Your Inner Fish Chapter 3 : Handy Gene

In this chapter of Your Inner Fish, I learned that we have hundreds of different cells which gives our tissues and organs their distinct shapes and functions. However, no matter how different all of these cells seem, they all contain the exact same DNA. The reason that the cells are not identical is that each of these cells contains different genes that are actually active. For example, a skin cell is different from neuron cell because different genes are active, so a different protein is made. Our body as a whole is essentially a composition of individual genes that turn on and off inside each cell during our development (The things that control the activity of our genes are known as genetic switches).

Many experiments were done in this chapter to examine organism development. One was done on chicken, and its purpose was to examine limb development. Saunders and Zwilling discovered that there are little patches at the end of a bud, where your fingers would eventually develop, that control the pattern of the bones that make up the limbs. They named this patch of tissue the zone of polarizing activity, or ZPA.

Normal development of a limb.

Development when ZPA is moved to other side of the bud.

In another experiment, scientist found a gene that made one end of a body segment of a fly look different from the other. They called this gene a “hedgehog”. When the same gene was later found in chickens, it was called the “Sonic hedgehog”.

"Sonic hedgehog"

The "Sonic hedgehog" gene was named after the video game character.

Briefly summarizing as much as I can for the rest of the chapter, the following is the main points. Scientists found that injecting Vitamin A at the right stage in development resulted in a mirror duplication of digits. Scientist attached dye to the “Sonic hedgehog” and found that only cells in a tiny patch at the end of the bud had gene activity, similar to the ZPA. Sonic hedgehog is active in ZPA tissue in every animal with limbs. Failure of “Sonic hedgehog” to turn on properly can result in extra fingers, paddle hands. Similarly, malfunctioning ZPA results in malformation in hands. Randy Dahn experimented on skates (very different structure and cartilage rather than bone) and found the “Sonic hedgehog” gene. Skate reacted the same way as a chicken did for everything, and when Vitamin A was injected in the skate, it produced a patch of tissue on the opposite side that contained the “Sonic hedgehog”, causing a duplication of bones. When a mouse “Sonic hedgehog” was placed in between the rods in a skate embryo, these rods developed differently, and depending on how close they were to the “Sonic hedgehog”.

Examples of malformation of the hands or feet.

 

            Conclusions drawn from all of the experiments were that the “Sonic hedgehog” makes fingers distinct from one another, and the finger formed depended on how close this “Sonic hedgehog” was to it. The author concludes that all appendages, whether fins or limbs, are built by the same genes. Building off of this, he argues that evolutionary transformation of fish fins into limbs did not involve the origin of new DNA, but rather just a shift of ancient genes into new ones. Lastly, Shubin, the author, states that the connection between living creatures is deep, leaving us to wonder what other things we may share with other species.