3.2.2 DNA

DNA is a polymer made of nucleotides. It is a unique molecule in biological terms in that it can store information. In this respect it is similar to the hard – drive of a computer or an architects diagram for a building.

DNA performs two very special roles:

  1. It is the molecule of inheritance – features are passed from parents to offspring and DNA is the method by which they are transmitted.
  2. It acts as a template, controlling and organising all the proteins in your body.

The examiner will expect you to be able to describe how the structure of DNA allows it to perform these roles successfully.

dnachain2(c) Jim Clark 2007

The diagram above shows the three structural elements of a DNA molecule.

  • In yellow there are the phosphate groups
  • In pink there is the deoxyribose sugar
  • In blue are the 4 nitrogenous bases, adenine, guanine, cytosine and thymine

The phosphate groups and deoxyribose sugars form a strong backbone that wraps around the nitrogenous bases. It is important to protect the bases because they contain coded information that tells the cell which proteins to construct and when to construct them. For this reason the bases are on the inside of the helical structure and thus protected from enzymes and other agents that may disrupt the sequence of bases.

The 4 nitrogenous bases are what allows the molecule to store information. Examiners will likely ask you questions about why the ratio of C:G bases and A:T bases remains constant through all species. This is because C always pairs with G so there must be the same number of C bases as G bases and the same holds true for A and T.

Where there are differences between species it is in the number of C and G bases and the number of A and T bases. One species might have 14% of its’ genome made up of cytosine whereas another species might have 20% of its’ genome made up of cytosine. This is because the order and number of bases on a strand of DNA changes from species to species. Thus DNA is able to encode different information for different species.

Here is a video from our lesson on DNA structure that explains how the structure of DNA relates to its function.

You’ve read through this tutorial – now complete these questions for your homework:

DNA questions

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