Question

DNA Replication - Describe the process of DNA replication in a well-organized manner. Start with the helicase and go through the entire process, step by step. Use these terms correctly: Helicase, DNA primase, single stranded binding proteins, DNA polymerase I, DNA Polymerase III, leading strand, lagging strand, okazaki fragments, ligase, topoisomerase, sliding clamp, clamp loader. Please use correct descriptions of the enzyme function. For example, do not say, “seals nicks.” Tell me what “seals nicks” means.

Answer 1

DNA replication is the process by which DNA duplicate itself. It occurs during the synthesis phase of cell cycle so that the daughter cells formed in the end of cell division receive equal amount of DNA.

The process begins with the uncoiling of double stranded DNA molecule into single stranded formed by the action of enzyme helicase.

Single stranded DNA molecule is maintained into single standard form by the binding of single stranded binding proteins. This prevents the two strands of DNA to combined together in a complementary manner.

This leads to the formation of a replication bubble. Ahead of replication bubble supercoiling occurs. This is relaxed by the action of enzyme topoisomerase.

DNA synthesis cannot be initiated on it's own by DNA polymerase 3. It requires a free hydroxyl group for the addition of deoxyribonucleotides to the growing daughter DNA molecule. This is provided by RNA primer. This short strand of RNA primer is synthesized by enzyme called as primase.

After the addition of primer, DNA polymerase three begins to add deoxyribonucleotides. This enzyme works only in 3' to 5' direction. Therefore the synthesis of DNA is continuous on the leading strand. It occurs only by the formation of single primer to which single long fragment of DNA molecule is synthesized. The Other strand of DNA is opposite in direction and DNA synthesis occurs in a discontinuous manner in this strand. This is called as lagging strand. The DNA synthesis occurs in this strand by the formation of multiple RNA primer. To each of the primer, single small fragment of DNA is synthesized. This is called as okazaki fragment.

For the application process to go on easily, proteins called as clamp loaders bind on the DNA and help DNA polymerase to attach to it. The whole assembly of clamp loader and DNA polymerase along with DNA is called as sliding clamp.

Once rhe process of polymerization by DNA polymerase 3 is completed, RNA primer are removed by DNA polymerase 1. The gap present between adjacent okazaki fragments is also filled by this enzyme. Adjacent fragments are connected to each other by the action of enzyme ligase. This enzyme seals Nick by the formation of phosphodiester Bond between those fragments.

This is how the process of DNA replication is completed in a Eukaryotic cell.

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