Principles of Biochemistry Volume 3 - A. Lehninger 1985

Molecular Mechanisms of Genetic Information Transfer
DNA Replication and Transcription
Sometimes DNA is replicated via a “rolling circle” mechanism

The DNA of certain Viruses replicates in one direction via a "rolling circle" mechanism, a variation of which is shown in Fig. 28-5. First, one of the two strands of the circular parental DNA is cleaved by an enzyme. Next, several new NUCLEOTIDES are added to the 3'-end of the cleaved strand. Elongation of the new strand on the circular template proceeds through the gradual Displacement of the 5'-terminal portion of the cleaved strand from the rolling circle template. As the new strand grows, the displaced 5'-tail becomes a linear template for the synthesis of a new complementary strand. This synthesis on the linear template continues until a daughter DNA strand complementary to one revolution of the circular template is formed. The double-stranded tail is then cleaved by an enzyme, and Replication can initiate again at the 5'-end. In this manner, multiple complementary copies of circular DNA can be produced from a circular template. The rolling circle mechanism is utilized in oocytes during the synthesis of rRNA genes; it allows for the generation of A large number of copies of these genes arranged in tandem, which in turn enables the simultaneous synthesis of abundant rRNA. This mechanism is essential for oocytes to produce numerous Ribosomes for rapid cellular Protein Synthesis during the accelerated Cytology/cytology/16.html">Early stages of embryonic development.

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Fig. 28-4. Replication of A eukaryotic chromosome. A. Schematic representation of a replicating DNA segment from Drosophila melanogaster eggs. Visible are the replication "bubbles" or "eyes" formed during replication occurring at multiple origins in accordance with the Bidirectional Replication fork model. B. Two replication forks form simultaneously at multiple sites (origins of replication). Replication continues until the Synthesis of the daughter strands (shown in red) is fully complete. The new duplexes then separate; each contains one parental strand (black) and one daughter strand (red).

Fig. 28-5. Rolling circle replication characteristic of several viral DNAs. A nick is introduced into one of the strands, after which nucleotides begin to attach to the 3'-end of this strand. As a result, the severed parental strand elongates complementarily to the other parental strand, displacing its own 5'-end. Then, as the displaced strand peels off the circle, its replication begins. Upon completion of the synthesis of the new daughter strand complementary to this unwinding strand, the newly formed linear duplex is cleaved by a nuclease, yielding linear viral DNA. The other newly formed (daughter) strand now repeats the replication process: its 3'-end is extended, while its 5'-end peels off and serves as a template for the synthesis of a new daughter strand. Thus, multiple new linear duplexes can be generated from a rolling circle template. Replication via this mechanism also occurs in eukaryotes during the synthesis of tandemly repeated rRNA genes. In this case, the new genes are not cleaved off, but remain part of a single continuous strand. See Sections 27.26 and 28.25.