|Feature Article - March 2019|
|by Do-While Jones|
Evolutionists claim this incredibly complex process evolved twice.
One of our readers alerted us to an article by Yang Gao (and his associates) published in the professional journal, Science, about how DNA replicates.
Before we tell you about Gao’s article, we need to establish some background about how the DNA molecule replicates.
DNA replication: The double helix is un'zipped' and unwound, then each separated strand (turquoise) acts as a template for replicating a new partner strand (green). Nucleotides (bases) are matched to synthesize the new partner strands into two new double helices. 1
As you will soon see, it isn’t quite that simple—but don’t worry about that yet.
You also need to know the difference between eukaryotes and prokaryotes.
Eukaryotes are organisms whose cells have a nucleus enclosed within membranes, unlike prokaryotes (Bacteria and Archaea), which have no membrane-bound organelles. 2
That’s a bigger difference than you might think. The editors of Science refered to eukaryotes and prokaryotes as being from “different worlds” in the title of their commentary on Gao’s article. Of course, that is hyperbole. Both kinds of cells exist on Earth. The editors exaggerated just to emphasize that eukaryotes and prokaryotes are so different that they could have come from different planets.
The introduction of the Science article about DNA replication begins and ends with these words:
In other words, nobody had ever actually seen a DNA molecule being unzipped to produce two identical DNA molecules—until now.
We determined cryo–electron microscopy (cryo-EM) structures of the T7 replisome and show how its essential enzymatic functions are coordinated in three dimensions. 4
They used an electron microscope to watch the process happen, and described in incredibly precise detail exactly what happened, and used some computer animation to demonstrate the process. Then they reached this rather unexciting conclusion:
It didn’t seem like a big deal to us—but the editors of Science realized the importance of the research and explained how important this is to the theory of evolution.
On page 835 of this issue, Gao et al., report the cryo–electron microscopy (cryo-EM) structure of the T7 bacteriophage replisome at a high atomic detail. The study not only advances our understanding of the helicase mechanism but also reveals an unexpected arrangement of the two DNA Pols in the replisome. Specifically, the two Pols sandwich the DNA helicase in an asymmetric manner; one DNA Pol is on top of the helicase, and one DNA Pol is below (see the figure). This architecture is unlike textbook illustrations of both DNA Pols trailing behind the helicase. 6
What biology students have been told since 1950 is wrong. Admittedly, that probably isn’t too important. Hopefully, every student knows there is some artistic license in every textbook illustration, so it isn’t a very big deal that textbook drawings aren’t perfectly accurate. The big deal is coming later in the article.
The Science editors said that replication of the DNA molecule is a complex task, and that Gao’s team used an electron microscope to watch it happen, and the textbooks were wrong. They took the following paragraph to say what we just said in one sentence:
Replication of the DNA genome is performed by a replisome complex composed of numerous proteins. Cells have duplex DNA genomes, and their replisomes duplicate both strands simultaneously. A functional replisome requires, at a minimum, a helicase to unwind the DNA duplex, two DNA polymerases (Pols) to replicate the two DNA strands, and a primase to form RNA primers that DNA Pols extend. The replisome functions at a Y junction, or replication fork, and is a complex task because DNA Pols can only extend DNA in a 3'-to-5' direction. Thus, as the helicase unwinds the antiparallel DNA strands, the DNA Pol on one strand (the leading strand) can go in the same direction as the helicase and replicate DNA continuously, but the DNA Pol on the antiparallel strand (the lagging strand) is generated in the opposite direction. This requires repeated priming and extension of the lagging strand discontinuously as a series of Okazaki fragments. This “semidiscontinuous replication” is shared by all cells. On page 835 of this issue, Gao et al. (1) report the cryo–electron microscopy (cryo-EM) structure of the T7 bacteriophage replisome at a high atomic detail. The study not only advances our understanding of the helicase mechanism but also reveals an unexpected arrangement of the two DNA Pols in the replisome. Specifically, the two Pols sandwich the DNA helicase in an asymmetric manner; one DNA Pol is on top of the helicase, and one DNA Pol is below (see the figure). This architecture is unlike textbook illustrations of both DNA Pols trailing behind the helicase. 7
Now, here is the big deal:
Multiprotein complexes carry out each step of the “central dogma” of genetic information flow: replication, transcription, and translation. Interestingly, proteins of transcription and translation are homologous among Bacteria, Archaea, and Eukarya and thus evolved from a common ancestor. By contrast, the DNA Pol, helicase, and primase of bacterial replisomes share no homology to their eukaryotic counterparts, implying that these replisome enzymes evolved independently, after the evolutionary split of bacteria and eukaryotes. The primordial cell possibly used a simpler process of DNA replication, or used an RNA genome.
Surprisingly, the asymmetric arrangement of two DNA Pols that sandwich the helicase was also demonstrated by EM for the eukaryotic replisome of the yeast Saccharomyces cerevisiae, albeit at lower resolution than the T7 study by Gao et al. Thus, although “worlds apart” in terms of their independent evolution, the core elements of bacterial and eukaryotic replisomes both contain a helicase between two DNA Pols, although the eukaryotic replisome requires a trimeric scaffolding factor (Ctf4) to help tether the top DNA Pol to the helicase.
Another unexpected feature of the bacterial (T7) and eukaryotic replisomes is that the top DNA Pol functions on the opposite strand: The DNA Pol at the top of T7 helicase replicates the leading strand, whereas the DNA Pol at the top of eukaryotic CMG helicase replicates the lagging strand (see the figure). This is because bacterial helicases encircle the lagging strand, whereas eukaryotic CMG helicase encircles the leading strand. It remains a mystery why this “mirror” arrangement evolved, but both arrangements share a pragmatic logic for replisome function. All replicative helicases split the duplex at their leading edge, with one strand going through the middle of the helicase ring and the other strand deflected off the top of the ring. Hence, a DNA Pol at the top of the helicase can immediately duplicate the separated strand. 6
A “central dogma” of the theory of evolution is that Bacteria, Archaea, and Eukarya evolved from a common ancestor—but the evidence suggests their replisome enzymes mysteriously evolved independently.
Let’s ponder that for a moment. All living cells contain DNA. You no doubt have heard about how much information is stored in the DNA molecule compared to how many books it would take to hold all that information. DNA is an incredibly efficient information storage medium. Evolutionists believe that all living cells inherited DNA from the first living cell. The first two things that first living cell had to learn were (1) to harness energy and (2) reproduce itself. If it hadn’t done that, it would have died alone.
So, according to evolutionists, the first living cell
miraculously fortunately, at a minimum, evolved “a process involving a helicase to unwind the DNA duplex, two DNA polymerases (Pols) to replicate the two DNA strands, and a primase to form RNA primers that DNA Pols extend.” But in eukaryotes the process is so different from prokaryotes, it is as if they came from “different worlds.” It is as if they were independently created. Perish the thought!
We wish we had noticed the difficulties this discovery poses for the theory of evolution first—but we didn’t. We don’t know if Gao’s team didn’t notice it either, or noticed it and chose not to mention it.
The body of Gao’s article does not contain the word, “evolution.” His article describes in precise detail exactly what they observed and how they observed it. We applaud him for just reporting what they found without speculating about why the process is different in eukaryotes and prokaryotes. That’s good science.
We also applaud the editors of Science for recognizing the implications of Gao’s observation and not sweeping the implication of the difference under the rug.
DNA replication is a complex process that evolutionists believe originated
miraculously unexplainably in the first living cell. It takes a lot of faith to believe that happened by chance. It takes even more faith to believe it happened twice by chance.
|Quick links to|
|Science Against Evolution
|Back issues of
of the Month
Illustration credit: Madeleine Price Ball. https://en.wikipedia.org/wiki/DNA_replication
3 Yang Gao, et al., Science, 22 Feb 2019, “Structures and operating principles of the replisome”, http://science.sciencemag.org/content/363/6429/eaav7003
6 Huilin Li and Michael E. O'Donnell, Science, 22 Feb 2019, “DNA replication from two different worlds”, pp. 814-815, http://science.sciencemag.org/content/363/6429/814