SAS' Cell Biology Page

DNA Replication

The purpose of cell division is to create a new daughter cell with a complete set of DNA copied from the parent cell, a process that occurs during the S phase of the cell cycle. For the daughter cells to work properly, their copy of DNA must be an exact copy. If an error happens, then the unlucky daughter cell will make the wrong protein. If that protein is an enzyme, then far more things may not work properly in the cell. So how does the parent cell make an exact copy of its DNA for the daughter cell?

If you think about the shape of the DNA molecule, the first step seems as obvious as making a photocopy from a book. The DNA molecule has to be uncoiled. This step is accomplished by the enzyme helicase. Helicase uncoils the DNA molecule by breaking the hydrogen bonds between the two strands. Helicase has the root that means helix, which is the shape of DNA.

The next step does the same thing as opening a book to the page that you want to copy, then laying it on the copier. The DNA molecule has to be unzipped, then the section of DNA which has to be copied must be read. This step is performed by the enzyme polymerase. Polymerase pulls the halves of DNA apart, then reads the nucleic bases, whether it is adenine, cytosine, guanine, or thymine.

Now here is the tricky part. Polymerase starts with the leading strand, starting at the end of the DNA molecule's leading strand, and reading the nucleic bases one-by-one until polymerase reaches the other end.

As polymerase reads the nucleic bases, it pulls complimentary bases in to make a base pair. If polymerase reads thymine, it pulls in thymine's complimentary base, adenine, making the base pair thymine-adenine. If polymerase reads guanine, it pulls in guanine's complimentary base, cytosine, making the base pair guanine-cytosine. And for adenine, thymine is pulled in and paired with the base. For cytosine, guanine is paired with the base.

What about the other strand? Isn't DNA made of two strands? Another polymerase starts reading the other strand. This strand is called the lagging strand because polymerase reads this strand second. The polymerase on the lagging strand makes base pairs in the same manner as the polymerase on the leading strand works. Well, there is one difference. The polymerase working on the lagging strand works discontinuously. The polymerase on the lagging strand adds bases to one section of the strand at one place, jumps ahead to add bases to a different section of the lagging strand. Then it may jump behind to add bases there. Its jumping all over the place on the lagging strand, making base pairs.

Once both polymerases have reached the end, the parent cell has two new DNA molecules, perfect copies of the original. Well at least most of the time. In several million copies, error, or mutations, do occur.