Reading Exercise: The Structure of DNA

DNA is found inside the nucleus of each cell. One very long, coiled up molecule of DNA is called a chromosome. Human cells have 46 chromosomes in total.

Each DNA molecule contains two strands that are connected by a pair of substances called bases. It looks like a ladder, where the bases form the rungs. In addition, the ladder is wound and looks a bit like a spiral staircase. We call this wound-ladder structure of DNA a “double helix”.

This double helix structure of DNA was discovered by the British scientists James Watson and Francis Crick who received the Nobel Prize for their work in 1962.

There are four bases in DNA, adenine, thymine, cytosine and guanine, normally just called A, T, C and G. When forming pairs to make the rungs of the ladder, A always pairs with T and C with G. We call this complementary base pairs. The base pairs are held together by a weak attraction called hydrogen bonding.

Furthermore, each base is attached to a sugar which in turn bonds to a phosphate group. The sugars and phosphate form the backbone of the DNA strands.

A gene is one section of DNA that codes for one single characteristic or protein. We all have very small differences in our genes caused by slightly different orders of the bases in our DNA. This means that everyone’s DNA is unique. It allows scientists to match DNA from cells to specific people. For example, it helps scientists to find out how people are related or it can be used by forensic scientists to identify criminals.


  1. Where is DNA found?
  2. What is a chromosome? How many chromosomes do humans have?
  3. What does the structure of DNA look like? What do we call it?
  4. Who discovered the structure of DNA?
  5. Name the four bases in DNA.
  6. What is complimentary base pairing?
  7. What holds the base pairs together?
  8. What is a gene?
  9. Why is everyone’s DNA unique?
  10. How can the knowledge that everyone’s DNA is unique help scientists?


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