DNA, CHROMOSOMES AND GENES
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)?……4!
Each chromosome contains a single immense molecule of DNA
that, in humans, has a length of up to 12 centimeters when
stretched out!
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.
Think of it this way: DNA is in genes, genes are on chromosomes.
When "mapping" all genes on all human chromosomes was first
seriously conceived, it was called the Human Genome Project - a
combination of gene and chromosome.