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Direction of DNA replication

  1. May 23, 2017 #1
    I am a medical student with an upcoming biochemistry exam, and I'm struggling with the concept of leading and lagging strands.
    My question is: Can the leading and lagging strand role be switched depending on the direction we look at the dsDNA? Or is the leading and lagging strand predetermined based on a set primer (or similar) on the leading strand that does not occur on the lagging strand, making it impossible for the lagging strand to be a leading strand?
  2. jcsd
  3. May 23, 2017 #2
    Not sure what you mean by dsDNA?

    But this might help

    DNA strands have a directionality, and the different ends of a single strand are called the "3' (three-prime) end" and the "5' (five-prime) end". By convention, if the base sequence of a single strand of DNA is given, the left end of the sequence is the 5' end, while the right end of the sequence is the 3' end. The strands of the double helix are anti-parallel with one being 5' to 3', and the opposite strand 3' to 5'. These terms refer to the carbon atom in deoxyribose to which the next phosphate in the chain attaches. Directionality has consequences in DNA synthesis, because DNA polymerase can synthesize DNA in only one direction by adding nucleotides to the 3' end of a DNA strand

    The 5' and 3' mean "five prime" and "three prime", which indicate the carbon numbers in the DNA's sugar backbone. The 5' carbon has a phosphate group attached to it and the 3' carbon a hydroxyl group. This asymmetry gives a DNA strand a "direction". For example, DNA polymerase works in a 5' -> 3' direction, that is, it adds nucleotides to the 3' end of the molecule (the -OH group), thus advancing to that direction.

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  4. May 25, 2017 #3


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    Consider a replication bubble which forms at origins of replications and initiates DNA synthesis in both directions:

    As you can see, the top strand templates leading strand synthesis for the replication complex that is traveling to the left, but it templates lagging strand synthesis for the replication complex traveling to the right. The same goes with the bottom strand which templates lagging strand synthesis in one direction, but leading strand synthesis in the other direction.
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