DNA Repair - Online Tutor, Practice Problems & Exam Prep

DNA replication is generally accurate, but errors can occur, leading to mutations that may cause diseases like cancer. DNA polymerases possess proofreading abilities, reducing the error rate from 1 in 100,000 to 1 in 10,000,000,000 base pairs. Other DNA repair enzymes assist in correcting any remaining errors. This proofreading and repair process is crucial for maintaining genetic integrity and preventing harmful mutations.

DNA Repair

In this video, we're going to begin our lesson on DNA repair. And so, it turns out that the DNA replication process is a nearly perfect process. Now although DNA replication usually is a perfect process that appropriately matches the nucleotides as they're supposed to, occasionally, mistakes can occur or errors can occur, such as DNA base pairing errors. And these DNA base pairing errors can occur throughout the DNA replication process. And so, for example, we know that 'A's or adenines are usually base paired with 'T's or thymines. However, if a mistake occurs or a DNA base pairing error occurs, then the 'A' may accidentally be paired up with a 'C' instead of the 'T'. And so, these DNA base pairing errors can actually occur at a rate of about 1 out of 100,000 base pairs, which at first glance may seem like a pretty low error rate. However, considering that there can be an enormous amount of DNA inside of a cell, millions if not billions of base pairs, and considering that DNA replication happens fairly frequently, this is actually a relatively high error rate. And so these unrepaired errors that occur can result in permanent mutations. And the mutations can potentially lead to diseases such as cancer. And so, it's really important for a cell to be able to repair all of these errors and so, moving forward, we're going to talk about some of these different DNA repair mechanisms.

But if we take a look at our example image down below, notice we have a cartoon here that is showing you how DNA errors somewhat resemble typos. And so, in this cartoon, notice that we have two people. The person on the right over here is actually a candidate that's applying for a job. And on the left-hand side over here, we have a boss that is looking to hire someone. And so, what you'll notice is that as this boss here is reading the resume from the candidate, the boss says, "This says that you are experienced in ruining small businesses." And so, notice that the candidate says, "Whoops, I meant to say running small businesses, not ruining." And so, notice that even just a single typo, this 'I' here exchanged for an 'N', or vice versa there, that can make a huge difference in the interpretation. And so, the same can occur with these mutations. Even just a single error, a single base pairing error could potentially lead to a very different result. And so, this here concludes our brief introduction to DNA repair and as we move forward, we'll get to learn a lot more. So stay tuned, and we'll see you in our next video.

DNA Proofreading & Repair Enzymes

In this video, we're going to briefly introduce DNA proofreading and DNA repair enzymes. DNA polymerases actually have a proofreading ability. This proofreading ability is really just an ability that these DNA polymerases have to find and fix many of the errors or mistakes that might occur during DNA replication. The proofreading ability of these DNA polymerases allows them to significantly reduce the error rate from 1 in 100,000 to just 1 in 10,000,000,000 base pairs, making mutations a lot less likely due to mistakes with DNA polymerases. However, they are still possible because 1 in 10,000,000,000 is still possible. Now, other DNA repair enzymes on top of the DNA polymerase proofreading can also be involved in helping to correct errors that may not have been fixed by proofreading. Below this text, our image shows a cartoon of a DNA polymerase proofreading the DNA sequence after replication. It says, "Looks like you made a typo here," which this proofreading ability allows for fixing those errors and mistakes in the DNA. Again, that will reduce the error rate to just 1 in 10,000,000,000 base pairs, which is significantly lower and makes mutations due to errors in DNA replication much less likely. This concludes our brief introduction to DNA proofreading ability and repair enzymes that can also help reduce error rates. We'll be able to get some practice applying these concepts as we move forward in our course. I'll see you all in our next video.

Researchers found a strain of E. coli bacteria that had mutation rates one hundred times higher than normal. Which of the following statements correctly describes the most likely cause of these mutations?

• A. The single-stranded binding proteins were malfunctioning during DNA replication.
• B. There were one or more mutations in the RNA primer.
• C. The proofreading mechanism of DNA polymerase was malfunctioning.
• D. The DNA polymerase was unable to add bases to the 3′ end of the growing DNA strand.

Here’s what students ask on this topic:

What is DNA proofreading and how does it reduce mutation rates?

DNA proofreading is a mechanism by which DNA polymerases correct errors that occur during DNA replication. These enzymes have the ability to identify and fix mismatched base pairs, significantly reducing the error rate. Initially, the error rate is about 1 in 100,000 base pairs, but with proofreading, it drops to approximately 1 in 10,000,000,000 base pairs. This drastic reduction in errors helps to maintain genetic integrity and minimizes the likelihood of mutations that could lead to diseases such as cancer.

How do DNA repair enzymes function in correcting replication errors?

DNA repair enzymes function by identifying and correcting errors that escape the proofreading activity of DNA polymerases. These enzymes scan the DNA for mismatches or structural abnormalities and initiate repair processes to correct these errors. For example, mismatch repair enzymes recognize and excise incorrectly paired bases, replacing them with the correct ones. This additional layer of error correction further reduces the mutation rate, ensuring the fidelity of genetic information.

Why is DNA repair important for preventing diseases like cancer?

DNA repair is crucial for preventing diseases like cancer because it maintains the integrity of the genetic code. Errors in DNA replication can lead to mutations, which, if not corrected, can accumulate and disrupt normal cellular functions. Some of these mutations can activate oncogenes or deactivate tumor suppressor genes, leading to uncontrolled cell growth and cancer. Therefore, efficient DNA repair mechanisms are essential for preventing the development of such diseases.

What are the different types of DNA repair mechanisms?

There are several types of DNA repair mechanisms, including mismatch repair, base excision repair, nucleotide excision repair, and double-strand break repair. Mismatch repair corrects errors that escape proofreading during DNA replication. Base excision repair fixes small, non-helix-distorting base lesions. Nucleotide excision repair removes bulky, helix-distorting lesions like thymine dimers. Double-strand break repair involves mechanisms like homologous recombination and non-homologous end joining to fix breaks in both DNA strands. Each mechanism plays a specific role in maintaining genetic stability.

How does the error rate of DNA replication change with and without proofreading?

Without proofreading, the error rate of DNA replication is approximately 1 in 100,000 base pairs. However, with the proofreading ability of DNA polymerases, this error rate is significantly reduced to about 1 in 10,000,000,000 base pairs. This reduction is achieved because the proofreading function allows DNA polymerases to identify and correct mismatched base pairs during replication, thereby enhancing the accuracy of DNA synthesis and reducing the likelihood of mutations.