DNA replication

• The first step in DNA replication is the unwinding of the double helix structure by the enzyme helicase.
• The leading strand is synthesized continuously, while the lagging strand is synthesized discontinuously as short fragments called Okazaki fragments.
• DNA polymerases are responsible for adding nucleotides to the growing chain during DNA replication.
• A replisome is a molecular machine of enzymes that replicate DNA
• As the DNA helicase unwinds the double helix, RNA primase lays down RNA primer
• DNA polymerase III synthesizes DNA by adding nucleotides to new DNA strand in 5-3 direction
• DNA polymerase I removes RNA primer of Okazaki fragments and fills in gaps with DNA
• DNA ligase joins the Okazaki fragments together by reforming the phosphodiester bonds
• Single strand binding protein stabilizes single stranded DNA before replication by preventing reannealing and degredation of individual strands
• The sliding clamp attaches DNA polymerase III to the DNA template so replication is more efficient
• parental strand can unwind by breaking the hydrogen bonds between the bases
• in semi-conservative replication each daughter strand remains paired with a complementary parent strand
• in the conservative model of replication both daughter strands pair up
• in the dispersion model of replication the daughter strands have a mixture of parental and newly synthesized DNA
• nucleotides are added to the new strand at the 3-OH end
• hydrolysis of phosphate provides energy for the formation of new phosphodiester bonds
• DNA polymerases require an RNA primer to synthesize a new DNA strand
• initiation in bacterial chromosomes is the unwinding and separation of template DNA strands at the origin of replication site
• elongation in bacterial chromosomes is the simultaneous synthesis of two DNA strands by DNA polymerase
• the end replication problem is when the 3 end is not replicated because of the need for RNA primer
• telomeres are the solution to the end replication problem as they are worn away after each replication and once gone the cell stops dividing
• telomerase is an enzyme that restores shortened telomeres but is not present in most eukaryotic cells
• DNA topoisomerase removes super coils that form ahead of the fork and relieves torque