Replicon

replicon is a unit of replication

1. replicon Concept and enzymes used in replication Mrs. Praveen Garg VITS College, Satna

2. A replicon is DNA molecule containing an origin of replication region essential for initiate to replication. Replicon is a segment of chromosome that can replicate with its own initiator and termination sequence, located in chromosome. One replicon has one origin of replication and termination sequences. Jacob, Brenner and Cuzin developed a  replicon model  as the unit in which the cell controls or regulate individual acts of replication. I t is also known as unit of replication. INTRODUCTION

3. The number of replicon per chromosome varries and depend on the size of the chromosome: Single copy replication control: A control system in which there is only one copy of a replicon present in it. Bacterial chromosome and plasmid have single replicon per chromosome. Multiple copy replication control: A control system in which there are multiple replicon present in it. Eukaryotic chromosome have larger and multiple replicon per chromosome.

4. Replicon concept is given by Jacob et al ., in 1963. The replicon initiates and completes synthesis once per cell cycle. For this method, an  initiator  protein interacted with a DNA sequence, called a  replicator , to start replication. The replicator can be identified genetically as a DNA sequence required for replication, whereas the  origin  is defined by physical or biochemical methods as the DNA sequence at which replication begins. Initiator proteins have now been identified for some replicons , such as the DnaA protein in  E. coli  and the  Origin Recognition Complex  in the yeast  Saccharomyces cerevisiae . CONCEPT

5. Replicons can be linear or circular. Replicated region appear as eye within non replicated DNA. Replication fork is initiated at the origin and then moves sequentially along DNA. Replicon is unidirectional when a single replication fork is created at an origin. Replicon is bidirectional when an origin creates two replication forks that move in opposite directions.

6. Prokaryotic Replicon A Bacterial genome is usually have a single circular replicon that replicate bidirectionally from a single origin. The origin of E.Coli is oriC , is 245bp in length. Two replication fork met around the circle but there are ter site that cause termination.

7. Eukaryotic genome have a multiple replicon (many oriC ) region, each with its own origin. The progression into S phase is tightly controlled. Individual replicon are activated at perticular time in S phase. Eukaryotic replicon are 40 to 100kb in length. Initiation of DNA synthesis in a replicon is highly regulated. Replication in higher eukaryotes depend upon position of nucleosome , type of histone modification, DNA methylation etc. Eukaryotic Replicon

8. ENZYMES USED IN REPLICATION Replication is a process in which each DNA strand synthesized its own complementary DNA. There are various enzymes used in the process of replication. The function of all enzymes is mostly similar to prokaryotes and eukaryotes. But the name of all enzymes is different in both.

9. Prokaryotic enzymes Eukaryotic enzymes Dna A Helicase Dna B Single standed binding protein Dna C DNA polymerase III Dna G DNA polymerase I Gyrase Topoisomerase Ligase Sliding clamp Clamp loader

10. Dna A It bind to 9 mer sequence and unwind to 13 mer sequences present at oriC site and forms an open complex during initiation of replication. It is ATP requiring process, cause double stranded DNA to melt forming ssDNA . It is first protein that bind to DNA to initiate replication.

11. It is primosome constituents and consist of six subunits. It unwinds DNA during replication. It is responsible for the extension of open complex during replication. It is essential for binding of Dna B protein to DNA strands. Its help to the formation of two bidirectional replication fork. After start to replication, it remove from the binding site. Dna B Dna C

12. Helicase enzyme often used to separate double stranded DNA molecule. This enzymes break the hydrogen bond between two strand using the energy by ATP hydrolysis. These are protein involved in unwinding of DNA molecule. It is hexamer protein and move with the replication fork. There are four type of helicase : Dna A, Dna B, Rep A protein, and Helicase II. The role of helicase is to unwind the duplex DNA and provide single stranded DNA for replication. DNA Helicases

13. It is called as DNA unwinding protein. Single stranded binding protein (SSB) bind to single stranded DNA to prevent annealing and to protect the DNA from being digested by nucleases. It is found in both prokaryotes and eukaryotes. It is tetramer structure, contain four sub unit. The major function of SSB is to prevent recoiling of DNA strand after its unwinding by helicases . Its stabilized the single stranded DNA. Its play major role in replication. Single Stranded Binding Protein

14. Primase enzyme involved in replication. DNA primase is a type of RNA polymerase which creates an RNA primer. DNA polymerase use this primer to replicate template ssDNA . Primase catalyzes the synthesis of short RNA segment called a primer complementary to ssDNA molecule. Primase play important role in replication because no known DNA polymerase can initiate the synthesis of DNA strand without an initial RNA or DNA primer. After completing replication, RNA primers is removed by 5’- 3’exonucleases. DNA Primase ( Dna G)

15. Gyrase is an enzyme that relieves strain while double stranded DNA is unwound by helicase . This cause negative supercoiling of DNA. Negative supercoiling of bacterial DNA by gyrase influences all the metabolic processes essential for replication. The ability of gyrase to relax positive supercoiling during DNA replication. Its work similar to topoisomerase II found in eukaryotes. DNA Gyrase

16. DNA ligase are responsible for connecting DNA segment during replication. It catalyze the phosphodiester bond between two DNA segment. It required to free OH group at 3’end of one DNA and Phosphate group at 5’ end of other DNA for creating phosphodiester bond. It is energy dependent process. DNA Ligase

17. It is responsible for removing supercoils in the helix. There are two types of topoisomerase : DNA Topoisomerase I: Reversibly cut a single stranded of double helix. They have both nuclease (strand cutting) and ligase (sealing) activity. They do not require ATP. DNA Topoisomerase II: It tightly bind to DNA double helix and break in both strand. It pass through the break and reseal the break. It introduce negative supercoils using energy as ATP. Topoisomerase

18. DNA is replicated by enzyme DNA polymerases. These enzyme utilize ssDNA as template on which complementary strand is synthesized using dNTPs . DNA polymerase require a free 3’OH group as a primer for synthesizing a new DNA strand. DNA polymerase synthesize DNA in 5’-3’ direction. It is categorized into three types in prokaryotes. DNA Polymerase I: Remove RNA primer or proof reading. DNA Polymerase II: Help in DNA repair. DNA Polymerase III: Add nucleotide in 5’- 3’ direction. DNA Polymerase

19. It is classify in to six types in eukaryotes. DNA polymerase α : Initiate replication and synthesize primers DNA polymerase β : replication with low fidelity DNA polymerase γ : polymerization in mitochondria DNA polymerase δ : elongation DNA polymerase ε : proofreading and filling gap

20. Slinding clamp hold DNA polymerase and prevent this enzyme from dissociate from the template DNA strand. Slinding clamp bind with DNA by the help of clamp loader. This is energy dependent process. Slinding Clamp

21. THANK YOU