The Disappearance of Snake Legs

Snakes have lost their legs about 150 million years ago due to a DNA composition change that created a mutation to get rid of legs. This has been confirmed two separate times by two separate groups of scientists. We know that snakes used to have legs because through dissection experiments of boas and pythons and other snakes, we can find tiny receded bone groups that seemed to be used as legs before.

"Mutations in DNA located near a gene key to limb formation keep that gene rom ever turning on"

The snakes at first were looked at under the mutation of the hedgehog gene that we learned about before in class. So they analyzed the sonic the hedgehog gene and they noticed that there were three deletions and a modified enhancer within snakes. In order to study this enhancer a special technique called CRISPR-Cas9 was used.

"CRISPR-Cas9 is a unique technology that enables geneticists and medical researchers to edit parts of the genome by cutting out, replacing, or adding parts to the DNA sequence"

When the snake enhancer was put into a mouse, the issue was that the mouse grew very stubby legs. When the snake enhancer was repaired with the missing deletions and then put into the mouse, it grew normally. This is molecular genetic evidence for snakes that used to have legs.

I am very excited to see how genetics is now affecting the idea of phylogenies and relationships between animals because for a long time scientists have been using taxonomy in order to project phylogeny, but the only way to truly study relationships is on the genetic and molecular level. Studies like this will continue amending the phylogenies we have developed thus far to be more accurate in telling us how things tacked back to their common ancestors and evolved.

Bigger Brains With Human DNA

The human version of a DNA sequence called HARE5 turns on a gene important for brain development (gene activity is stained blue), and causes a mouse embryo to grow a 12 percent larger brain by the end of pregnancy than an embryo injected with the chimpanzee version of HARE5. Credit: Silver lab, Duke University

Duke scientists showed that they could point out differences in genetic code between chimpanzees and humans which would lead to early brain development in mouse embryos. They found that humans have a difference in regulatory gene dubbed HARE5 that when introduced into mouse embryo would lead to a 12% larger brain than that of an embryo which was treated with the HARE5 sequence from a chimpanzee.

Initially, the group screened enhancers expressed primarily in the brain tissue and early development. This gave them 106 candidates, but only 6 of them were near genes thought to be involved with brain development so they chose these to work with. They were named 'human-accelerated regulatory enhancors,' HARE1 through HARE6. HARE5 then became the prime candidate for their work as its chormosomal location is near a gene called Frizzled 8, which is part of a well-known molecular pathway in the brain for development and disease. The human HARE5 and the chimpanzee HARE5 differed by 16 letters of genetic code. The human HARE5 injected mice had a 12% larger brain than the chimpanzee HARE5 injected mice and showed the neocortex, a region of brain involved with language and reasoning, to be affected.

I found this piece very interesting because it gives us a part of the genetic puzzle as to why and how we have bigger brains. Also, I did not know it was possible for such specific sequences of DNA to be both taken from and inserted into other species. Furthermore, these findings may help to our understanding of diseases like autism and Alzheimer's which is present in humans, but not chimpanzees.

Original Article: http://www.sciencedaily.com/releases/2015/02/150219133104.htm

Human DNA Placed Into Mouse Embryos

Evolution of the human brain has always been a topic of interest for scientists over the years. Many have wondered how human brains have become so complex and why chimpanzee brains have lagged behind even though chimpanzees have almost all of the same genes that humans have. Scientists at Duke University have tackled this question and obtained some interesting results. More specifically, these scientists have found differences between chimpanzee and human genetic codes and observed how these differences affected embryonic brain development in mice.

For the study scientists focused on shorts pieces of DNA called enhancers that are a part of every genome. These enhancers regulate gene activity and are sometimes human specific. Until now none of these human specific enhancers had been shown to influence brain development directly.  The scientists searched through chimpanzee and human genomes to find enhancers that are expressed in brain tissue and early in development. Enhancers that were largely different between the two species were of high importance. In the beginning, 106 enhancers were narrowed down as being potentially important for discovering the differences in chimp and human brains. Out of these 106, 6 were thought to be involved in brain development. The enhancers were named HARE1-6, standing for human accelerated regulatory enhancers.  HARE5 showed the most promise as it is located near a gene, Frizzled8, which is known for its role in brain development and disease. The researchers directed their attention onto this enhancer and postulated that it enhanced Frizzled8 since HARE5 and Frizzled8 make contact in the brain.  

The HARE5 in humans and chimpanzees only differ by only 16 base pairs. However, the human enhancer was active earlier and more active in general in the mouse embryos than the chimpanzee enhancer. The activity differences between the two enhancers were detected at a critical time in brain development. The mouse embryos with the human HARE5 ultimately ended up with more neurons than those with the chimpanzee HARE5. As the mouse embryos developed more and more and came closer to the end of gestation the difference in size of the brains became noticeable. The mice that had human HARE5 had brains that were 12% larger in area than those that had the chimpanzee HARE5. The part of the brain that was affected was the neocortex. This part of the brain is involved in language and reasoning. 

This feat was astonishing, just narrowing down the enhancers in itself was a difficult and cumbersome task. Many other scientists have tried to do what the Duke University researchers did and failed. As a result of this successful study, a genetic reason as to why humans have bigger brains than chimpanzees has been discovered.

I feel that this is so interesting and awesome that it has finally been discovered. Humans are so closely related to primates, yet so different. It is interesting to realize that all of the differences are most likely due to some small change in the genetic code. I mean if just one different enhancer could make mice brains larger then imagine what a few differences could amount to. It is astonishing to think about the complexity of genes and how they work together to produce many different characteristics. 

Link to Article: http://www.sciencedaily.com/releases/2015/02/150219133104.htm

Additional Link: http://www.latimes.com/science/sciencenow/la-sci-sn-dna-chimp-human-brains-20150218-story.html