DNA Replication

DNA Structure: It has a double helix structure made of sugar-phosphate backbones, with nucleotide bases that form the structure which is connected via hydrogen bonds. Adenine always pairs with thymine; guanine always pairs with cytosine. One strand points the 5’-3’ and the other 3’-5’, so that DNA replication can occur. The 5’ end always has a phosphate group and 3’ end always has a sugar group. This allows for complimentary base pairing and the synthesis of new DNA strands. During DNA replication, the leading strand is replicated continuously, and the lagging strand is replicated in fragments.

Replication Fork: The direction of replication is in the 5’-3’ direction. DNA polymerase is only able to add nucleotides to the 3’ end. The leading strand is synthesized continuously in the 5’-3’ direction which moves in the same direction as the replication fork. The lagging strand, however, is synthesized discontinuously in the 5’-3’ direction away from the replication fork and forms Okazaki fragments. Okazaki fragments are synthesized on the lagging strand in the 3’-5’ direction, away from the replication fork.

Enzymes: DNA Polymerase III synthesizes DNA in prokaryotic cells and works with DNA helicase and primase at the replication fork. DNA ligase is an enzyme that joins Okazaki fragments together into complete strands so that the lagging strands can be complete. Primase is needed to synthesize primers for each Okazaki fragment. Topoisomerase enzymes work to relieve torsional strain caused by the unwinding of DNA, in DNA replication, DNA gyrase is used. Single-strand binding proteins stabilize the single stranded regions. Helicase makes replication more efficient as it unwinds the DNA prior to DNA polymerase. This action requires ATP for energy supply, and it forms the single strands that SSBs coat to protect. DNA polymerase I erases the RNA primers and fills gaps in the DNA strands and has 5’-3’ exonuclease activity that replaces nucleotides in the growing DNA strand.

Semiconservative Replication: Each new DNA molecule consists of one original strand and one new strand as the DNA double helix is separated from itself and one new strand is synthesized which ensures genetic continuity.

Origins of Replication: The location differs in prokaryotic and eukaryotic cells. In eukaryotic cells, it occurs in the nucleus and the DNA material cannot be free in the cytoplasm, but because prokaryotic cells do not have a nucleus, it occurs in the cytoplasm. This makes eukaryotic replication slower as it requires full transcription to mRNA to occur before translation into nucleotides can occur. In prokaryotic cells, this can occur simultaneously, but it results in less complex organisms. They also differ in DNA structure as prokaryotic cells have circular DNA which creates for a single origin of replication and eukaryotes have linear DNA, so they often have multiple points of replication. Most eukaryotic cells do not have telomeres which protect the ends of the chromosomes.