Where's the DNA?
Normal DNA
I, Helfiss G. Telmene, present to you an average cell with a few of its average features. Nothing too outrageous, just a cell membrane, a nucleus, and some chromosomes. Not to patronize, but the border of the blue field is the cell membrane. If you were a single-cell organism, then this is the edge of you. However, if you are reading this, then I assume that you are a multicellular organism, made up of trillions of cells, each performing different tasks that keep you happy, healthy, and alive. The yellow field shows the nucleus. The nucleus is the hub of all cellular activity. If your cell came with instructions, which it does, these instructions would be found in the nucleus.
The wavy black lines in the nucleus are the chromosomes. Do you remember when I mentioned cellular instructions in the nucleus? These are the instructions. However, they are SO MUCH MORE than just cellular instructions. Let’s start with the most obvious and average case – the “A” chromosomes. Notice how the normal “A” chromosomes are always found in pairs of two. Granted, each pair might differ in size and each pair has different set of instructions, but in the end they simply come in pairs of two and contain all the biological information you need to function properly.
Mitochondrial DNA
I now present to you a mitochondrion named Michael O’Gabriel. He is one of hundreds of mitochondria in each of your cells found outside of the nucleus. You probably know about mitochondria as “the powerhouse of the cell”. True, mitochondria create cellular energy from the food that you eat, but under closer examination they are far more interesting. Mitochondria are one of the only organelles that contain their own DNA. Unlike the DNA found in your nucleus and the nucleus of all animals, plants, and fungi, mitochondrial DNA are very similar bacterial DNA; you know, the bacteria 
that makes you sick and cause you endless suffering. Mitochondrial DNA does a few things, but not much. For one thing, mitochondrial DNA tends to influence its own reproduction. For the most part, this is great. When you need more energy, the mitochondrial reproduces. This works because all the mitochondria in all of your cells have the same DNA. However, this can fall apart when the mitochondria start to vary in you. Note the slight color differences among the illustrated mitochondria. When this happens, some mitochondria might become selfish, such as Mr. O’Gabriel.
"B"Chromosomes
I now present to you a chromosomal curiosity. Let me introduce you to Madam Bea. Madam Bea a “B” chromosome and she is a genetic anomaly. There is DNA in your “A” chromosomes, but there can also be DNA in “B” chromosomes. “B” chromosomes are essentially extra chromosomes that are found in some individuals and some cells within an individual.
But then you say, “Wait a minute! I thought that all cells have the same chromosomes. Are you telling me that some of my cells can have “B” chromosomes but not others?” And I say, “YES! You narrow-minded-recalcitrant nimrod.” What’s more, these little deviants like to party. You might find a single “B” chromosome, 2, 3, 4, or even 20 “B’s” in the same cell. This is 
due to the fact that Madam Bea and her “B” chromosomes are not like your “A” chromosomes, which are always paired. The number of “B” chromosomes in a cell is highly variable. In addition, some cells have no “B” chromosomes.
So, what is the deal Madam Bea? What does having “B” chromosomes entail? To what horrible fate am I bound? Well, to be honest, you do not have much to worry about. And you can take solace in the fact that if you are reading this, the “B” chromosomes that you may or may not have did not affect you in any noticeable way.
All Your DNA
So here it is, all of your DNA. You have the predictable “A” chromosomes, the surprisingly complex Mitochondrial DNA, and the always diverse and unusual “B” chromosomes. Together, they make you who you are. So, praise them you genetic stud or feel free to blame the “A”, “B”, and Mitochondrial DNA for all of your problems (at least your genetic ones).