DNA replication is the most beneficial process in the living world. True cell replication is a complex process. The process can be discussed under three headings. Double helix separation, complementary strand formation and new DNA formation.
1. Separation of the double helix
(i) At the beginning of replication, base pairs are released at specific sites in the DNA and bubble-like ori points or replicon points or initiation points are formed. Ori spots are usually formed at sites where adenine and thymine are abundant in DNA. Because adenine and thymine have two hydrogen (A=T) bonds. Adikosha has one and Prakharkosha has multiple ori points.
(ii) First the helicase enzyme binds to the Ori point and starts unwinding the double helix. The helicase enzyme then uses energy from ATP to break the hydrogen bond. A Y-shaped structure is formed at the point where the two strands diverge. This is called replication fork.
(iii) After opening the patch, the two strands separated due to tension or attraction tend to gather again by re-adjusting the patch. The enzyme topoisomerase cuts the strand near the replication fork. As a result, the attraction of forming patches and gathering of the formula is lost. In primary cells, gyrase enzyme breaks down the attractive pull of formation and aggregation of sutra patches. The severed formula is then reattached.
(iv) The two formulas are complementary to each other. So it wants to re-attach by forming a hydrogen bond. Single Strand Binding Protein (SSBP) does not allow hydrogen bonds to form. As a result the two formulas cannot be rejoined.
(v) Replication forks move in opposite directions and balloon or eye-like structures are formed in the intervening space. It is called Replication eye or Replication bubble. Many replication bubbles are formed simultaneously. The bubbles elongate and coalesce, separating the two sutras. The separation of the two strands of DNA is called denaturation.
2. Creating complementary chains
(i) RNA primase enzyme uses the formula two as a template and creates smaller primers. The R3 end of the primer has a free -OH group.
(ii) DNA polymerase enzyme adds new nucleotides to the 3 -OH end of the primer. Nucleotides tend to join 5-3 carbons. As a result, two new sources started to emerge. One is the leading formula and the other is the lagging formula.
(iii) The strand that grows towards the replication fork is called the leading strand. Leading formulas continuously create counterpoints. The strain that grows in the opposite direction of the replication fork is called the lagging strain. The lagging formula produces unpaired segmental counterparts.
(iv) Each term of the lagging formula is called Okazaki. Okazaki of protocells is composed of 1000-2000 nucleotides and Okazaki of true cells is 100-200 nucleotides. Japanese scientist Reiji Okazaki and his wife Tsuneko Okazaki discovered it.
3. Creation of new DNA
(i) Exonuclease enzyme removes the primers complementary to the new sequence and the empty sites are filled with complementary nucleotides.
(ii) Incorrect nucleotides are removed by nuclease enzymes and correct nucleotides are added by DNA polymerase enzymes. This mismatch repair (MMR) is called DNA proof reading. Only one in 1,000 genes in humans can be a mismatch.
(iii) Ligase enzyme joins Okazaki. Purine and pyrimidine bases are joined by hydrogen bonds. As a result, two new DNAs are created.