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DNA ligase seals the nicks between Okazaki fragments by forming phosphodiester bonds.
Ref: NCERT Class 12 Biology Chapter 5: Molecular Basis of Inheritance DNA Replication - Semiconservative and Machinery
DNA polymerase III synthesizes the lagging strand in short Okazaki fragments during DNA replication.
Ref: NCERT Class 12 Biology Chapter 5: Molecular Basis of Inheritance DNA Replication - Semiconservative and Machinery
DNA ligase joins Okazaki fragments by forming phosphodiester bonds between them.
Ref: NCERT Class 12 Biology Chapter 5: Molecular Basis of Inheritance DNA Replication - Semiconservative and Machinery
Okazaki fragment synthesis inevitably leaves gaps between fragments after primer removal. Pol I synthesizes DNA to fill those short single-strand gaps using upstream 3' OH as primer ensuring precise replacement of RNA with DNA in 5' to 3' direction. Once filling complete, adjacent fragments remain connected by single phosphodiester nick because polymerase cannot create final bond between 3' hydroxyl and 5' phosphate. DNA ligase using NAD+ cofactor in bacteria hydrolyzes to close nick forming continuous phosphodiester backbone. Requirement for both enzymes couples gap filling and final ligation.
Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 5: Joining Okazaki fragments by Pol I and DNA ligase
Okazaki fragments, 1000 to 2000 nucleotides long in bacteria and 100 to 250 in eukaryotes, are short nascent DNA segments observed during brief pulse labeling experiments. They appear exclusively on lagging strand because its template synthesized opposite to fork movement. Each fragment begins with 10 to 12 nucleotide RNA primer made by primase that provides essential 3' OH for Pol III extension. Polymerase extends until reaching previous fragment then displaces or awaits processing. Leading strand shows no such fragments under normal physiological conditions.
Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 5: Okazaki fragments on lagging strand
Both template strands are antiparallel, but replication fork moves unidirectionally. Leading strand polymerase follows helicase smoothly synthesizing continuously. Lagging strand template runs 5' to 3' toward fork, requiring polymerase to synthesize away from fork advancement. As helicase unwinds parental duplex, new single-stranded DNA exposed discontinuously behind fork. Primase synthesizes short RNA primers at intervals, Pol III extends each into Okazaki fragment that later becomes processed and joined. This semi-discontinuous mode elegantly solves antiparallel constraint without violating fundamental 5' to 3' polymerization chemistry.
Ref: NCBI Bookshelf, Molecular Cell Biology, Lodish, Figure 11-21: Discontinuous synthesis of lagging strand
