MOL - DNA/RNA Structure [LESSON]
DNA/RNA Structure
History of DNA
It may surprise you to learn that, not even a century ago, even the most educated members of the scientific community didn’t know that DNA was genetic material. It was widely accepted that proteins would turn out to be the genetic material. Let’s look at some of the experiments that helped to shape our understanding of this concept.
Griffith Experiment
In 1928, British bacteriologist Frederick Griffith conducted a series of experiments using Streptococcus pneumoniae bacteria and mice. Griffith wasn't trying to identify the genetic material, but rather, trying to develop a vaccine against pneumonia. In his experiments, Griffith used two related strains of bacteria, known as R and S. He named them R and S because of their appearance under the microscope (rough and smooth).
Griffith injected live S bacteria into mice and noticed that the mice died. He did the same with the R strain and noticed that the mice survived. Then, he heat killed the S strain and the mice survived. Finally, he injected a combination of live R strain and heat-killed S strain, neither of which had previously killed the mice, and the mice died.
Why did the combination of live R and heat-killed S kill the mice?
Griffith concluded that the R-strain bacteria must have taken up what he called a "transforming principle" from the heat-killed S bacteria, which allowed them to "transform" into smooth-coated bacteria and become virulent. He did not yet know what it actually was.
Avery, McCarty, and MacLeod
In 1944, three Canadian and American researchers, Oswald Avery, Maclyn McCarty, and Colin MacLeod, set out to identify Griffith's "transforming principle." To do so, they began with large cultures of heat-killed S cells and, through a long series of biochemical steps (determined by careful experimentation), progressively purified the transforming principle by washing away, separating out, or enzymatically destroying the other cellular components. The data strongly suggested to the three of them that it was DNA but Avery was extremely cautious to announce this for fear that it was still some other contaminating substance.
Hershey and Chase
In their now-legendary experiments, Hershey and Chase studied bacteriophage, or viruses that attack bacteria. The phages they used were simple particles composed of protein and DNA, with the outer structures made of protein and the inner core consisting of DNA. Hershey and Chase knew that the phages attached to the surface of a host bacterial cell and injected some substance (either DNA or protein) into the host. This substance gave "instructions" that caused the host bacterium to start making lots and lots of phages—in other words, it was the phage's genetic material.
To establish whether the phage injected DNA or protein into host bacteria, Hershey and Chase prepared two different batches of phage. In each batch, the phage was produced in the presence of a specific radioactive element, which was incorporated into the macromolecules (DNA and protein) that made up the phage. Remember from Module 1 that ONLY nucleic acids contain phosphorus and ONLY proteins contain sulfur. Hershey and Chase tagged these elements to figure out which one was transferred.
When Hershey and Chase measured radioactivity in the pellet and supernatant from both of their experiments, they found that a large amount of the radioactive phosphorus appeared in the pellet, whereas almost all of the radioactive sulfur appeared in the supernatant. Based on this and similar experiments, Hershey and Chase concluded that DNA, not protein, was injected into host cells and made up the genetic material of the phage.
Other relevant scientists that you may be interested in learning about include James Watson, Francis Crick, Rosalind Franklin, and Erwin Chargaff. You are not required to know specifics but may want to check them out on your own time.
Click through the DNA Research Timeline activity below.
DNA Structure
The two main types of nucleic acids are deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). DNA is the genetic material found in all living organisms, ranging from single-celled bacteria to multicellular mammals. DNA bears the hereditary information that’s passed on from parents to children, providing instructions for how (and when) to make the many proteins needed to build and maintain functioning cells, tissues, and organisms. It’s amazing to think that inside the same organism, all cells have the same DNA! Adenosine triphosphate (ATP) is also considered to be in this biomolecule category. The role of nucleic acids is to transfer the genetic code from DNA to a messenger RNA (mRNA) molecule in order for the ribosome to read it and create a functional protein. Since structure determines function, if there is a mistake in the genetic code, then the correct protein cannot be produced and it cannot do its job in the cell.
Nucleic acids are built from repeating monomers called nucleotides. A nucleotide has three parts: a 5-carbon sugar, a phosphate group (PO43-), and a nitrogenous base. The four possible nitrogenous bases in DNA are guanine, cytosine, adenine, and thymine. It is helpful to remember the mnemonic device G-CAT because G pairs with C and A pairs with T. RNA does not contain thymine but has an additional nitrogenous base called uracil. Another difference between DNA and RNA is the sugar found in each nucleotide. DNA contains the sugar deoxyribose and RNA contains the sugar ribose. You can see the differences in structure in the image below.
Each strand of DNA is joined together in the middle by hydrogen bonds between the bases. Guanine and cytosine (G-C) base pairs have three hydrogen bonds and adenine and thymine (A-T) base pairs have two hydrogen bonds. Please watch the following video for a nice summary of nucleic acid structure.
Watch the Introduction to Nucleic Acids and Nucleotides video below.
As shown above, the two strands of DNA run in opposite directions. The term antiparallel means one is upside down compared to the other. The sugar-phosphate backbone is located on the outside, and the bases are in the middle. The strands run from the 5’ to 3’ direction, and this is named for the position of the carbon in the sugar to which the free functional group is attached. We’ll look at this more in the replication lesson.
Try the DNA Directionality activity below.
Make sure you found all the differences between DNA and RNA by sorting into this DNA RNA Venn diagram activity (select the arrow after each question to move to the next question):
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