In this animation we’ll see the remarkable way our DNA is tightly packed up so that 6
feet of this long molecule fits into the microscopic nucleus of every cell.
The process starts when DNA is wrapped around special protein molecules called histones.
The combined loop of DNA and protein is called a nucleosome. Next the nucleosomes are packaged
into a thread. The end result is fiber know as chromatin.
The fiber is then looped and cooled again, leading finally to the familiar shapes known
as chromosomes, which can be seen in the nucleus of dividing cells.
Chromosomes are not always present. They form around the time cells divide when the two
copies of the cells need to be separated.
Using computer animation based on molecular research we are now able to see how DNA is
actually copied in living cells. You’re looking at an assembly line of amazing miniature
bio-chemical machines that pulling apart the DNA double helix and cranking out a copy of
each strand.
The DNA to be copied enters the production line from the bottom left. The whirling blue
molecular machine is called helicase. It spins the DNA as fast as a jet engine as it unwinds
the double helix into two strands. One strand is copied continuously and can be seen spooling
off to the right.
Things are not so simple for the other strand because it must be copied backwards. It is
drawn out repeatedly in loops and copied one section at a time. The end result is two new
DNA molecules.
What you are about to see is DNA’s most extraordinary secret; how a simple code is
turned into flesh and blood. It begins with a bundle of factors assembling at the start
of a gene. A gene is simply a length of DNA instruction stretching away to the left. The
assembled factors trigger the first phase of the process; reading off the information
that will be needed to make the protein.
Everything is ready to go. 3-2-1-Go! The blue molecule racing along the DNA is reading the
gene it’s unzipping the double helix and copying one of the two strands. The yellow
chain snaking out of the top is a copy of the genetic message and it’s made of a close
cousin of DNA called RNA.
The building blocks to make the RNA enter through an intake hole. They are matched to
the DNA letter by letter to copy the A’s, C’s, T’s, and G’s of the gene. The only
difference is that in the RNA copy the letter T is replaced is replaced with a closely related
building block known as U.
You are watching this transcription process in real time. It’s happening right now in
almost every cell in your body.
When the RNA copy is complete it snakes out into the outer part of the cell. Then, in
a dazzling display of choreography all the components of a molecular machine lock together
around the RNA to form a miniature factory called a ribosome. It translates the genetic
information in the RNA into a string of amino acids that will become a protein.
Special transfer molecules, the green triangles, bring each amino acid to the ribosome. The
amino acids are the small red tips attached to the transfer molecules.
There are different transfer molecules for each of the twenty amino acid. Each transfer
molecule carries a three letter code which is matched with the RNA in the machine.
Now we come to the heart of the process. Inside the ribosome the RNA is pulled through like
a tape. The code for each amino acid is read off, three letters at a time and matched to
three corresponding letters on the transfer molecules. When the right transfer molecule
plugs in, the amino acid it carries is added to the protein chain.
Again, you are watching this in real time.
And after a few seconds the assembled protein starts to emerge from the ribosome. Ribosomes
can make any kind of protein. It just depends on what genetic message you feed in on the
RNA. In this case the end product is hemoglobin.
The cells in our bone marrow churn out a hundred trillion molecules of it per second, and as
a result our muscles, brain, and all the vital organs in our body receive the oxygen they
need.