What makes who we are --- Explanations of two pictures

I. Morning Glory

   As we known, DNA is copied from the “parent” cell to the “daughter” cell. Despite this process usually produces accurate copies, errors do occur. When an error does occur, the new combination of DNA sequences is a mutation.

   DNA can be modified in more ways that only by random mutations. By doing so, “jumping genes” are formed --- the whole sequence of DNA that moves from one place to another over times of environmental stress. Normal morning glory favors the color of blue over the color of white. Yet, due to the different growing environment, helpful mutation occurs that causes DNA retro-transposon happens; therefore, in the picture, the morning glory contains more of the color of white than the color of blue.

Normal Morning Glory

!! Mutation occurs!

II. Handy Genes

   As mentioned in Chapter 3 of Your Inner Fish, “Our limbs exist in three dimensions: They have a top and bottom, a pinkie side and a thumb side, a base and a tip. The bones at the tips, in our fingers, are different from the bones at the shoulder. Like wise, our hands are different from our thumbs.” What DNA actually makes a pinky different from a thumb? How does our body know to develop in this way? In order to find out these answers, Randy Dahn, a researcher in Dr. Shubin’s laboratory did experiments on the embryos of sharks and skates by injecting a form of Vitamin A.

Nevertheless, in the 1950’s and 1960’s a number of biologists did amazingly creative experiments on chicken eggs to understand how the pattern of the skeleton forms. By cutting up embryos and moving around tissues, biologists were able to discover that two little patches of tissue essentially control the development of the pattern of the bones inside limbs.

   On the other hand, Mary Gasseling did another experiment that could explain why the infant’s hand in the picture looks differently. In the picture, the infant has two more extra fingers growing out from the index finger. How so? This is because of ZPA (the zone of polarizing activity, also known as the patch of tissue that control the development of the pattern of the bones inside limbs.) Although ZPA causes fingers to look differently, something else inside ZPA controls how fingers form and what they look like, which is Sonic Hedgehog.

    Sonic Hedgehog is active in the ZPA tissue. If Sonic Hedgehog hasn’t turn on properly during the eighth week of one’s own development, then one either would have extra fingers or one’s fingers would look alike. Furthermore, moving a little patch of the ZPA tissue would cause the fingers to duplicate and supplying Vitamin A at the right concentration and at the right stage, fingers would form mirror-image duplication. This is why the infant’s hand looks differently in the picture.

Normal hand

Mirror - image