Fig. 1.3.

The process of DNA replication. The DNA is densely packed and packaged in the chromosomes. The process requires the action of several factors and enzymes. DNA helicase unwinds the double helix. Topoisomerase relaxes DNA from its super coiled nature. Single-strand binding proteins bind to single-stranded open DNA and prevent its reannealing and maintains strand separation. DNA polymerase is an enzyme which builds a new complimentary DNA strand and has proofreading activity. DNA clamp is a protein which prevents dissociation of DNA polymerase. Primase provides a short RNA sequence for DNA polymerase to begin synthesis. DNA ligase reanneals and joins the Okazaki fragments of the lagging strand. DNA duplication follows semiconservative replication, where each strand serves as template which leads to the production of two complimentary strands. In the newly formed DNA, one strand is old and the other one is new (semiconservative replication). DNA polymerase can extend existing short DNA or RNA strand which is paired to template strand and is called primer. Primer is required as DNA polymerase cannot start the synthesis directly. DNA polymerase is capable of proofreading, that is, correction of wrongly incorporated nucleotide. One strand is replicated continuously with single primer, and it is called as leading strand. Other strand is discontinuous and requires the addition of several primers. The extension is done in the form of short fragments called as Okazaki fragments. The gaps are sealed by DNA ligase. Replication always occurs in 5′–3′ direction