Overview

The mode of chromosome replication varies from life form to life form and fits with chromosome structure and with the course and tempo of reproduction.

Chromosome replication begins at origins of replication and stops at termination sequences; small chromosomes typically have a single origin. Replication may be bidirectional or unidirectional. Linear chromosomes may have interior or terminal origins. Most linear chromosomes have telomeres, which are terminal sequences that are replicated in special ways. The large, linear chromosomes of eukarya are divided into many replicons (simultaneously replicated regions), each having an origin at its center.

Chromosomes made of RNA or single-stranded DNA replicate by exceptional mechanisms.

Replication of Circular Chromosomes

Theta Replication

Many circular chromosomes – bacterial chromosomes, most plasmids, and some dsDNA viral chromosomes – replicate by the theta mechanism, so named because the shape of partly replicated chromosomes resembles the Greek letter theta. Theta replication is usually bidirectional, but in some chromosomes it is unidirectional – e.g., colE1 phage.

Replication begins at the chromosome's origin (Figure 13.1). In Escherichia coli the origin is called oriC, which consists of 245 bp and contains short repeated sequences: four copies of a 9-bp sequence and three copies of a 13-bp sequence. First, about 30 copies of initiator protein (dnaA) bind to the four 9-bp sequences of oriC. DnaA helps the DNA to melt, making a single-stranded region. Helicase + dnac bind to the three 13-bp sequences and open up the double-stranded DNA (dsDNA) further, after which two primosomes are assembled and bidirectional replication starts (Figure 13.2).