Course: SAT 170 Introductory Biotechnology

Unit 3 Reading and Multimedia Assignment

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Page 8 of 13
DNA Replication Initiation
DNA replication begins at origins of replication, where helicase begins unwinding the DNA, creating a replication fork.
In eukaryotes DNA replication is controlled within the cell cycle. As the cell grows and divides, it progresses through stages. DNA replication occurs during the synthesis or S phase. Most bacteria, or prokaryotes, do not go through a well-defined cell cycle but instead continuously copy their DNA. During rapid growth, this can result in the concurrent occurrences of multiple rounds of replication.

DNA replication begins with the unzipping of the parent molecule as the hydrogen bonds between the base pairs are broken. The process is initiated at particular points in the DNA, known as origins, which are targeted by proteins that separate the two strands and initiate DNA synthesis. Origins of replication contain DNA sequences recognized by replication initiator proteins. These initiator proteins recruit other proteins to separate the two strands at the origin, forming a bubble. As DNA synthesis continues, the original DNA strands unwind on each side of the bubble, forming a replication fork with two prongs at each end of the bubble.

In bacteria, which have a single origin of replication on their circular chromosome, this process eventually creates a theta structure, resembling the Greek letter theta: θ. Replication begins at the origin and continues in both directions until the molecule is completely copied. In contrast, eukaryotes have longer linear chromosomes and initiate replication at multiple origins, creating many bubbles at the same time. As replication continues, the bubbles expand, moving the origins, with replication proceeding in both directions until the bubbles meet and fuse. The multiple initiation sites and origins of replication increase the rate of DNA replication in eukaryotes.
Helicases are a class of enzymes that move directionally along DNA, untwisting and separating the two strands at the Y-shaped replication forks. Once DNA strands are separated, single-stranded binding proteins, recruited by the initiator proteins, hold the strands open and prevent them from binding back together. Topoisomerase enzymes unwind and wind DNA to facilitate DNA replication and control the synthesis of proteins. Topoisomerases bind to either single-stranded or double-stranded DNA and cut the phosphate backbone of the DNA. This intermediate break allows the DNA to be untangled or unwound, and at the end of these processes, the DNA is reconnected again.

RNA primers are created on the exposed separated template strands. Primase catalyzes the synthesis of these RNA primers, which are short RNA segments complementary to a single-stranded DNA template. Primase is of key importance in DNA replication because no known DNA polymerases can initiate the synthesis of a DNA strand without an initial RNA primer. In bacteria, primase binds to the DNA helicase forming a complex called the primosome. Primase is activated by the DNA helicase, where it then synthesizes a short RNA primer approximately 11 nucleotides long to which new nucleotides can be added by DNA polymerase.