Читать книгу You: Having a Baby: The Owner’s Manual to a Happy and Healthy Pregnancy - Michael Roizen F. - Страница 33

One of the most miraculous processes in nature, aside from the formation of such things as the Grand Canyon and the hammerhead shark, has to be how we grow from a single fertilized egg cell to the trillions of cells that make up a new person.

Human cells have twenty-three pairs of chromosomes, structures that hold our DNA. The DNA acts as a complete set of instructions that tells our bodies how to develop. Individual genes are short sequences of these instructions that regulate each of our traits. (See figure 1.2.) As you might imagine, given the fact that virtually every person in this world looks different from every other, the nearly infinite possible combinations of maternal and paternal DNA are what give us our individuality. When maternal brown eyes and maternal red hair get paired with paternal blue eyes and paternal blond hair, there are four possible combinations for offspring, right? Brown eyes – blond hair, brown eyes-red hair, blue eyes-blond hair, blue eyes – red hair. Extrapolate that scenario out to twenty-three chromosomes, and the possible combinations become mind-boggling, unless scientific notation is your thing: 2²³, or about 8.3 million, combinations—meaning that there’s about a 1 in 8 million chance that the same mother and the same father would have two kids with the exact same coding (excluding identical twins). (See figure 1.3.)

But that’s only part of the story. Consider identical twins. They get dealt exactly the same DNA, but they may develop different traits down the line: One may have allergies and the other may not, one may develop a particular disease and the other may not, one may be able to play the piano without ever learning how to read music, while the other can’t carry a tune with a dump truck. What accounts for these differences? Something in their environment—potentially as early as in utero—affected the expression of their genes differently. That something is called epigenetics.

Figure 1.2 All Wound Up Storing more data than any computer, each chromosome contains all the information needed to give you a base for your physical and emotional characteristics. What we can learn from epigenetics is that you have the power to influence the course of biological destiny.

Here’s how it works:

Each cell in the human body contains about 2 meters of DNA that’s packed into a tiny nucleus that’s only about 5 micrometers in diameter. That’s the rough equivalent of stuffing two thousand miles of sewing thread into a space the size of a tennis ball. As with thread, DNA is wound around spools of proteins called histones. Not all of your DNA gets expressed, or used to create proteins, in every cell; in fact, most of the spools of DNA in each cell are stored away, some never to be seen or heard of again.

A good way to visualize the process: Let’s say that you and your partner each comes to your relationship with a set of favorite family recipes. You may contribute a blue-ribbon chili recipe, and your significant other may bring a killer lemon meringue pie to the table. But it’s not just two recipes, it’s hundreds, maybe thousands. (The human genome has some twenty to thirty thousand genes, after all.) Some on index cards, some in books, some on torn-up shreds of cocktail napkins. So what do you do with all these cranberry mold recipes? Stuff each and every one of them in the kitchen drawer. Now it’s hard to sift through them, you don’t have access to many of them, and you really can’t find what you want. Unless …(you knew there was an “unless” coming) you get them organized, say, by sticking hot pink Post-it notes on the recipes you really want to access quickly. You tag your favorite recipes, so you can quickly search, find, and put them into action.

That’s the way epigenetics works.

Genes are like recipes—they’re instructions to build something. Both mom and dad contribute a copy of their entire recipe book to their offspring, but for many genes, only one copy of each recipe will be used by the baby. Mom and dad have the same recipes (one for eye color, one for hair color, one for toenail growth rate, and so on), except they may have slightly different versions of those recipes (they’re called alleles). For example, eye genes are either brown or blue or green. For such genes, you express only the gene from your mom or dad—that is, only one copy is active, but not both. In some cases, neither copy will need to be expressed: Eye color matters only to eye cells; a liver cell doesn’t need either mom’s or dad’s eye color gene to be cranking away.

Figure 1.3 Cell Power On day one, we start with a single fertilized egg (called a zygote). Then cells divide and divide and divide, forming the biological structure of the fetus. By the end of the pregnancy, cells have divided a whopping forty – one times to end up with trillions of cells.

So how does a cell turn off the 24,999 genes it doesn’t need and turn on the few it does? Every cell—and there are around 200 different types in the body—needs to know which few genes are relevant for it, and, of those genes, whether mom’s or dad’s is going to be expressed. As with the kitchen drawer full of recipes, the genes alone are useless unless there’s a way to find what you need when you need it.

There is. Your body puts biological Post-it notes called epigenetic tags on certain genes to determine which genetic recipes get used. This tagging happens through a couple of chemical processes (such as methylation and acetylation), but guess what? Actions you take during your pregnancy can influence these processes and determine where the Post-it notes go and which genes will be expressed, ultimately affecting the health of your child. (See figure 1.4.)

When DNA gets tagged, it changes from being tightly wound around those histone proteins to being loosely wound, making the genes accessible and able to be expressed. At any given time, only 4 percent of your genes are in this accessible state, while the rest can’t be actively used in the body. By determining which genes are turned off and which are turned on, epigenetics is what makes you unique.

Here’s a point that will help you put epigenetics in perspective: We share 99.8 percent of the same DNA as a monkey, and any two babies share 99.9 percent of the same DNA. Heck, we even have 50 percent of the same DNA as a banana.^* So genes alone cannot explain the diversity in the way we look, act, behave, and develop. How those genes are expressed plays a huge role in how vastly different we are from monkeys and how explicitly and subtly different we are from one another.

Figure 1.4 The DNA Drawer The way genes are expressed is a little like pulling recipes out of a drawer. They may be there, but it can take some work to find them.