because it runs the opposite direction of the dna polymerase

The enzyme that synthesizes DNA, DNA polymerase, can only add nucleotides to an already existing strand or primer of DNA or RNA that is base paired with the template.

DNA Polymerase: recognizes the RNA primers and extends them in the 5' to 3' direction.

2. DNA polymerase then incorporates a dNMP onto the 3" end of the primer and initiates lagging strand synthesis. The polymerase extends the primer for about 1,000 nucleotides until it comes in contact with the 5' end of the preceding primer. These short segments of RNA/DNA are known as Okazaki fragments.


DNA synthesis occurs only in 5'-3' direction.?

DNA synthesis requires a primer usually made of RNA. A primase synthesizes the ribonucleotide primer ranging from 4 to 12 nucleotides in length. DNA polymerase then incorporates a dNMP onto the 3' end of the primer initiating leading strand synthesis. Only one primer is required for the initiation and propagation of leading strand synthesis.


SparkNotes: DNA Replication and Repair: DNA Replication

Chromosomes are very long DNA molecules, and the replisome constantly encounters proteins and DNA lesions in its path. We have studied replisome collisions with proteins. When the replisome collides with RNA polymerase traveling in the same direction, the replisome remains on DNA and takes over the RNA without pause. The replisome pauses upon collision with RNA polymerase in the opposite direction, but recruits a specialized protein that displaces the RNA polymerase. We have studied how the replisome deals with DNA lesions incurred by sunlight or oxidative damage. In our collaboration with Myron Goodman (University of Southern California), we discovered a new class of DNA polymerase (Pol V) that functions with the clamp to extend DNA across a lesion, resulting in a mutation. This class of polymerase, referred to as translesion synthesis (TLS) polymerases, is now found in all cell types.

A summary of DNA Replication in 's DNA ..

In 1969, Jovin et al. elucidated the amino acid composition (Jovin et al. 1969a, b). That same year, DeLucia and Cairns isolated an E. coli strain with a mutation that affected the DNA polymerase and surprisingly found that the mutant synthesized DNA normally. This discovery cast doubts on the role of DNA polymerase in replication and led groups to search for other replication enzymes. At the same time, Klenow and colleagues showed that the treatment of DNA polymerase with the proteolytic enzyme subtilisin (type Carlsberg) resulted in an increase of polymerase activity and decrease of exonuclease activity. The resulting DNA polymerase was isolated and was named the “Klenow fragment” (Klenow and Henningsen 1970a, and Klenow and Overgaard-Hansen 1970).

polymerase is capable of only one direction, ..

The overall architecture of proteins at the bacterial replication fork, called a replisome, has emerged from these studies (see the movie). At the center is the clamp loader; it has protein arms that bind the Pol III cores for simultaneous synthesis of both strands of duplex DNA. The clamp loader arms also bind the DnaB helicase, a hexamer that encircles one DNA strand. The leading-strand Pol III continuously travels with the helicase during DNA unwinding, but due to the antiparallel structure of DNA, the lagging strand is synthesized in the opposite direction of DNA unwinding. Thus the lagging strand is made as a series of small fragments, each started by an RNA primer (e.g., by primase). RNA primers are extended by Pol III, resulting in a DNA loop. When the polymerase finishes, it bumps into a fragment made previously and disengages from its β ring, allowing it to recycle to the next upstream RNA primer onto which the clamp loader has loaded a new clamp.

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