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DNA Replication (Semi Conservative Model)

DNA replication follows a semi-conservative pattern. As each strand in the double helix is synthesized, a complementary strand is synthesized.

DNA replication

  • DNA replication follows a semi-conservative pattern. As each strand in the double helix is synthesized, a complementary strand is synthesized.
  • As DNA slowly synthesizes in a 5' to 3' direction, it is made by an enzyme called DNA polymerase. DNA polymerases require a template and a primer (starter).
  • The leading strand of DNA is made from a continuous piece of DNA during replication. A small piece of the other strand is made up of lagging.
  • Replication of DNA is not solely dependent on DNA polymerase. Additionally, it is required by DNA primase, DNA helicase, DNA ligase, and topoisomerase.
It is difficult to replicate DNA, or to copy a cell's DNA! When one of your trillions of cells divides, your genome's 3 billion base pairs must be copied perfectly every time. DNA replication mechanisms are the same for all organisms.

As DNA replication occurs semi-conservatively, every strand in the double helix serves as a template for the synthesis of a new, complementary strand. The process goes from a single starting molecule to two daughters, with each of the new double helices consisting of one new strand and one old strand.

In simple words, all about DNA replication is all given above but the best part is when the part about the process in detail starts. To avoid the problems like the disease called cancer, cells need to be very quick to make DNA with minimum errors possible. The process of DNA replication is made smooth and accurate by the combination of various enzymes and proteins.

DNA polymerase

It is essential for DNA replication that DNA polymerase is present. A DNA polymerase synthesizes DNA by adding one nucleotide at a time to the chain while incorporating only nucleotides that are complementary to those in the template.


Given below are some of the important key features of DNA polymerase:
  • There is always a need for templates.
  • A DNA strand can only be extended 3' by adding nucleotides.
  • Thus, to make a DNA chain from scratch, scientists must start with a pre-existing nucleotide chain or short stretch of nucleotides called a primer.
  • A proofreader removes almost all of the "wrong" nucleotides accidentally added to a chain of nucleotides.
Nucleotides require energy to be added. Nucleotides contain three phosphates, and, like ATP, they carry energy via these phosphates. An incoming nucleotide is used to form a bond with the growing chain after the phosphate bond is broken.

The replication of DNA takes place by two DNA polymerases in prokaryotes such as E.coli - DNA Pol I, and DNA Pol III. DNA Pol III plays a very important role in making DNA.

Replication of DNA

The DNA replication process starts at specific places on the chromosome, known as the origins of replication, which can be identified by their sequence. A chromosome of E. coli has only one point of replication, like most bacteria. A/T base pairs make up most of the DNA origin's 245 base pairs (in which there are fewer hydrogen bonds than G/C base pairs), which makes separating these DNA strands easier. A specific protein recognizes the origin, binds to the site, and opens the DNA. Two Y-shaped structures in the DNA called replication forks create a replication bubble. As replication progresses, replication forks will move in opposing directions.


The first replication enzyme that loads at replication's origin is Helicase. The Helicase enzyme's job is to "unwind" DNA (break hydrogen bonds between the nitrogenous base pairs) to advance replication forks. Separated strands of DNA are bound together by proteins called single-strand binding proteins, preventing double helices from forming.

Primer and primase

Adding nucleotides to an existing DNA strand can only be achieved by DNA polymerases. A DNA polymerase adds the first nucleotide to the 4' end of the DNA molecule in a replication fork, by attaching a nucleotide to the -OH group at the 3' end. The enzyme named primase makes it simpler. As RNA primers, DNA polymerases can work on short stretches of complementary RNA that provide 3 'ends for DNA polymerase to work on. Primers typically have a length of five to ten nucleotides. Primers initiate DNA synthesis, that is, get it going. A DNA strand is created by extending RNA primers one by one after DNA polymerase extends the template strand.

Leading and lagging strands

Among E. coli's DNA polymerase subunits, DNA polymerase III is responsible for most of the synthesis. Each DNA polymerase III molecule is hard at work on one of the two new DNA strands at a replication fork. The 5'-3' direction is the only direction in which DNA polymerases can make DNA. There is always one strand running from 5' to 3' in DNA double helixes, while the other runs from 3' to 5'. Due to this, the two strands need to be generated in slightly different ways, since they are also antiparallel with their templates. The easy new strand, which runs 5' to 3' toward the replication fork, is running 5' to 3'. DNA polymerase moves in conjunction with the replication fork, creating a continuous strand of DNA. Continuously synthesized is the leading strand.


Another new strand, which runs between 5' and 3' away from the fork, presents more challenges. DNA polymerase must separate from the fork and reattach on the newly exposed strand as the fork advances. This strand is always fragmented. Lagging strands are tricky strands that are made by fragmenting. They are known as Okazaki fragments because they were discovered by a Japanese scientist named Okazaki. One primer can extend the leading strand, but a primer is required for each short Okazaki fragment on the lagging strand.

In addition to the proteins and enzymes mentioned above, DNA replication requires additional enzymes and proteins. The sliding clamp holds DNA polymerase III molecules in place while they synthesize DNA. Slide clamps are ring-shaped proteins that keep the DNA polymerase of lagging strands from floating away when the polymerase re-starts at a new Okazaki fragment. During DNA replication, topoisomerase is also important for maintaining DNA integrity. While the DNA is being opened up, this enzyme prevents the DNA double helix from being tangled up too tightly. During this process, temporary nicks are made in the helix to release tension, and then the nicks are sealed to prevent long-term damage. The final step is to remove any RNA or gaps from the DNA if we want it free of RNA. Another polymerase, DNA polymerase I, get rid of the RNA primers for DNA synthesis. The enzyme DNA ligase seals up the remaining nicks after the primers have been replaced.
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Ankur Choudhary is India's first professional pharmaceutical blogger, author and founder of Pharmaceutical Guidelines, a widely-read pharmaceutical blog since 2008. Sign-up for the free email updates for your daily dose of pharmaceutical tips.
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