fork.

TLS Online TPP Program

#Id: 3187

#Unit 3. Fundamental Processes

Replication starts at a specific origin in the circular duplex DNA. Initially, though, only one of the two parental strands (the H strand in mammalian mitochondrial DNA) is used as a

template for synthesis of a new strand. 

Synthesis proceeds for only a short distance, displacing the original partner (L) strand, which remains single-stranded,

TLS Online TPP Program

#Id: 3188

#Unit 3. Fundamental Processes

A single D loop is found as an opening of 500 to 600 bases in mammalian mitochondria.

TLS Online TPP Program

#Id: 3189

#Unit 3. Fundamental Processes

Some mitochondrial DNAs possess several D loops, reflecting the use of multiple origins. The same mechanism is employed in chloroplast DNA, where (in complex plants) there are two D loops.

TLS Online TPP Program

#Id: 3190

#Unit 3. Fundamental Processes

To replicate mammalian mitochondrial DNA, the short strand in the D loop is extended. The displaced region of the original L strand becomes longer, expanding the D loop

TLS Online TPP Program

#Id: 3191

#Unit 3. Fundamental Processes

In mtDNA, The existence of D loops exposes a general principle: An origin can be a sequence of DNA that serves to initiate DNA synthesis using om strand as a Template.