Credit: SciTechDaily.comRecent research study challenges the long-standing understanding of the end-replication issue in DNA, exposing two distinct concerns rather than one.Half a century earlier, scientists Jim Watson and Alexey Olovnikov independently understood that there was an issue with how our DNA gets copied. Credit: Sarah CaiThe Leading-Strand ProblemSince the description of the DNA double helix, it is known that DNA has two complementary strands running in opposite directions– one from 5 ′ to 3 ′; the other from 3 ′ to 5 ′.” The DNA duplication machinery can not fully duplicate the end of a direct DNA, much the same method that you cant paint the floor under your feet,” says Hiro Takai, senior personnel scientist in the de Lange laboratory and lead author on the paper.CST– Polα/ primase, the enzyme that solves the recently discovered end-replication problem.” De Lange called Joseph T. P. Yeeles, a biochemist who studies DNA duplication at the Laboratory of Molecular Biology in Cambridge (the exact same laboratory where Watson and Crick worked on the structure of the DNA double helix). He was able to determine how much DNA is lost due to the lagging-strand end-replication issue, exposing how many CCCAAT repeats require to be added by CST– Polα-primase to keep telomeres intact.Implications and Future DirectionsThe results alter our understanding of telomere biology– needing modification of the books.
Discoveries about the end-replication problem show both telomerase and the CST– Polα-primase complex are vital for chromosome protection, suggesting a modification in the science of telomeres and prospective influence on hereditary conditions. Credit: SciTechDaily.comRecent research study challenges the enduring understanding of the end-replication issue in DNA, revealing two distinct issues rather than one.Half a century ago, scientists Jim Watson and Alexey Olovnikov independently understood that there was an issue with how our DNA gets copied. A quirk of direct DNA replication dictated that telomeres that protect the ends of chromosomes ought to have been growing much shorter with each round of duplication, a phenomenon called the end-replication problem.Telomerase: A Solution EmergesBut a service was forthcoming: Liz Blackburn and Carol Greider discovered telomerase, an enzyme that adds the telomeric repeats to the ends of chromosomes. “Case closed, everybody believed,” says Rockefellers Titia de Lange.Now, new research published in Nature suggests that there are two end-replication issues, not one. Even more, telomerase is just part of the service– cells also utilize the CST– Polα-primase complex, which has been extensively studied in de Langes laboratory. “For numerous decades we believed we understood what the end-replication issue was and how it was fixed by telomerase,” says de Lange. “It ends up we had actually missed out on half the problem.” CST– Polα/ primase, the enzyme that solves the recently discovered end-replication problem. Credit: Sarah CaiThe Leading-Strand ProblemSince the description of the DNA double helix, it is known that DNA has two complementary hairs running in opposite instructions– one from 5 ′ to 3 ′; the other from 3 ′ to 5 ′. When DNA is reproduced, the 2 strands are separated by the replication equipment, likewise called the replisome. The replisome copies the 3 ′ to 5 ′ hair without disturbance, a process referred to as leading-strand synthesis. However the other strand is synthesized in other words backwards steps from numerous fragments (Okazaki fragments) that are later sewn together.The procedure is relatively direct up until completions of the chromosomes. When copying the telomere, leading-strand DNA duplication ought to copy the CCCTAA repeats to generate the TTAGGG repeat strand, while lagging-strand synthesis should do the opposite, making new CCCTAA repeats. The end-replication issue develops since leading strand synthesis stops working to replicate the tail end of the telomere, leaving a blunt leading-end telomere without it characteristic and important 3 overhang. Telomerase fixes this issue by including single-stranded TTAGGG repeats to the telomere end. When it comes to the lagging-strand, DNA synthesis should not have a problem. It could start the last Okazaki fragment somewhere along the 3 overhang.” The DNA duplication machinery can not totally replicate the end of a linear DNA, similar method that you cant paint the flooring under your feet,” says Hiro Takai, senior staff researcher in the de Lange laboratory and lead author on the paper.CST– Polα/ primase, the enzyme that resolves the freshly found end-replication problem. Credit: Sarah CaiThe Lagging-Strand ProblemAs descriptions of biological processes go, this design looked water tight. Until Takai made an unexpected discovery while dealing with cells that did not have molecular equipment called the CST– Polα-primase complex. He and others had previously revealed that CST– Polα-primase can renew CCCTAA repeats at telomeres that had been assaulted by DNA-degrading enzymes referred to as nucleases. This new data exposed something unanticipated: not just was the leading hair in need of assistance– he discovered proof that completion of the delayed strand might likewise not be manufactured by the replisome.Takais work suggested that the end-replication issue was twice as serious as formerly believed, impacting both strands of DNA. “The results just didnt fit with the model for telomere replication,” de Lange states. “At that point, Hiro and I understood that either his outcomes were not ideal or the model was wrong. As his outcomes appeared really strong to me, we needed to review the design.” De Lange got in touch with Joseph T. P. Yeeles, a biochemist who studies DNA duplication at the Laboratory of Molecular Biology in Cambridge (the exact same laboratory where Watson and Crick dealt with the structure of the DNA double helix). Yeeles agreed that it would be excellent to take a close take a look at how the replisome acts at the end of a direct DNA template. Could the replisome usage a 3 overhang to make the last Okazaki fragment, as was proposed?The outcomes of Yeeles in vitro duplication experiments were very clear. The replisome does not create Okazaki fragments on the 3 overhang; it really stops lagging-strand synthesis long before the leading hair reaches the 5 end. This second end-replication problem means that both hairs of DNA will reduce with each department. Telomerase was only avoiding this from taking place at the leading strand and Hiros data suggested that CST– Polα-primase repaired the second end-replication problem, that of the delayed strand.Takai spent the next 4 years developing new assays to confirm Yeeles findings in vivo. He was able to determine just how much DNA is lost due to the lagging-strand end-replication issue, exposing how numerous CCCAAT repeats need to be added by CST– Polα-primase to keep telomeres intact.Implications and Future DirectionsThe results change our understanding of telomere biology– needing modification of the textbooks. But the findings might likewise have medical ramifications. Individuals who acquire anomalies in CST– Polα-primase suffer from telomere conditions, such as Coats plus syndrome, which is characterized by an eye condition and problems in the brain, bones, and GI system. Through a better understanding of how we maintain our telomeres, strides might one day be made in attending to these ravaging disorders.Reference: “Cryo-EM structure of the human CST– Polα/ primase complex in a recruitment state” by Sarah W. Cai, John C. Zinder, Vladimir Svetlov, Martin W. Bush, Evgeny Nudler, Thomas Walz and Titia de Lange, 16 May 2022, Nature Structural & & Molecular Biology.DOI: 10.1038/ s41594-022-00766-y.