Humans come in many shapes and sizes, but we’re all very similar at the DNA level. In fact, the genomes of any two people are more than 99% the same. Still, the tiny fraction of the genome that varies among humans is very important. DNA variations are part of what makes each of us unique. They affect the color of your eyes, hair and skin. What’s more, they influence your risk of disease and your response to drugs.
Our genomes also are likely to contribute to some of the ways we feel, think and act. But keep in mind that many other things, such as how you were raised and your access to medical care, can influence your behaviors and your health.
If you could peer inside your cells, you’d see your genome contained in 46 tightly packed bundles of DNA — 23 came from your mother and 23 from your father. These DNA bundles, called chromosomes,provide the instructions that enable a one-cell embryo to develop into a 100 trillion-cell adult.
But DNA isn’t just about growth. It instructs cells throughout your life — telling them how to respond to the foods you eat, the germs you encounter and the pollutants to which you are exposed. Ultimately, DNA even influences how you age.
To understand DNA’s instruction manual, let’s look at its structure. If you unwind the DNA molecule packed into each chromosome, it looks like a twisted ladder. The rungs of this ladder are made from four types of chemical building blocks. These blocks — adenine, thymine, cytosine and guanine — are abbreviated with the letters A, T, C and G.
Depending on how many of these building blocks are stacked together and the order in which they are arranged, DNA can produce many different types of organisms. It takes about 3 billion pairs of A’s, T’s, C’s and G’s to write the instructions needed to build a human. So, every time the human body produces a sperm or an egg, 3 billion DNA letters must be copied and packaged so they can be passed along to future offspring
Reference: National Human Genome Research Institute ~ A Guide to your Genome