DNA: Structure & Function

Taryn VanderVelde

Introduction

The structure and function of DNA (or deoxyribonucleic acid) has quite an extensive background. It all started with a man by the name of Gregor Mendel around 150 years ago. Mendel hypothesized the existence of a molecule that passes genetic information from generation to generation. A couple of decades later, scientists discovered DNA. Scientists also sequenced the genome of a lot of species (humans, crops, etc). This helped to understand diseases such as breast cancer, cystic fibrosis, and muscular dystrophy, which are all caused by genetic mutations. DNA has made it possible to determine lineage (for example, King Tut), solve crimes, and convict criminals.

Frederick Meischer

In 1868, Frederick Meischer decided to examine the pus from his patients. Within the pus, he found a substance in the white blood cells which he called ‘nuclein’. At this time, people thought the hereditary material was proteins, but Meischer thought that his discovery could actually be the hereditary material. The composition and function of this substance was not discovered for another 50 years and the structure was not determined until years after that.

Frederick Griffiths

During World War I, a pneumonia epidemic struck Europe and Frederick Griffiths decided to research the disease. He conducted an experiment in 1928 using two strains of pneumonia bacterium, the S-strain and the R-strain. The S-strain had a capsule that surrounded each cells, which made the bacterial colonies look smooth. The cells from the R-strain did not have a capsule, so the bacterial colonies looked rough and irregular. When Griffiths injected mice with the S-strain, they died from pneumonia, but when he injected them with the R-strain, they lived with no signs of pneumonia. After this, he injected mice with heat-killed S-strain cells and the mice lived, which shows that the S-strain cells must be alive in order to kill the mice. Mice injected with the heat-killed S-strain cells and the live R-strain cells died. This shows that live R-strain cells changed to deathly S-strain cells with some factor from the dead S-strain cells. Even though Griffith could not explain exactly what was going on, he knew that some kind of hereditary substance had passed from the dead S-strain cells to the live R-strain cells. He called it transformation and he called the factor that was responsible the transforming principle.

Avery, McLeod, and McCarty

Oswald Avery, Colin McLeod and Maclyn McCarty together carried out experiments that were similar to Frederick Griffiths. They used S-strain and R-strain bacteria as well, but they treated the dead bacteria from the heat-killed S-strain with DNA, RNA, and proteins because they knew that these were the possible transforming substances. They did this to destroy one of the three enzymes to show which one was the transforming substance. Their results clearly showed that DNA was the transforming substance, but they didn’t want to make their results known because everyone still thought that proteins were the transforming substance

Hershey and Chase

Alfred D. Hershey and Martha Chase wanted to find out whether proteins or DNA functioned as the genetic material. They conducted an experiment using bacteriophage - the outside of bacteriophage is made of proteins and the inside of it has DNA. DNA molecules contain phosphorus and proteins only have a little bit. They dyed the proteins with radioactive sulphur and the DNA with radioactive phosphorus. The radioactive material remained outside of the protein cell, but the radioactive DNA had entered the cells. Hershey and Chase concluded that DNA carried genetic information and proteins didn’t.

Wilkins and Franklin

Rosalind Franklin and Maurice Wilkins used X-ray crystallography to study the shape of the DNA molecule. Wilkins DNA was not prepared properly, so he didn't get any good results from the crystallography. However, Franklin had some really well prepared DNA samples and she discovered that the pattern that the X-rays produced was in the shape of an X. She suggested that DNA was a double helix and it rotated in a clockwise direction. She also suggested that the sugar-phosphate backbones of DNA faced the outside and not the inside of the molecule. She could not explain how the nitrogenous bases were connected to the center of the helix, which is the reason why she didn’t want to publish her findings.

Watson and Crick

Without telling her, Wilkins gave information from Franklin’s discoveries to James Watson. Using Franklin’s discoveries, Watson and Crick built a DNA molecule and they claimed that the information was their own

Their model showed that:

Each strand of the helix had a phosphate and sugar backbone
Nitrogenous bases were attached to backbone and directed toward the centre
Strands twisted around in clockwise direction
In the centre, each nitrogenous base on the one strand was bonded with another nitrogenous base on the other strand

Their model proved that DNA could only be stable if the strands ran antiparallel (opposite directions). The model also proved that the nitrogenous bases are connected by hydrogen bonds, which kept the two strands together. The purine and pyrimidine base pairs are linked to each other - thymine bonds with adenine by two hydrogen bonds and guanine bonds with cytosine by three hydrogen bonds, which is called complementary base pairing. The symmetry of DNA is the key to it's structure and its ability to divide and convey genetic information. Watson and Crick’s model revolutionized our understanding of life and led to many other scientific breakthroughs

The Chemical Composition of DNA

Scientists were still trying to figure out the molecular composition of Frederick Meischer’s nuclein. In 1920, while trying to figure out the molecular composition, a man by the name of Phoebus Levene said that the DNA molecules contain three major components: deoxyribose sugars, phosphate groups, and nitrogenous bases. Each nucleotide sub-unit contains a nitrogenous base which is attached to a deoxyribose sugar, which is connected to a phosphate group. The four nitrogenous bases were identified as Adenine (A), and Guanine (G), which were both double-ring structures called purines and Thymine (T) and Cytosine ( C ) which were both single-ring structures called pyrimidines. Edward Chargaff discovered that human DNA contained 30.9% A, 29.4% T, 19.9% G and 19.8% C. This information was vital for future research. At this time, scientists still didn’t know the structure of DNA molecules, how cells produced DNA, and how DNA was passed from parents to offspring.

References

DiGiuseppe, M., & Adam-Carr, C. (2012). Nelson biology 12: university preparation. Scarborough, ON: Nelson Thomson Learning.

The Editors of Encyclopædia Britannica. (2017, May 01). DNA. Retrieved November, 2017, from https://www.britannica.com/science/DNA

Pray, L. A. (n.d.). Discovery of DNA Double Helix. Retrieved November, 2017, from https://www.nature.com/scitable/topicpage/discovery-of-dna-structure-and-function-watson-397

Rettner, R. (2013, June 06). DNA: Definition, Structure & Discovery. Retrieved November, 2017, from https://www.livescience.com/37247-dna.html

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