In DNI semi-conservative replication the template strand goes in direction 3' -5', while the leading strand goes in direction 5' - 3'. Adenine pairs with timine, and cytocine pairs with guanine. The complementary strand in this example is 3'- G C G T A C A T C G C T -5'.
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DNI replication is semi-conservative because each new molecule carries an original DNI strand and a new one. The fact that the new molecule is composed of an original strand makes it semi-conservative. The old existing strands are used to synthesize the new complementary strand.
If the original strand -template strand- goes in direction 3´ to 5´, the complementary strand -leading strand- grows continuously in direction 5´ to 3´.
So here we have the leading DNI strand.
5'- C G C A T G T A G C G A -3'
First, we will place the bases that pair with the ones of this strand.
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Nitrogenated bases form nucleic acids correspond to purines and pyrimidines.
- Adenine and guanine derive from purines,
- Timine and Citocine derive from Pyrimidines.
In the DNA molecule,
- Adenine (Purine) forms pairs with Timine (Pyrimidine) ⇒ Two hydrogen bonds join A-T
- Guanine (Purin) pairs with Cytosine ⇒ Three hydrogen bonds join G-C.
- leading strand C G C A T G T A G C G A
- original strand G C G T A C A T C G C T
Once we pair the bases, we need to recognize the orientation of the strains.
From the left to the right,
- the leading strand ⇒ 5' → 3'
- the original strand ⇒ 3' → 5'
- leading strand 5'- C G C A T G T A G C G A -3'
- original strand 3'- G C G T A C A T C G C T -5'
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