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DNA Replication and Repair

DNA Replication and Repair

Replicating DNA strands are the basic building blocks of all living cells. The replicating machinery consists of three main components: the polymerase, which converts RNA into double-stranded DNA; the helicase, which joins together the two strands of DNA; and the transcription factors, which control how much each strand gets transcribed. These three components work in concert to make new copies of a gene or genes.

The first step in DNA replication is called “priming.” When a cell divides, it makes two daughter cells. One daughter cell contains the original nucleus (the genetic material) from one parent cell and a small amount of genetic material from the other parent cell.

The second daughter cell contains only the genetic material from its mother’s egg. The third daughter cell contains no genetic material at all!

When these daughter cells divide, they separate into two daughter cells with different genetic material. These two daughters have a 50/50 chance of having children with each other or not. If both daughters do have offspring, then the probability that any one child will inherit the same DNA sequence is 1 in 2^32.

After four “generations” the possibility of having any DNA sequence at all is very small.

The process of DNA replication and repair is similar in many ways to the process of making a backup copy of an important word-processing file. Most of the time this process is handled by a specialized machine (called the “polymerase”), which is started by an “initiation factor.” The initiation factor binds to a specific sequence (a certain type of letters) in the strand of DNA.

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The initiation factor is designed to bind to a specific strand of DNA. When this initiation factor binds to the proper site on the cell’s DNA, the polymerase immediately starts building a new strand of DNA that is an exact copy of the original strand.

For various reasons, sometimes the initiation factor doesn’t bind properly.

So what happens then? Does the cell die?

No. The polymerase just builds the wrong strand of DNA. When this other strand copies an incorrect strand, the result is a mutation (a mistake in the cell’s DNA). In order to keep the cell alive, the cell has to correct these mutations. So the cell has machinery to recognize when it has the wrong DNA strand and machinery to build the right strand. When these two types of machinery work properly, then all is well.

The initiation factor and the polymerase are two of a number of important proteins that copy DNA. (There are many different types of proteins. Each one is designed for a specific purpose, such as building bones, digesting food, or helping the cell reproduce.) These three types of proteins are just part of a very large group of proteins that maintain the cell’s DNA.

In other words, there are more than three ways to fix a mistake in the cell’s genes.

Most of these other ways aren’t quite so complicated. For example, some small mistakes can be easily fixed by a second type of protein known as an “editorial enzyme.” This kind of enzyme detects small mistakes in the newly-made DNA strand, and then cuts that strand of DNA in a precise place and then glues on a new piece of DNA.

This second way is much simpler than the complex method used by the initiation factor, the polymerase, and other more exotic proteins. In fact, there are at least a dozen different kinds of “editorial enzymes,” each one designed to fix a specific mistake in the newly-made DNA strand.

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Sometimes these editorial enzymes cut the wrong piece of DNA. When this happens, the cell has still more machinery to repair the newly-made DNA strand. This machinery includes yet another type of proteins known as “fill-in enzymes.” These fill-in enzymes find and glue new pieces of DNA onto the newly-made strand.

(These fill-in enzymes can only fix very small mistakes because if the mistake is too large, then the newly-made strand can’t be repaired).

In summary, our DNA molecules are being constantly bombarded by small and large mistakes. Fortunately, Mother Nature has created a large variety of different types of proteins to correct these mistakes. The DNA maintainance process is very complex, requiring at least three different types of specialized

proteins for each step in the process. This complexity is required to deal with the variety of mistakes that can occur in DNA.

The complexity of the maintenance process is most easily understood by comparing it to how we make a pie. If we wanted to bake a pie from scratch, we would have to get all of our ingredients and tools ready first. For example, we couldn’t start making the pie until we had turned on the oven and set the temperature.

We also couldn’t bake the pie until we had measured out all of the ingredients and had them sitting on the counter.

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This is similar to what happens inside our cells when new DNA is made. First, the cell has to have all of the right “ingredients” ready for when the cell divides. The DNA polymerase “mixes” these ingredients together and then watches over this process to make sure that everything goes smoothly.

Just like when we make a pie, we also have to wait for the oven to preheat before we can put the pie inside and cook it. In cells, this means that all of the “ingredients” of new DNA have to be present in high enough quantities before the cell will divide.

As you can see, DNA replication must be a very precise process. Otherwise, mistakes would occur and these mistakes would cause genetic diseases or cancer.

The next time you eat a piece of pie or look at sunflowers, remember all of the complex and elegant machinery that exists inside each and every one of your cells.

 

Sources:

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