DNA, CHROMOSOMES AND GENES
How do these terms relate to one another?
DNA? Chromosomes? Genes? Aren't these just different terms for the same thing? Well, yes and no. To begin to figure out the difference between these terms, we'll have to learn a little bit about the life of a cell and it's nucleus.
When a cell is not actively dividing, its nucleus contains chromatin, a tangle of fibers composed of protein and DNA. When the time comes for the cell to divide into two new cells, the DNA is duplicated via Mitosis so that each new cell can receive a complete copy of all the genetic material in the "parent" cell. This makes perfect sense if you think of it in practical terms. What if you and another person were both in charge of putting together a bicycle. As part of the deal, you and your friend would each have a bike in your own separate room without being able to contact each other. But the person asking you and your friend to put the bicycles together only had one set of directions, so the directions were torn in half and each of you received one half. It's pretty obvious that, unless you're a whiz at putting bicycles together, there's going to be some trouble unless each of you has your own complete set of directions!
During cell division chromatin organizes itself into chromosomes. Each chromosome contains a DNA molecule, and each DNA molecule is made up of many genes-individual segments of DNA that contain the instructions needed to direct the synthesis of a protein with a specific function.
Different organisms differ in their complexity and therefore have different numbers of chromosomes and genes. Up to a certain point (and there are some exceptions), the more complex an organism is, the more chromosomes it has. A frog, for example, has 26 chromosomes (13 pairs), whereas a human has 46 chromosomes (23 pairs). Why do we make a reference to there being 23 pairs? Why don't we just say 46 chromosomes? This is because the 46 chromosomes in human somatic cells are actually 23 matched pairs of homologous chromosomes! A good analogy is a pair of socks...with one exception. Each member of the pair comes from one parent; so they may be the same size and shape, but they don't carry exactly the same information.
There are two categories of cells in multicellular organisms based on their chromosomes: Somatic (another word for "body" cells) and Gametes (reproductive cells). In human gametes, each sperm cell has 23 chromosomes, and each egg cell has 23 chromosomes through the process of Meiosis. This is called the haploid number and is represented by the letter n. When these two gametes get together, they form a complete human whose somatic cells have 46 chromosomes. This is the diploid number and is represented by 2n. So, since the frog has a diploid number of 26 (2n=26), its haploid chromosome number is 13 (n=13). A fruit fly has a diploid number of 8. So what is the number of chromosomes in its gametes (the haploid number)?
Each chromosome contains a single immense molecule of DNA that, in humans, has a length of up to 12 centimeters when stretched out!
Hey! This is 12 centimeters!
(on my screen, at least...but you get the idea)
As a matter of fact, all the DNA in one human cell (on all 46 chromosomes) is about two meters long, yet fits into a cell nucleus which is 2-3 micrometers (that's .000002 meters wide!).
Yet, the DNA must still be in such a state as to allow for enzymes to replicate the molecule or initiate the production of a protein. The 23 pairs of human chromosomes are estimated to include about 20,000 - 25,000 genes. Each gene codes for ONE protein; we'll see how this process works later in the tutorial.