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DNA replication in eukaryotes

Key points of  DNA replication in eukaryotes

  1. The genomic DNA of eukaryotic is 10 to 1000 times larger than the bacterial genomic DNA.
  2. The genomic DNA of eukaryotes is organized into more than one chromosome. The chromosome number is species-specific.
  3. Each chromosome is divided into several thousands of replicons, the size of the replicon varies from 30-kilobase pairs (kbp) to 300 kbps.
  4. Each replicon contains its own origin, the number of origins in a chromosome will be equal to the number of replicons present, because of the presence of multiple replicons, replication can be initiated at multiple sides in a chromosome, in the case of bacteria, the replication will be initiated at the single position because of the presence of a single origin.
  5. All the origins in the chromosome may be used or some of the origins may be skipped, the skipping of origins occurs when the replication fork from one replicon carries the replication into the adjacent replicon.
  6. An origin in replication can be used only once during a given cell cycle, this s essential to prevent chromosomal aberrations.
  7. The use of an origin depends on the availability of licensing proteins to form the origin recognition complex (ORC) since the licensing can be used only once for cell cycle, an origin can be used only once.
  8. In the cell cycle, DNA replication occurs only during the S-phase, which is the period between the two gap phases, G, and G2.
  9. The highly diffuse euchromatin region undergoes replication during the early S- phase, whereas the constitutive heterochromatin which is highly condensed will undergo replication in the late S- phase.
  10. Every nucleotide present in DNA should undergo replication within the S- phase.
  11. Unlike bacteria where a specific DNA primase carries out the synthesis of primers, while in eukaryotes the primase activity is associated with DNA pol-α which synthesis the primer needed for leading and lagging strand synthesis.
  12. DNA polymerase-α is a unique DNA polymerase, in that it can synthesize RNA primers as well as the Okazaki fragments during the lagging strand synthesis.
  13. Multiple replicase enzymes participate in replication and distinct DNA polymerases are involved in the synthesis of the leading and lagging strand, DNA polymerase-α replicates the lagging strand whereas DNA polymerase- δ and ε are involved in the formation of the leading strand.
  14. All the replicases of eukaryotes are inhibited by Aphidicholin.
  15. DNA gyrase is absent in eukaryotes and type-2 DNA topoisomerase capable of removing positive supercoiling is involved in the formation of relaxed DNA ahead of the replication fork.
  16. Eukaryotic chromosomes lack specific termination sequences for the termination of replication, the DNA polymerase either falls off from the template at the terminal region or is released from the template when the adjacent forks are fused.
  17. The primer excision and replacement of the primer with the deoxynucleotides is complex in eukaryotes.
  18. The deoxynucleotides at the primer region are inserted by the gap-filling activity of the DNA polymerase-β.
  19. Eukaryotic DNA ligases require ATP as a cofactor, DNA ligase activity is needed for the joining of Okazaki fragments into the lagging strand.
  20. Telomerase required for the replication of telomeres.
  21. A telomere is a chromosome end that contains highly repetitive DNA and is anchored in proteins.
  22. The telomere contains a highly specialized secondary structure in the form of G-Quartex, the presence of such a secondary structure prevents the deterioration of chromosomes from ends and also prevents the fusion of chromosomes.
  23. In normal cells, the telomeres cannot be replicated and therefore the site of the telomere decreases from one generation to another, this reduction in the telomeric size contentious until the chromosome reaches a critical stage after which DNA cannot be replicated and the cell undergoes senescence and death.
  24. Even though the somatic or vegetative cells cannot replicate telomeres and therefore have a finite number of generations in cultures, the stem cells and germline cells are capable of replicating the telomeres, due to the presence of telomerase.
  25. The enzyme telomerase is a transcriptase that contains an RNA template that is complementary to the telomeric sequence and a protein enzyme capable of telomeric synthesis.
  26. The RNA template of telomerase is used for the extension of the DNA by several hundreds of nucleotides and after the extension, the secondary structure formation takes place due to the abundance of guanine nucleotides.
  27. The telomeric sequence is specific for species, for example, vertebrates have a telomeric sequence (TS) hexanucleotide repeats of TTAGGG.
  28. The immortal nature of most human cancer cells arises due to the overexpression of the telomerase, one of the changes in gene expression during the transformation of a normal cell into a tumor cell is the activation of telomerase expression.