DNA Replication is the process by which DNA makes a copy of itself during cell division. It is semi conservative replication as both the copies contains one old strand and one new strand.
- Substrates : The four deoxynucleoside triphosphates ; dATP(deoxy-adenosine triphosphate) , dGTP(deoxy-guanosine triphosphate) , dCTP(deoxy-cytidine triphosphate) and dTTP(deoxy-thymidine triphosphate) are needed as substrate for DNA synthesis.
- Template : A template is required to direct the addition of the approptiate complementary deoxynucleotide to the newly synthesized DNA strand. In semi conservative replication , each strand of parental DNA serves as template. Then , each template strand and newly synthesized complementary strand serve as the DNA in daughter cells.
- Primer : DNA synthesis cannot start withour primer , which prepares the template strand for the addition of nucleotides. Primase enzyme synthesized RNA primer.
- Enzymes :
- DNA Helicase : It breaks the hydrogen bond formed between the bases of nucleotides , helps in unwinding of helix.
- Topoisomerase : It is responsible for causing nick in the DNA strand to release the tension created during unwinding of DNA.
- DNA Polymerase : Catalyses leading and lagging strand synthesis by adding complementary nucleotides in the template DNA.
- DNA Ligase : It seals the gaps in the synthesized DNA (i.e bind the fragments of DNA strand together.
- Repair Enzymes : It cuts off the wrong base of nucleotides and rejoin the correct base in newly formed DNA.
- SSB Proteins : These protein molecules which attach tightly to the exposed single stranded DNA in order to stabilize the single stranded DNA long enough for replication.
- Initiation of DNA replication : DNA consist of many replication units called replicons . Replication starts at replicon known as origin or site of replication ( Ori-Site). In prokaryotic cell ,there is only one ori-site where as in eukaryotic cell it is more than one.
- Activation of deoxyribonucleotides : Inactive deoxyribo-nucleotide monophosphate present in nucleoplasm are changed into active deoxyribo-nucleotides triphosphate with the help of enzyme phosphorylase and ATP. This process is called phosphorylation.
dNMPs ⟶ dNTPs + H3PO4
- Unwinding of DNA helix : The unwinding of the DNA double helix takes places by the help of helicase enzymes which breaks the hydrogen bonds formed between bases of nucleotides by using energy ATP . Each unwinding parental DNA strand acts as a template DNA strand . When two strands unwind and separate incompletely , they form a Y- shaped structure known as replication fork . SSB protein stablized each separated strands .
As DNA two strands are separated , DNA topoisomerase cut the phosphodiester link of DNA one strand of helix to prevent supercoiling thus allows the helix to rotate around the remaining unbroken strand . The enzyme then quickly reseals the link.