This story attracted my attention last week at work. I align teaching resources to state standards and benchmarks. One area of standards is in cell biology, heredity, and DNA. This is not the field of my training or education. I come from the physical sciences. But, I have learned enough about this field to be reasonably competent. It is my hope that many others who read this diary will contribute their expertise and help answer technical questions that arise. I thank you ahead of time.
Living systems owe their existence to a pair of information-carrying molecules: DNA and RNA. These fundamental chemical forms possess two features essential for life: they can encode and pass on genetic information, and they can adapt over time, through processes of Darwinian evolution. A long-debated question is whether heredity and evolution could be performed by molecules other than DNA and RNA.
John Chaput, a researcher at ASU’s Biodesign Institute, who recently published an article in Nature Chemistry describing the evolution of threose nucleic acids, joined a multidisciplinary team of scientists from England, Belgium and Denmark to extend these properties to other so-called Xenonucleic acids or XNA’s.
The group demonstrates for the first time that six of these unnatural nucleic acid polymers are capable of sharing information with DNA. One of these XNAs, a molecule referred to as anhydrohexitol nucleic acid or HNA was capable of undergoing directed evolution and folding into biologically useful forms.
Their results appear in the current issue of Science.
More below the DNA squiggle
Both RNA and DNA coding uses four nucleotides to pass genetic information. The information determines hereditary traits and the recipe for building proteins from the 20 naturally occurring amino acids. How this system started is one of the most debated areas of biology.
According to one hypothesis, the simpler RNA molecule preceded DNA as the original informational conduit. The RNA world hypothesis proposes that the earliest examples of life were based on RNA and simple proteins. Because of RNA’s great versatility—it is not only capable of carrying genetic information but also of catalyzing chemical reactions like an enzyme—it is believed by many to have supported pre-cellular life.
For RNA to spontaneously occur through a random mixing of chemicals is unlikely. “This is a big question,” Chaput says. “If the RNA world existed, how did it come into existence? Was it spontaneously produced, or was it the product of something that was even simpler than RNA?”
Chaput, and the work of the other researchers, gives alternatives that could have acted as 'chemical stepping-stones' to the emergence of life. Their research strengthens the case that something like this may have taken place.
Our Present Understanding of DNA
Video segments from the DNA Learning Center of Cold Spring Harbor will be used to illustrate some of the concepts. They have a YouTube channel with many videos available to the public, students, and teachers.
The DNA is a double helix structure. The genetic code is set by the rungs of the ladder. The rungs are comprised of four chemicals.
DNA became clear in 1953 thanks to the work of James Watson, Francis Crick, Maurice Wilkins and Rosalind Franklin. By studying X-ray diffraction patterns and building models, the scientists figured out the double helix structure of DNA. Here, Watson talks about discovering the model.
The order, or sequence, of these bases determines what biological instructions are contained in a strand of DNA. For example, the sequence ATCGTT might instruct for blue eyes, while ATCGCT might instruct for brown.
Each DNA sequence that contains instructions to make a protein is known as a gene. The size of a gene may vary greatly, ranging from about 1,000 bases to 1 million bases in humans. The complete DNA instruction book, or genome, for a human contains about 3 billion bases and about 20,000 genes on 23 pairs of chromosomes. The large structure is tightly coiled to save space within the nucleus.
DNA Replication for Cell Division
Before a cell divides, the double helix unwinds and the two strands of the DNA molecule in the nucleus separate. Each strand is then used as a template for the construction of new DNA molecules. This process is called replication. The first video illustrates the concept. The second video uses computer graphics to illustrate an assembly line of amazing miniature biochemical machines that are pulling apart the DNA double helix and cranking out a copy of each strand.
DNA Transcription for Protein Manufacture
DNA contains the instructions needed for an organism to develop, survive and reproduce. DNA sequences must be converted into messages that can be used to produce proteins, which are the complex molecules that do most of the work in our bodies. The instructions to make proteins in a two-step process. First, enzymes read the information in a DNA molecule and transcribe it into an intermediary molecule called messenger ribonucleic acid, or mRNA.
Next, the information contained in the mRNA molecule is translated into the "language" of amino acids, which are the building blocks of proteins. This language tells the cell's protein-making machinery the precise order in which to link the amino acids to produce a specific protein. This is a major task because there are 20 types of amino acids, which can be placed in many different orders to form a wide variety of proteins.
This video using computer graphics shows the mechanism of the transcription process which produces an mRNA strand ready for the next step of protein production.
What About Synthetic DNA and RNA?
Nucleotides of DNA are made of four bases - A, G, C, and T. Attached to the bases are sugars and phosphates. (See the graphic at the top of the diary.)
First, the researchers made building blocks of six different genetic systems by replacing the natural sugar component of DNA with one of six different polymers, synthetic chemical compounds. These xeno-nucleic acids, or XNAs, replaced the sugar groups that make up the sides of the ladder.
The team then evolved enzymes, called polymerases, that can make XNA from DNA, and others that can change XNA back into DNA.
The copying and translating ability allowed for genetic sequences to be copied and passed down. This is an artificial heredity.
The team also found that HNA, one of the six XNA polymers, could respond to selective pressure in a test tube and evolve into different forms. This shows that "beyond heredity, specific XNAs have the capacity for Darwinian evolution," according to the study. Heredity and evolution appear to be possible for synthetic DNA and RNA.
Future Uses of the Technology?
According to Chaput...
Nucleic acid aptamers, which have been engineered through in vitro selection to bind with various molecules, act in a manner similar to antibodies—latching onto their targets with high affinity and specificity. “This could be great for building new types of diagnostics and new types of biosensors,” Chaput says, pointing out that XNAs are heartier molecules, not recognized by the natural enzymes that tend to degrade DNA and RNA. New therapeutics may also arise from experimental Xenobiology.