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Pregnancy Timeline by SemestersFemale Reproductive SystemFertilizationThe Appearance of SomitesFirst TrimesterSecond TrimesterThird TrimesterFetal liver is producing blood cellsHead may position into pelvisBrain convolutions beginFull TermWhite fat begins to be madeWhite fat begins to be madeHead may position into pelvisImmune system beginningImmune system beginningPeriod of rapid brain growthBrain convolutions beginLungs begin to produce surfactantSensory brain waves begin to activateSensory brain waves begin to activateInner Ear Bones HardenBone marrow starts making blood cellsBone marrow starts making blood cellsBrown fat surrounds lymphatic systemFetal sexual organs visibleFinger and toe prints appearFinger and toe prints appearHeartbeat can be detectedHeartbeat can be detectedBasic Brain Structure in PlaceThe Appearance of SomitesFirst Detectable Brain WavesA Four Chambered HeartBeginning Cerebral HemispheresEnd of Embryonic PeriodEnd of Embryonic PeriodFirst Thin Layer of Skin AppearsThird TrimesterDevelopmental Timeline
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April 30, 2012--------News Archive Return to: News Alerts


NYU biologists findings call for reconsideration of a decades-old biological theory.
The morphogen theory posits that proteins control how traits are arranged, with different
amounts of proteins activating genes to create specific physical features.
It has been used to explain why a tiger has stripes, among other phenomena. ©iStockPhoto.com/nialat

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Long-held Gene Theory Doesn't Quite Hold Up

Biologists have discovered new mechanisms that control how proteins are expressed in different regions of embryos

New York University (NYU) scientists also shed additional insight into how physical traits are arranged in body plans.

Their findings, which appear in the journal Cell, call for reconsideration of a decades-old biological theory.

The researchers investigated the morphogen theory, which posits that proteins controlling traits are arranged as gradients, with different amounts of proteins activating genes to create specific physical features.

This theory was first put forth in the 1950s by mathematician and World War II code breaker Alan Turing and refined in the 1960s by Lewis Wolpert. It has been used to explain why a tiger has stripes, among other phenomena.

But some biologists have raised questions about the theory, which proposes that physical features are tied to absolute concentrations of proteins within the morphogen gradient.

If a certain critical mass of protein is present, then a given physical feature—for example, cells that make the skin on your forehead—will appear. If less than that critical mass is present, a different structure—say, the skin that makes your eyebrows—will appear, and a boundary will be formed between the two structures.

But alternative views have suggested that physical features are not necessarily the result of a specified number of proteins, but, rather, come from more complex interactions between multiple gradients that work against one another.

The NYU biologists explored this process by studying the fruit fly Drosophila, a powerful model for studying genetic development as it is amenable to precise genetic manipulations. They focused on one protein, Bicoid (Bcd), which is expressed in a gradient with highest levels at the end of the embryo that will become the mature fly's head.

The researchers, headed by Stephen Small, chair of NYU's Department of Biology, examined a large number of target genes that are directly activated by Bcd. Each target gene is expressed in a region of the embryo with a boundary that corresponds to a specific structure.

By examining DNA sequences associated with these target genes, the NYU researchers discovered binding sites for three other proteins—Runt, Capicua, and Kruppel—which all act as repressors.

All three proteins are expressed in gradients with highest levels in the middle part of the embryo, and thus are positioned in exactly the opposite orientation compared to the Bcd activation gradient.

By changing the spatial distribution of the repressor genes and by manipulating their binding sites, Small and his colleagues showed that these repressors antagonize Bcd-dependent activation and are absolutely critical for establishing the correct order of boundaries that are found in a normal embryo.


In other words, contrary to Turing's theory, a single gradient of proteins does not have sufficient power to form the same body plan in each member of a species; however, if there are multiple gradients that work against each other, then the system becomes robust enough for normal development.


While the results raise questions about morphogen theory, the researchers explained that their findings did not "falsify" it, but, rather, suggest it needs some additional refinement.

The study's other co-authors were Constance Mei, an NYU undergraduate at the time of the study who received her bachelor's degree in 2011; current NYU doctoral students Hongtao Chen and Zhe Xu; and Danyang Yu, who received her doctorate in biology from NYU in 2009 and who is now an assistant professor of biology at Farleigh Dickinson University.

Original article: http://www.nyu.edu/about/news-publications/news/2012/04/26/long-held-genetic-theory-doesnt-quite-make-the-grade-biologists-find-.html