elephants and Escherichia Coli,

man and mushroom. All so different in form and function,

but amazingly the same in how their form and function are determined.

First, all these organisms are made of one or more cells,

and every cell of every living thing on earth

contains all the information it takes

to create and duplicate and make variations of itself.

That information is stored in a very long but quite simple

double molecule called DNA,

or Deoxyribonucleic Acid.

And the DNA of every living organism is made of chains of four smaller molecules

called nucleotides. What dictates the difference between a man and a mushroom

is the sequence of these nucleotides in the long DNA chain.

The four differing nucleotide parts, called bases,

are made of a few carbon, oxygen, hydrogen, nitrogen and phosphorus atoms,

and the molecules look like this.

And each of these four bases is attached to an identical backbone molecule,

a sugar called deoxyribose - the "D" in DNA - and a phosphate group.

Let's simplify these nucleotides and show them like this.

So, a single sequence of nucleotides joined by their common sugars would look like this.

And the DNA molecule where such sequences are stored looks like this.

But how does a simple molecule dictate the form and function of millions of different living things?

You can think of DNA as a great library of information,

information that is used to do one thing and one thing only:

direct the building of different protein molecules.

And it's the proteins that build the cells and keep them functioning

and changing and reproducing. Here's where the familiar word 'gene' comes in.

If your DNA is a library of information, a gene is a book in that library.

A gene is a segment of the DNA molecule.

Let's say your body needs a particular protein, like insulin.

To get it, some of your cells send a protein signal through the bloodstream

to the cells in your pancreas, where insulin is made.

That signal protein tells other proteins in the cell's nuclei

to open up a part of the DNA double helix, the insulin gene,

and start making insulin proteins.

As soon as enough insulin has been produced,

another signal protein comes to the pancreas' cells that tells them to stop making insulin.

It's like looking up a book in the DNA library about insulin,

and then putting it back when you're done.

There are genes in DNA for visible and invisible things that make up your body,

like genes for eye color, protein pigments, for skin color,

for hair color, for stopping and starting bone growth,

for your blood type, for how many fingers or arms and legs you have,

for proteins that influence how long you live.

Your DNA probably contains between 25 thousand and 40 thousand genes,

while the DNA of a worm or a plant or a fruit fly

contains about 12 thousand to 20 thousand genes.

Some of those genes have quite different sequences of nucleotides than yours,

and some are similar to yours.

Though it happens infrequently,

our own nucleotide sequences can change

as the result of spontaneous or environmental damage

which might remove or shift a nucleotide position.

This changes the gene involved, and can then change the protein.

Most of these changes, called mutations,

have very little effect on the organism or its descendants.

some are mildly damaging,

and a few can make the organism better-suited to its environment.

It is these tiny changes in DNA gene sequences, happening over millions of years,

that create the differences among living organisms, from geckos to grasshoppers.

worms to watermelons, elephants to Escherichia Coli, and man to mushroom.