In the lagging strand, DNA is made in the direction _________ the replication fork and is made as __________.

Question

BlackTom AI · Accepted Answer

DNA synthesis on the lagging strand proceeds away from the replication fork, producing Okazaki fragments.

1. Relevant knowledge points and concepts
- DNA strands are antiparallel with 5' and 3' ends. 
- DNA polymerases synthesize new DNA only in the 5'→3' direction. 
- At a replication fork the two parental strands present opposite orientations; one daughter strand (leading) can be synthesized continuously, the other (lagging) must be synthesized discontinuously.
- Short RNA primers are required to start each fragment; DNA ligase joins fragments into a continuous strand.

2. Step-by-step reasoning process
- The replication fork moves along the parental duplex, exposing two single-stranded templates with opposite polarity.
- Because DNA polymerase can add nucleotides only to a free 3' end (synthesizes 5'→3'), whichever template has its 3' end oriented toward the fork can be copied continuously in the same direction as fork movement — that is the leading strand.
- The opposite template has its 3' end oriented away from the fork; to maintain 5'→3' synthesis on that template, new DNA must be made in short segments that start near the fork and extend away as the fork opens further.
- Each short segment requires a new RNA primer; these segments are the Okazaki fragments and are later joined by ligase.

3. Intermediate steps shown
- Parental template orientations: if the fork opens to the right, the template whose 3'→5' runs to the right is copied continuously (leading). The other template, whose 5'→3' runs to the right, is copied in segments oriented 5'→3' but extending leftward relative to the fork movement — i.e., away from the fork — creating Okazaki fragments.

4. Why the correct answer is correct
- The constraint that polymerase only works 5'→3' forces the lagging strand to be synthesized discontinuously in the direction away from the moving fork. These discontinuous pieces are historically named Okazaki fragments.

5. Why other options are incorrect
- "toward, one continuous strand": describes the leading strand, not the lagging strand.
- "away from, one continuous strand": contradicts the 5'→3' synthesis requirement; continuous synthesis away from the fork would be impossible.
- "toward, Okazaki fragments": Okazaki fragments are produced away from the fork, not toward it.

6. Conclusion
This reasoning confirms the provided correct answer: away from, Okazaki fragments.

In the lagging strand, DNA is made in the direction _ the replication fork and is made as __.单项选择题

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In the lagging strand, DNA is made in the direction _________ the replication fork and is made as __________.单项选择题