Structure of DNA

Learning Objectives

By the end of this section, you will begin to: Discuss and apply biological theories and concepts of the molecular and genetic of life, including cell division, inheritance, and gene regulation by showing that you can:

  • Describe the structure of DNA.

DNA is the genetic material passed from parent to offspring for all life on Earth. The three letters “DNA” have now become associated with crime solving, paternity testing, human identification, and genetic testing. DNA can be retrieved from hair, blood, or saliva. With the exception of identical twins, each person’s DNA is unique and it is possible to detect differences between human beings on the basis of their unique DNA sequence.

DNA analysis has many practical applications beyond forensics and paternity testing. DNA testing is used for tracing genealogy and identifying pathogens. In the medical field, DNA is used in diagnostics, new vaccine development, and cancer therapy. It is now possible to determine predisposition to many diseases by analyzing genes.

In the 1950s, Francis Crick and James Watson worked together to determine the structure of DNA at the University of Cambridge, England. Scientists Linus Pauling and Maurice Wilkins were also actively exploring this field. Pauling had discovered the secondary structure of proteins using X-ray crystallography. In Wilkins’ lab, researcher Rosalind Franklin was using X-ray diffraction methods to understand the structure of DNA. Watson and Crick were able to piece together the puzzle of the DNA molecule on the basis of Franklin’s data because Crick had also studied X-ray diffraction (Figure 1), however, Watson and Crick gained access to Franklin’s data without her knowledge or approval. In 1962, James Watson, Francis Crick, and Maurice Wilkins were awarded the Nobel Prize in Medicine. Unfortunately, by then Franklin had died of ovarian cancer, likely caused by exposure to X-rays, and Nobel prizes are not awarded posthumously (after death). This is actually a really interesting story of sexism in the sciences and there’s a movie called “The Secret of Photo 51” that you can find on YouTube if you’re interested in learning more.

Figure 1 (a) James Watson, Francis Crick, and Maclyn McCarty  (b) the X-ray diffraction pattern of DNA
Figure 1 (a) James Watson, Francis Crick, and Maclyn McCarty (b) the X-ray diffraction pattern of DNA created by Rosalind Franklin (credit a: modification of work by Marjorie McCarty, Public Library of Science)

Based on Rosalind Franklin’s X-ray diffraction photograph, and work by other scientists, Watson and Crick proposed that DNA is made up of two strands of nucleotides that are twisted around each other to form a right-handed helix. The nucleotides are joined together in a chain by covalent bonds known as phosphodiester bonds. Scientists already knew that nucleotides contain the same three important components: a nitrogenous base, a deoxyribose (5-carbon) sugar, and a phosphate group. The nucleotide is named depending on the nitrogenous base: adenine (A), thymine (T), cytosine (C), and guanine (G). Watson and Crick had key pieces of information available from other researchers such as Chargaff’s rules. Chargaff had shown that of the four kinds of monomers (nucleotides) present in a DNA molecule, two types were always present in equal amounts and the remaining two types were also always present in equal amounts. This meant they were always paired in some way, which help scientists figure out the overall structure.


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OpenStax, Concepts of Biology. OpenStax CNX. May 18, 2016


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Mt Hood Community College Biology 102 Copyright © 2016 by Lisa Bartee and Christine Anderson is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.


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