The DNA replication is semi-conservative is proved by an experiment conducted by Meselson and Stahl in 1958. To perform their experiment they use heavy nitrogen (`""^(15)N`) in E. coli. The process of replication. in living cells requires a set of enzymes. The main enzyme is DNA dependent DNA polymerase. The DNA-A dependent DNA polymerase catalyse polymerization only in one direction, that is `5 to 3'`. In eukaryotes, the replication of DNA takes place at the S-phase of the cell cycle.
DNA replication is:

Options

A. Semi-conservative, continuous
B. Semi-continuous, conservative
C. Semi-conservative, semi-discontinuous
D. Conservative

Explanation

**Step-by-Step Solution:**

1. **Understanding Semi-Conservative Replication**:
- DNA replication is termed semi-conservative because each new DNA molecule consists of one original (parental) strand and one newly synthesized strand. This was demonstrated by Meselson and Stahl in their 1958 experiment using heavy nitrogen isotopes.

2. **Meselson and Stahl's Experiment**:
- They used heavy nitrogen (`15N`) to label the DNA in E. coli. When these bacteria replicated, the new DNA strands incorporated lighter nitrogen (`14N`), allowing the researchers to distinguish between the old and new strands through density gradient centrifugation.

3. **Role of Enzymes in DNA Replication**:
- The process of DNA replication requires various enzymes, with DNA-dependent DNA polymerase being the main enzyme. This enzyme catalyzes the addition of nucleotides to the growing DNA strand.

4. **Direction of Polymerization**:
- DNA polymerase synthesizes DNA in a specific direction: from the 5' end to the 3' end. This means that nucleotides are added to the 3' end of the growing strand.

5. **S-Phase of Cell Cycle**:
- In eukaryotic cells, DNA replication occurs during the S-phase (Synthesis phase) of the cell cycle, which is when the cell prepares to divide.

6. **Leading and Lagging Strands**:
- During replication, one strand (the leading strand) is synthesized continuously in the 5' to 3' direction, while the other strand (the lagging strand) is synthesized discontinuously in short segments known as Okazaki fragments.

7. **Conclusion**:
- Therefore, DNA replication is characterized as semi-conservative (each new DNA molecule contains one old and one new strand) and semi-discontinuous (the leading strand is continuous, while the lagging strand is made in fragments).

**Final Answer**: DNA replication is semi-conservative and semi-discontinuous.

---