Meet DNA Polymerase!

DNA Polymerase lives and works in the cell’s nucleus where it is the star of DNA replication.

A sequence of events:
1. One cell with one copy of DNA. 
2. DNA replication results in one cell with two copies of DNA.
3. Cell division results in two cells, each with one copy of DNA.
In the cell nucleus, DNA Polymerase makes a copy of DNA to prepare for cell division.

When a cell divides into two cells, it needs to make a copy of its DNA so that each resulting cell has a whole copy. To make sure that the copy is the same as the original, DNA Polymerase reads the code on one DNA strand and strings together DNA pieces (called nucleotides) to make a complimentary DNA strand.

Diagram of DNA replication process:
1. Double-stranded DNA separates into two single strands.
2. DNA Polymerase attaches to the DNA strands and makes a second complimentary strand for each.
3. The result is two copies of double-stranded DNA.
DNA Polymerase makes complimentary DNA strands for each original.
Cartoon of a partially closed hand mimicking DNA Polymerase's shape. DNA is shown in the palm of the hand, with the thumb and fingers circling around it.
DNA Polymerase is shaped like a hand.

DNA Polymerase is said to have a structure like a hand, with fingers, a thumb, and a palm.

The palm is where the biggest job happens: this is where the new DNA strand is put together. When a DNA piece enters the palm, the fingers close to draw the piece into position. The thumb attracts the new DNA strand so it doesn’t leave before it is completed.

Three-dimensional molecular model of DNA Polymerase. DNA Polymerase adopts a shape like a gently closed hand. In the "palm" rests DNA made of one original (template) strand and one newly-made strand. An ion near DNA helps the reaction. 

The "thumb" grips the new DNA strand and the "fingers" open and close to let DNA pieces inside the palm. The DNA pieces can approach the palm by a channel between the fingers and the thumb.

But DNA Polymerase is not just content with a first draft. Getting this job right is key to making sure the cell (and by extension, you!) have the right instructions. Bad instructions could be deadly. So to be extra careful, DNA Polymerase proofreads its work.

When DNA Polymerase makes a mistake, the palm sends the bad DNA away to the proofreading area. There, DNA Polymerase snips off the mistake just like a “backspace” key. Once the mistake is removed, the DNA can move back to the palm where copying resumes.

DNA Polymerase "types" out the word "REPLICATION!"
Along the way, it makes a mistake, deletes the mistaken letters, and finishes "typing."

(REPLI,
REPLICA,
REPLICASH,
REPLICA,
REPLICATI,
REPLICATION!)

It is surely a good thing that DNA Polymerase is so careful with its work. It has the important job of conserving the genetic instructions that define life. If it copies the instructions incorrectly in an unlucky part, the cell could lose a critical worker that it literally cannot live without. In reproduction, DNA Polymerase’s diligence in proofreading is responsible for keeping the ova and the sperm viable and preventing genetic diseases in the next generation. What important work!

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