Introductory Biology

9.3 The Replication of DNA

The DNA inside of you has been copied almost perfectly from 4 billion years ago. There is continuity and change in DNA, where mainly it is continuity but a little bit of mutation is involved. When the mother has a offspring, the information is passed on by either re-shuffling or mutation. In every cell, there are about 3 billion nucleotides.

DNA replication is proceeded when the two strands separate and each parent strand acts like a template for the newly synthesized daughter strand, which produces an exact copy of the original form. All of these nucleotides can only be added to the sequence of the reflecting parent strand that result two identical daughter strands and so you are basically cloning the information in yourself. To replicate DNA, you need all four nucleotides because if any one part is left out, DNA replication cannot be done.

The replication of DNA takes place in the cell cycle, during the s phase of interphase in eukaryotes. First, the DNA helicases unwind/unzip the double helix and produce replication forks (the place where two strands are separated) as they break hydrogen bonds between the complementary base pairs, to separate the two strands of the double helix. Next, the DNA polymerases move on each of the DNA strands as it adds complementary nucleotides to the nitrogen bases that re exposed, using the base pairing rules (A=T G=C). Lastly, the DNA polymerases continue to copy until a replication of the original DNA double helix has been made into two identical copies.

DNA polymerases checks for errors in the strands and proofread the newly synthesized strands. They can remove the incorrect nucleotides by replacing it with the correct complementary base. This enzyme helps decrease the rate of mutation, which has an error rate of 1 per billion nucleotides. Though mutation can make variations, which can lead to natural selections.

Multiple replication forks increase the speed of replication in eukaryotes. In a eukaryotic cell each chromosome is replicated in about 100 sections (each about 100,000 nucleotides long), for humans and is a 8 hour long process.  Bacterias (prokaryotes) only have one starting point and two replication forks because it contains so little DNA and only takes about 20 minutes.

Matt Meselson and Frank Stahi had suggested that during the DNA replication, each strand separates and acts as a template. In 1958, they published the results using the nitrogen isotopes with different densities. The first generation (Generation 0) contains N15, a heavy isotope of nitrogen. Generation 1 has N14, an intermediate weighted isotope of nitrogen and a higher band in the tube. Generation 2 is half and half of intermediate and light. Generation 3 is more in the light form. This is called a semi-conservative replication, where one old half of strand makes it equal to another new strand. The heavier strand is closer to the bottom of the tube. Other forms of replication include conservative replication and disperative replication. Conservative replication has one completely old strand and another completely new strand.

One thought on “9.3 The Replication of DNA

  1. Yurika,

    Excellent entry for 9.3 on DNA replication. You’ve covered the main ideas of how genetic information is transferred well by giving a lot of detail.

    Mr. F.

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