# DNA Replication The process by which a DNA molecule makes an identical copy of itself is called **DNA replication**. This is a form of self-synthesis that occurs during the S-phase of the cell cycle. The original molecule is called the **parent DNA**, and the two new molecules are called **daughter DNA**. DNA replication ensures that genetic information is passed from one cell generation to the next. For more details on how DNA serves as the hereditary material, refer to the chapter on DNA as the Hereditary Material. ## Models of DNA Replication Three models were initially proposed to explain how DNA could replicate. | Model | Description | Outcome | | :--- | :--- | :--- | | **Semi-conservative** | The two parental DNA strands separate, and each serves as a template for a new complementary strand. | Each new DNA double helix consists of one **parental** (old) strand and one **newly synthesized** strand. (This model was proven correct) | | **Conservative** | The entire parental DNA double helix serves as a template for a completely new double helix. | One daughter molecule contains both original parental strands, and the other daughter molecule contains two entirely new strands. | | **Dispersive** | The parental DNA molecule is broken into fragments, and these fragments are used alongside new nucleotides to synthesize two new DNA molecules. | Both daughter DNA molecules are a mix of old and new DNA fragments interspersed along each strand. | ## The Meselson and Stahl Experiment (1958) This experiment was designed by Matthew Meselson and Franklin Stahl to determine which of the three replication models was correct. Their results conclusively supported the **semi-conservative model**. **Methodology:** 1. *E. coli* bacteria were grown for several generations in a medium containing a heavy isotope of nitrogen, $^{15}\mathrm{~N}$. This heavy nitrogen was incorporated into the DNA of the bacteria. 2. The bacteria were then transferred to a new medium containing a lighter isotope, $^{14}\mathrm{~N}$. 3. Samples of bacteria were collected at different time points: - **0 minutes:** Immediately after transfer (parent generation) - **20 minutes:** After one round of replication (first generation) - **40 minutes:** After a second round of replication (second generation) 4. DNA from each sample was extracted and centrifuged in a **cesium chloride (CsCl) density gradient**. This technique separates molecules based on their density. **Results and Interpretation:** | Sample Time | DNA Composition | Centrifuge Result | Interpretation | | :--- | :--- | :--- | :--- | | **Control ($^{14}\mathrm{~N}$ only)** | Both strands are light ($^{14}\mathrm{~N}$). | A single light band at the top. | Establishes the position of light DNA. | | **0 minutes ($^{15}\mathrm{~N}$ only)** | Both strands are heavy ($^{15}\mathrm{~N}$). | A single heavy band at the bottom. | Establishes the position of heavy DNA. | | **20 minutes (1st gen.)** | Hybrid DNA (one $^{15}\mathrm{~N}$ strand, one $^{14}\mathrm{~N}$ strand). | A single intermediate band. | Rules out the **conservative model**, which would predict one heavy and one light band. | | **40 minutes (2nd gen.)** | Half hybrid DNA, half light DNA. | Two bands: one intermediate and one light. | Conclusively supports the **semi-conservative model**. The **dispersive model** would have predicted a single band that gradually became lighter over generations. | ## Mechanism of DNA Replication DNA replication is a complex process involving multiple enzymes and proteins, divided into three main phases. ### 1. Initiation Phase - **Origin of Replication:** Replication begins at specific nucleotide sequences on the DNA molecule. Prokaryotes typically have one origin, while eukaryotes have multiple origins. - **Replication Bubble:** Enzymes unwind the DNA at the origin, creating a "bubble." Each end of the bubble is a Y-shaped **replication fork**. - **Key Enzymes and Proteins:** - **DNA Helicase:** Unwinds the DNA double helix by breaking the hydrogen bonds between base pairs. - **DNA Gyrase (a topoisomerase):** Relieves the tension and supercoiling created ahead of the replication fork as the DNA unwinds. - **Single-Stranded Binding (SSB) Proteins:** Bind to the separated DNA strands to prevent them from re-annealing (pairing up again). ### 2. Extension/Polymerization Phase - This phase involves the synthesis of new DNA strands using the original strands as templates. - **Primer and Primase:** - **DNA Polymerase** can only add nucleotides to an existing 3'-OH group; it cannot start a new strand from scratch. - **Primase** (an RNA polymerase) synthesizes a short RNA sequence called a **primer**, which provides the necessary 3'-OH starting point. **DNA Polymerases:** | Enzyme | Function | | :--- | :--- | | **DNA Polymerase I** | Replaces the RNA primers with DNA nucleotides during the termination phase. | | **DNA Polymerase II** | Involved in repairing DNA damage. | | **DNA Polymerase III** | The primary enzyme for synthesizing the new DNA strands. It adds nucleotides in a **5' to 3' direction**. It also has a **proofreading** function to remove incorrectly added nucleotides. | - **Leading vs. Lagging Strand:** - Because DNA strands are anti-parallel and synthesis only occurs in the 5' to 3' direction, the two new strands are synthesized differently. - **Leading Strand:** Synthesized **continuously** in one long piece, moving towards the replication fork. - **Lagging Strand:** Synthesized **discontinuously** in short segments, moving away from the replication fork. These short segments are called **Okazaki fragments**. A new primer is needed for each fragment. ### 3. Termination Phase - The final step to complete the two new DNA molecules. - **DNA Polymerase I:** Removes the RNA primers from the lagging strand (and the single primer from the leading strand) and fills the resulting gaps with DNA nucleotides. - **DNA Ligase:** Joins the Okazaki fragments on the lagging strand by forming phosphodiester bonds, creating a continuous strand. ## Possible Questions and Answers **Q: What is semi-conservative replication?** A: It is the process where each new DNA molecule consists of one original (parental) strand and one newly synthesized strand. **Q: Why is a primer necessary for DNA replication?** A: DNA Polymerase III can only add nucleotides to a pre-existing 3'-OH group. The RNA primer, synthesized by primase, provides this starting point. **Q: What is the difference between the leading and lagging strands?** A: The **leading strand** is synthesized continuously in the 5' to 3' direction towards the replication fork. The **lagging strand** is synthesized discontinuously in short Okazaki fragments, also in a 5' to 3' direction, but away from the replication fork. **Q: What are Okazaki fragments?** A: Short, newly synthesized DNA fragments that are formed on the lagging template strand during DNA replication. They are later joined together by DNA ligase.