Dr. Israa ayoub alwan Lec -5- AL-Ma’moon University College Medical Laberatory techniques Department Molecular biology/ Second stage Dr. Israa ayoub alwan Lec -5-

DNA Replication

DNA Replication DNA replication occurs during S phase of the cell cycle . When DNA replicates, it unwinds, breaks, builds a new nucleotide chain, and mends .Enzymes called helicases unwind and hold apart replicating DNA, enabling other enzymes to guide the assembly of a new DNA strand. Human DNA replicates about 50 bases per second. To get the job done, a human chromosome replicates simultaneously at hundreds of points along its length, and the pieces join. A site where DNA is locally opened, resembling a fork, is called a replication fork.

DNA replication begins when a helicase breaks the hydrogen bonds that connect a base pair . Binding proteins hold the two strands apart. Another enzyme, primase, then attracts complementary RNA nucleotides to build a short piece of RNA, called an RNA primer, at the start of each segment of DNA to be replicated. The RNA primer is required because the major replication enzyme, DNA polymerase (DNAP), can only add bases to an existing nucleic acid strand. (A polymerase is an enzyme that builds a polymer, which is a chain of chemical building blocks.) Next, the RNA primer attracts DNAP, which brings in DNA nucleotides complementary to the exposed bases on the parental strand; this strand serves as a mold, or template. New bases are added one at a time, starting at the RNA primer.

The new DNA strand grows as hydrogen bonds form between the complementary bases. The nucleotides are abundant in cells, and are synthesized from dietary nutrients. DNAP works directionally, adding new nucleotides to the exposed 3 ′ end of the sugar in the growing strand. Overall, replication proceeds in a 5 ′ to 3 ′ direction, because this is the only chemical configuration in which DNAP can add bases. How can the growing fork proceed in one direction, when both parenta lstrands must be replicated? The answer is that on at least one strand, replication is discontinuous. It is accomplished in small pieces from the inner part of the fork outward, in a pattern similar to backstitching. Next, an enzyme called a ligase then seals the sugar-phosphate backbones of the pieces, building the new strand. These pieces, up to 150 nucleotides long, are called Okazaki fragments, after their discoverer .

DNA polymerase also “proofreads” as it goes, excising mismatched bases and inserting correct ones. It also removes the RNA primer and replaces it with the correct DNA bases. Yet another enzyme, called an annealing helicase, rewinds any sections of the DNA molecule that remain unwound. Finally, ligases seal the entire sugar-phosphate backbone. Ligase comes from a Latin word meaning “to tie.” As a human body grows to 100 trillion or so cells, DNA replication occurs about 100 quadrillion times. Some of the DNA sequence encodes protein, but most does not.  

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