Test: Replication - Grade 12 MCQ

- There were three models for how -organisms might replicate their DNA: semi-conservative, conservative, and dispersive.
- The semi-conservative model, in which each strand of DNA serves as a template to make a new, complementary strand, seemed most likely based on DNA's structure.
- The models were tested by Meselson and Stahl, who labeled the DNA of bacteria across generations using isotopes of nitrogen.
- From the patterns of DNA labeling they saw, Meselson and Stahl confirmed that DNA is replicated semi-conservatively.

Semiconservative replication of DNA was first demonstrated in E.coil. According to the semiconservative mode proposed by Waston and Crick, each strand of the two double helices formed would have one old and one new strand. The semiconservative nature of DNA of replication was proved by the experiment of Meselson and Stahl (1958).

Okazaki fragments form during DNA replication because DNA is anti parallel and can only be synthesized in one direction (3' to 5'). Because of this, at each replication fork, there is a leading strand, that is synthesized in the 3' to 5' direction, and a lagging strand, synthesized in the 5' to 3' direction. Cells do not have a mechanism for 5' to 3' DNA synthesis, so instead they use short segments, called Okazaki fragments, of 3' to 5' synthesis, and then join them together.

Thermo Scientific Phusion High-Fidelity DNA Polymerase was created by fusing a dsDNA-binding domain to a Pyrococcus-like proofreading polymerase. Due to this unique fusion technique, Phusion DNA Polymerases have extremely low error rates and are considered a gold standard for high-fidelity PCR.

In 1957, Meselson and Stahl gave experimental evidence for the semi-conservative replication. It was done as follows-

E. coli was grown in a medium having heavy isotope of nitrogen i.e. ¹⁵N nitrogen (heavy). Then it was grown on a medium having ¹⁴N nitrogen (light) and after one, two, or three generations the DNA samples were tested.

They were mixed with cesium chloride and it was centrifuged. After one generation, the double stranded DNA was found to form a band at an intermediate position between the bands of  DNA having ¹⁵N nitrogen below it and  

¹⁴N nitrogen above it. Hence the separation was according to the density of the DNA strand.

Thus the correct answer is option A.

The S phase of a cell cycle occurs during interphase, before mitosis or meiosis, and is responsible for the synthesis or replication of DNA. In this way, the genetic material of a cell is doubled before it enters mitosis or meiosis, allowing there to be enough DNA to be split into daughter cells. The S phase only begins when the cell has passed the G₁​ checkpoint and has grown enough to contain double the DNA. S phase is halted by a protein called p16 until this happens.

The replication occurs in three steps in both prokaryotes and eukaryotes. These are include-
• Initiation
• Elongation
• Termination
The elongation steps involve the synthesis of leading and lagging strands.
The helicase and Dna B unzips the double-stranded DNA for replication, making a forked structure. The primase then performs the task of generating short strands of RNA that bind to the single-stranded DNA to initiate DNA synthesis with the help of DNA polymerase. This enzyme can work only in the 5′5′ to 3′3′ direction, the replication of the leading strand is done continuously whereas lagging-strand replication is discontinuous, with short Okazaki fragments being formed and later linked together.
So, the leading strand is the one that replicates in the 5′→3' direction continuously.

• The E. Coli has a total of 4.6 × 10⁶ base pair. It completes its process of replication within 18 minutes.
• This means that the average rate of polymerisation of E. Coli DNA is 2000 base pairs per second.

Semiconservative replication of DNA was first demonstrated in E.coil. According to the semiconservative mode proposed by Waston and Crick, each strand of the two double helices formed would have one old and one new strand. The semiconservative nature of DNA of replication was proved by the experiment of Meselson and Stahl (1958).

Har Gobind Khorana was awarded the Nobel prize in Medicine in 1968. He introduced the first synthetic gene using DNA ligase and polymerase enzymes.