DNA Replication-Process of duplication of DNA

DNA Replication

DNA replication is the process by which a cell duplicates its DNA in preparation for cell division. The process involves a series of steps that are carried out by a variety of enzymes and proteins.

Steps in DNA replication

Steps In DNA Replication
Pic: Steps In DNA Replication, photo Credit:pixabay.com
  • Initiation:
    • The process of DNA replication begins at specific sites on the DNA molecule called origins of replication. At these sites, the double helix is unwound by an enzyme called helicase, which creates a replication fork.
  • Priming:
    • The next step is the priming of the DNA strands. The enzyme primase synthesizes short RNA primers that provide a starting point for DNA synthesis.
  • Elongation:
    • Leading and Lagging strands: DNA polymerase synthesizes the new DNA strand in the 5' to 3' direction. This means that one of the strands, called the leading strand, is synthesized continuously in the direction of the replication fork. The other strand, called the lagging strand, is synthesized in fragments that are later joined together.
  • Okazaki fragments:
    • The fragments that make up the lagging strand are called Okazaki fragments. These fragments are short stretches of DNA that are synthesized in the opposite direction to the movement of the replication fork.
  • Joining of fragments:
    • The fragments of the lagging strand are joined together by an enzyme called ligase, which seals the gaps between the fragments.
  • Termination:
    • DNA replication continues until the replication forks meet at a specific site on the DNA molecule called the termination site. At this point, the process of DNA replication is complete, and the cell has duplicated its DNA in preparation for cell division.

DNA replication is Semi conservative?

This means that each newly synthesized DNA molecule contains one strand from the original DNA molecule and one newly synthesized strand. This was first demonstrated by Meselson and Stahl in 1958 through a series of experiments. In their experiment, they grew E. coli bacteria in a medium containing a heavy isotope of nitrogen, 15N. The bacteria incorporated this heavy nitrogen isotope into their DNA, making it heavier than normal DNA. They then transferred the bacteria to a medium containing a lighter isotope of nitrogen, 14N, and allowed them to divide and replicate their DNA. Meselson and Stahl then analyzed the DNA using a technique called density gradient centrifugation, which separates DNA molecules based on their density. If DNA replication were conservative, the original heavy DNA molecule would have remained intact, and a new light DNA molecule would have been synthesized. On the other hand, if DNA replication were dispersive, the heavy and light isotopes would have been mixed, and the DNA molecules would have contained a mixture of both. However, Meselson and Stahl found that the DNA molecules formed two distinct bands, one containing heavy DNA and one containing hybrid DNA molecules with both heavy and light nitrogen. This result supported the hypothesis of semiconservative replication, which predicted that each new DNA molecule would consist of one heavy strand and one light strand. This finding was later confirmed by several other experiments and is now widely accepted as the mechanism of DNA replication in all living organisms.

Significance of DNA Replication

The accurate replication of DNA is essential for the proper functioning of cells and the prevention of mutations that can lead to genetic disorders and diseases such as cancer. Mutations that occur during DNA replication can lead to changes in the genetic code, which can affect the structure and function of proteins and other molecules. This, in turn, can disrupt cellular processes and lead to diseases.

Conclusion

DNA replication is a complex process that involves a series of steps that are carried out by a variety of enzymes and proteins. The process is essential for the transfer of genetic information from one generation to the next and is highly regulated to ensure that the genetic code is faithfully copied. The understanding of the steps of DNA replication is crucial for researchers to develop new treatments for genetic diseases and to gain insight into the evolution of life on earth.

References

  • Kornberg, A., & Baker, T. A. (1992). DNA replication (2nd ed.). W. H. Freeman.
  • Alberts, B., Johnson, A., Lewis, J., Raff, M., Roberts, K., and Walter, P. (2002). Molecular biology of the cell (4th ed.). Garland Science.
  • Bell, S. P., & Dutta, A. (2002). DNA replication in eukaryotic cells. Annual Review of Biochemistry, 71, 333-374.
  • Kunkel, T. A., & Erie, D. A. (2015). DNA mismatch repair. Annual Review of Biochemistry, 84, 193-217.

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