5 Methods of DNA Repair Enzymes
You might’ve wondered why no two people are alike at some point in your life. One of the key foundations that make you, well, you, is DNA. Because DNA is so important, it’s vital for our bodies to have ways to fix it when it becomes damaged. In fact, your DNA is damaged and repaired every single day, and you probably don’t even realize it.
We’re going to look at what DNA is, and then we’re going to jump into how enzymes are used to fix DNA damage. Don’t worry. As daunting as it sounds, you’ll have a solid understanding of the five ways DNA is repaired by the end of this blog!
What is DNA?
DNA, short for deoxyribonucleic acid, is a type of molecule that carries genetic information. Genetic information is like your body’s instruction manual for how to grow and develop. It controls the way you look, your health and risk for diseases, and the way your body works. The genetic information DNA carries makes sure that every single cell knows how to keep the body functioning.
Every living organism has a unique genome, or set of DNA, which means everyone has slightly different genetic information. Within your body, every single cell has a copy of all of your DNA. Cells use information from the DNA to produce proteins through transcription and translation. It is important that cells can make these proteins properly because proteins carry out many bodily functions. Therefore, the DNA must be used to make them accurate and undamaged.
What is DNA Damage?
DNA damage occurs when a section of DNA changes in a way that makes it unable to function normally. DNA damage can look like missing, distorted, or damaged sections of DNA. When DNA becomes damaged, cells don’t “read” the DNA the same way they would if they were undamaged.
As a result, the proteins produced from reading that DNA might not work properly, or even at all. The incorrect DNA could also be passed down when the cell divides, leading to a mutation. Thus, damaged DNA needs to be fixed as soon as possible.
DNA damage happens both naturally and because of environmental factors. Smoking, too much exposure to UV light, certain chemicals, and free radicals in your body can all promote DNA damage. Luckily, your body has many different ways to repair DNA using specialized enzymes.
Base Excision Repair
One of the ways your body can repair a small portion of DNA is through a process called base excision repair. The process of base excision repair replaces a damaged base while the cell is in its growth stage. Enzymes called DNA glycosylases first identify the damaged base. Then, DNA glycosylases cut the damaged base out of the DNA sequence.
Next, enzymes called endonuclease and phosphodiesterase work together to cut out the section where the damaged base was. Once they're done, the enzyme DNA polymerase comes and “reads” the base that was complementary to the old, damaged base. It determines what base should fill in the missing spot and adds back in a new base.
Finally, an enzyme known as DNA ligase fills in the gaps and seals the DNA strand in the spot where the damaged base was removed and replaced.
Nucleotide Excision Repair
Although base excision repair is effective, it's only used for replacing one damaged base. If a larger section of DNA warps, nucleotide excision repair kicks in. The damage that nucleotide excision repair fixes mostly results from environmental factors. For example, UV light or carcinogens that distort the shape of the DNA.
First, proteins called UVrA and UVrB look for damaged sections in the DNA chain. Once they identify a distorted section, the enzyme endonuclease cuts out and removes the entire damaged strand. DNA polymerase then fills in the missing bases.
Finally, DNA ligase seals the gaps between the preexisting and the newly added sections of the DNA strand.
Mismatch Repair
When pairing bases in your DNA, adenine is always paired to thymine, and cytosine is always paired to guanine. But what if adenine is accidentally paired with cytosine? While DNA is being copied by DNA polymerase, the enzyme sometimes makes a mistake and pairs the wrong bases together. That’s when mismatch repair comes into play.
First, a specialized enzyme called exonuclease chops off a section of the strand where the mismatched base is. In doing so, it removes not only the mismatched base but also some of the bases around it. DNA polymerase then returns and replaces the incorrect base with the correct one. It also adds back in the extra bases that were removed by the exonuclease.
Finally, DNA ligase fixes the breaks between the old and new strand sections.
Homologous Recombination
Sometimes there can be damage to both strands of DNA, known as double-strand breaks. When this happens, your body can use homologous recombination to repair the broken strands. To begin this process, a similar, undamaged section of DNA is found in the cell.
Next, DNA polymerase fills in the damaged strand with the correct bases, using similar DNA base pair sequences as a template. Next, a helicase enzyme attaches the now-repaired section back to its strand. This process is then repeated with the other broken strand.
Finally, DNA ligase seals the new and old sections of the once-damaged DNA together.
Non-Homologous End Joining
The final major method of DNA repair is called non-homologous end joining. Like homologous recombination, this method is used to repair double-strand breaks. To begin non-homologous end joining, any damaged sections are cut out of the DNA.
Then, DNA polymerase bridges the gaps where the damaged sections were with new base pair sequences. These new sequences are not necessarily the same base pairs that were there before the double-strand break happened; they are used simply to connect the broken strands. After the base pairs are added, DNA ligase fuses together the broken DNA sections.
Non-homologous end joining is a quick fix. However, since it just cuts out the damage and joins the strands back together, mutations could form as a result of the DNA being slightly changed. As a result, it’s not ideal unless homologous recombination isn’t possible.
Hopefully, you now have a better understanding of how DNA is repaired! Let's recap the purposes of the five methods of repairing DNA. Base excision repair is for replacing a single base.
Nucleotide excision repair is for replacing multiple bases next to each other. Mismatch repair is for replacing bases that have been matched incorrectly. Homologous recombination is for fixing double-strand DNA breaks.
Finally, non-homologous end joining is for repairing double-strand breaks when a similar DNA section isn’t available. Keep in mind that these are the five major ways enzymes repair DNA, but there are other methods out there.
Want some more examples of how DNA damage can happen and be fixed? Or, do you want a more in-depth explanation of this blog? Sign up for UPchieve today for free, personalized one-on-one help with an UPchieve tutor!