By now, we all know that time-telling was developed to give some structure to both the business and social aspects of life. We also know that nature is a time-keeper, as the solar system and seasons continue through the same cycles. And we all know that our own lives are, to an extent, driven by our own biological clocks and circadian rhythms. Expounding on this idea, scientists gained curiosity about the smaller elements of nature, wondering if they also work in clock-like fashion. Their findings are fascinating, so we decided to dedicate a blog post to the subject of these tiny clocks, called “molecular clocks.”
So, what exactly is a molecular clock? A molecular clock differs from a regular timepiece in the sense that it counts the time between mutations or systematic biological changes – and, of course, it doesn’t have a face with numbers. In an informational article published by Penn State University, the molecular clock concept is explained. Blair Hedges, a professor of biology at the institution, explains that the name implies regular clock-like changes in DNA mutations. Even though changes are random, they appear to have a sequence that maintains a constant rate. Hedges notes that as time passes, the number of differences in any two sequences of genes increases steadily.
The original molecular clock concept is credited to Emile Zuckerkandl, a biologist, and Linus Pauling, a chemist. Their efforts of developing the concept started in 1962. They noted that in alignment with time, the amount of amino acid differences seen in different species’ hemoglobin changes systematically. Zuckerkandl and Pauling used their generalized observation to show that any specific protein’s rate of evolutionary change was constant throughout time and different lineages. Together with E. Margoliash’s 1963 discovery of genetic equidistance, the formulation of the molecular clock discovery moved forward. Genetic equidistance is the difference between cytochrome C residue differences of any two species, reflecting the idea that they are equal in comparison with one another in their current rate from their evolutionary beginning. The idea would be calculated similar to a proportion formula.
Fathers of the Molecular Clock: Biologist Emile Zuckerkandl (Left) and Chemist Linus Pauling (Right)
An interesting point considering the theory of genetic equidistance was recently published by Konrad Lorenz Institute for Evolution and Cognition Research, penned by Shi Huang of Xiangya Medical School in China. Huang points out that while Margoliash’s theory is good, it does not allow for time overlaps. If the species compared vary from a result of macroevolution, Huang notes that there are a large number of overlapped mutant amino acid positions in three-pair sets of species. However, simpler organisms tend to have a much smaller number of overlaps, which he says are consistent with neutral theory or chance.
Penn State’s Professor Hedges also makes the point that in order to work properly, any clock must be calibrated – including a molecular clock. The process for this, he says, includes taking a known factor, such as a fossil record and determining the mutation rate. Rapidly-changing genes are used to determine events in more recent history, while slowly-changing genes are beneficial in determining ancient history’s divergences. In an example, he explains that if five mutations are seen every million years and there are 25 mutations in a sequence, the sequential result is five million years for the divergence. Hedges points out that different genes evolve and mutate at different rates, providing flexibility to date history’s events in the life of each species. Another interesting fact he presents is that genes with more vital functions evolve slower than those that have less vital functions.
A Molecular Science Tag Cloud
Today, scientists also attempt to use the molecular clock concept to determine information about species of which they have little to no record of. Species of biological matter that do not form well in a fossil state lack evidence and sequence information about mutations and changes. The information gleaned from studying species that do show their evidence clearly through fossils is applied to the species that lack it, based on relation with current information. The molecular clock also benefits science by assisting in putting evolutionary events into a chronological order. By comparing species, scientists can determine at what point in time the two shared an ancestor, thus discovering their links to one another.
In an introductory Evolution learning plan from the University of California, Berkeley, it is stated that this method has been used by evolutionary biologists in the past several decades to determine where humans and chimpanzees diverged. Considering the lengthy amount of time in which scientists are attempting to gauge history, they are trying to compare the molecular clock rates with other relative or absolute timing methods used in science for optimal accuracy.
It is important to remember that this theory isn’t flawless. Not only does it still have years to go before it can be considered fully reliable, but there are already many inconsistencies noted. The inconsistencies are, of course, noted more in the species studied, not in the plan itself. In a publication by the University of North Carolina, a previous research study in July of 1997 is referenced. This research was performed by Francisco J. Ayala. His study focused on two genes, SOD and GDPH. He notes that when compared with Drosophila species, the GDPH protein evolves at a slower rate than it does when compared with mammals, animal phyla, Dipteran families or multicellular kingdoms. SOD evolution is fast between the Drosophila species, but remains constant between mammals and Dipteran families. It is slightly slower in animal phyla and slower yet in multicellular kingdoms. When calculated, the SOD and GDPH rates don’t support the molecular clock theory. It is unknown how frequently proteins evolve in such erratic sequences as these examples. However, they are good reminders that many scientific theories must have exceptions.
Diagram of a Molecular Clock
Molecular clocks may not be applied to every relation of species, but there are some encouraging effects of this beneficial finding. Based on proven similarities in molecular clock theory in a specific gene related to causing breast cancer, scientists at UT Southwestern Medical Center may have discovered a bit of evidence that will lead to better future prevention of this deadly cancer.
Their findings were published in a May 2008 issue of Cancer Epidemiology Biomarkers & Prevention. They studied methylation, which is a biological chemical process that contributes to the precancerous changes and ultimate risk of developing breast cancer in women. The process of mehtylation is seen as a form of molecular clock, counting the systematic cell divisions, which vary from one woman to the next. The number of cell divisions correlates with the risk for breast cancer. The scientists conducted a study in which they observed women whose ovaries were still cycling, looking for the methylation of tumor-suppressing genes. Their findings showed that methylation was definitely a form of molecular clock in this aspect. They also noted that women who bore children had lowered amounts of the tumor-suppressing gene, which contradicts the popular belief that women who have children face less risk of developing breast cancer. Methylation can be slowed or stopped, so there may be promising advances in the world of cancer research, thanks to the molecular clock theory.
Research certainly shows a mixed set of opinions about this molecular clock theory. Although it may not be applied to everything, it is important to keep in mind that everything in science is simply experimental until it becomes proven.
The theory of the molecular clock is relatively new, so future advances can be expected.
Scientists have made much more progress studying species that make clear fossils, but those that do not still remain “unknown” until a concrete formula can be developed for them.
Until more research findings surface, we can only conclude that molecular clocks are one other new kind of “clock” that show a great deal of potential for helping humankind.
Clocks That Are Molecular is a post from: Alarm Clock Blog, the official blog of the original Online Alarm Clock.
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