What are Okazaki fragments and how are they processed?

• A. Long continuous strands on leading strand
• B. RNA sequences at telomeres
• C. Enzymes that unwind the helix
• D. Short RNA-primed DNA segments on lagging strand; primers removed and replaced with DNA; ligated to form continuous strand

Answer: (D)

Okazaki fragments are short RNA-primed DNA segments synthesized on the lagging strand during DNA replication. Because DNA polymerases can only add nucleotides in the 5' to 3' direction, the lagging strand is made discontinuously in these fragments as the fork opens. Each fragment starts with an RNA primer laid down by primase, and DNA polymerase extends the fragment from that primer.

After a fragment is completed, the RNA primer must be removed and replaced with DNA. In bacteria, DNA polymerase I removes the RNA primer and fills in the gap with DNA. In eukaryotes, RNase H (and other nucleases like FEN1) removes the primer, and a DNA polymerase fills in the space. Finally, DNA ligase seals the sugar-phosphate backbone, joining adjacent fragments into one continuous strand.