DNA Structure and Replication, Polymerase Chain Reaction

Edited by Jamie (ScienceAid Editor), Taylor (ScienceAid Editor)

DNA is an extremely important molecule in Biology. It carries the genetic code, instructs protein synthesis, can replicate itself and is thought to be the basis of all life on earth!


DNA is made up of repeating units called nucleotides they are made up of three different components:

  1. 1
    Deoxyribose sugar.
    5 carbon monosacharide.
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  2. 2
    PO43- molecule
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  3. 3
    A Base.
    The base can either be adenine, thymine, guanine or cytosine (more on these below).
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The nucleotide is represented in the diagram below, where each component is shown by a different shape.

diagram of a nucleotide

The nucleotides form the polymer DNA or deoxyribonucleic acid [dee-ox-ee-ri-bo-new-clay-ik]. Its structure is the double helix. It's best described as a twisted ladder, where the sides are the phosphate-sugar chain, whilst the rungs are the bases.

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The Bases

As was mentioned above, there are 4 different bases in DNA:

  • Adenine
  • Thymine
  • Guanine
  • Cytosine.

They are each abbreviated to their first letter. An important feature of bases in DNA, is complimentary base pairing. This is where only AT and GC will sit next to each other as a 'rung' in the DNA double helix. The reason for this has to do with the hydrogen bonds that hold the two strands together as below:

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two strands of DNA nucleotides

DNA Replication

One of the special characteristics of DNA is that it can replicate. This is useful when making new cells in the cell cycle. The process is said to be semi-conservative, since it conserves (or keeps) one of the original strands in the new DNA molecule.

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diagram of DNA replication
  1. 1
    The first stage is when the two strands in the DNA unwind, and the hydrogen bonds holding them together are broken.
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  2. 2
    Complimentary Base Pairing.
    Next, free nucleotides that are floating around, match up with the bases and complimentary base pairing takes place. As these nucleotides are matched up, an enzyme called DNA polymerase makes them bond to each other, therefore connecting and building the new strand.
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  3. 3
    Finally, the new strand is complete and it twists into the double helix shape!
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Polymerase Chain Reaction (PCR)

Artificial DNA replication often called, molecular photocopying, is a process used by scientists to make copies of DNA that may be needed for genetic testing and research - copying tiny bits of DNA. From a small amount of DNA, you can basically make tons of copies for testing and research.

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The process is quite simple. The following ingredients are added to a test tube:

  1. 1
    The original DNA sample.
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  2. 2
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  3. 3
    DNA polymerase and primers (bind to DNA and instructs replication to begin).
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  • The DNA is heated to 95°C for a few minutes. This causes the DNA strands to separate.
  • Then it is cooled with primers, so they can bind to the DNA.
  • Finally the solution is raised to about 70°C and the polymerase enzyme acts to add the complimentary bases.

This process is repeated many times so that hundreds and thousands of copies are made of this piece of DNA: and there we have it, very simple really.

The enzymes used in this process are from bacteria that live in very warm conditions (by sea vents for example). This is because they have enzymes that easily adapt to the high temperatures, so when the sample is heated to separate DNA, the enzymes will not denature.

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Categories : Genetics

Recent edits by: Jamie (ScienceAid Editor)

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