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Chapter 14: Problem 13

Distinguish between proofreading and mismatch repair.

Short Answer

Expert verified
Answer: The main differences between proofreading and mismatch repair during DNA replication are the timing of the processes, the enzymes involved, and the mechanisms of error correction. Proofreading occurs during replication and is carried out by DNA polymerase, while mismatch repair takes place after replication and involves separate sets of proteins. The mechanism of error correction in proofreading involves the removal of mispaired nucleotides by DNA polymerase, whereas mismatch repair proteins replace a longer segment of the new DNA strand. Both processes contribute to the overall fidelity of DNA replication and prevent the accumulation of mutations in the genome.

Step by step solution

01

Define Proofreading

Proofreading is a process that occurs during DNA replication, where DNA polymerase checks the accuracy of the nucleotide bases it has added to the new strand of DNA. If an incorrect nucleotide has been added, DNA polymerase detects the error, removes the incorrect nucleotide, and replaces it with the correct one.
02

Define Mismatch Repair

Mismatch repair is a separate process that occurs after DNA replication has been completed. Proteins involved in mismatch repair recognize any errors in the new DNA strand that were not corrected during proofreading, e.g., mismatched base pairs or small insertions/deletions. These proteins then excise the incorrectly paired section of DNA and replace it with the correct nucleotides, using the original DNA strand as a template.
03

Compare Timing of Processes

One primary difference between proofreading and mismatch repair is the timing of the processes. Proofreading occurs during DNA replication, whereas mismatch repair takes place after replication has been completed.
04

Compare Involved Enzymes

Another significant difference is the enzymes involved in each process. In proofreading, DNA polymerase carries out both the synthesis of the new DNA strand and the correcting of any errors. In mismatch repair, a separate set of proteins, such as MutS, MutL, and MutH in bacteria or MSH and MLH proteins in eukaryotes, are responsible for detecting and fixing errors in the new DNA strand.
05

Compare Mechanisms of Error Correction

Finally, the mechanisms of error correction also differ. In proofreading, DNA polymerase identifies and corrects errors through its 3' to 5' exonuclease activity, which removes the mispaired nucleotide and allows the polymerase to continue synthesis. In contrast, mismatch repair proteins replace a longer segment of the new DNA strand, including the incorrect nucleotide, by excising it and then resynthesizing the correct sequence using the original DNA strand as a template. In summary, proofreading and mismatch repair are two distinct processes that help maintain the accuracy of DNA replication. Proofreading occurs during replication and is performed by DNA polymerase, whereas mismatch repair takes place after replication has been completed, and involves a separate set of proteins. Both processes contribute to the overall fidelity of DNA replication and help prevent the accumulation of mutations in the genome.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Proofreading
During the process of DNA replication, proofreading plays a crucial role. It is responsible for checking the accuracy of the newly formed DNA strand. The main enzyme responsible for this is DNA polymerase. This enzyme ensures each nucleotide added is the correct match for the template strand.

Proofreading occurs as DNA replication happens. If DNA polymerase recognizes a mistake, it uses its proofreading ability to remove the incorrect nucleotide. Only then does it insert the correct nucleotide, maintaining the integrity of the genetic code.

Proofreading is like an immediate quality check that happens in real-time during the synthesis of DNA. This process prevents many potential errors from being passed on, thus maintaining genetic stability.
Mismatch Repair
Mismatch repair is a mechanism that takes place after DNA replication has concluded. It acts as an additional line of defense to correct errors that were missed by proofreading.

Special proteins are involved in the mismatch repair process. In bacteria, proteins such as MutS, MutL, and MutH detect mismatches or small errors like insertions or deletions. In eukaryotes, the MSH and MLH proteins perform similar tasks.

When a mismatch is found, these proteins remove a section of the newly synthesized DNA strand that includes the error. The correct sequence is then synthesized using the original template strand. Mismatch repair ensures that replication errors not fixed immediately are corrected, further reducing mutation rates.
DNA Polymerase
DNA polymerase is an essential enzyme during DNA replication. It performs two primary functions: synthesizing the new DNA strand and proofreading each nucleotide added.

This process is vital during replication to ensure accuracy. DNA polymerase adds nucleotides one by one to the growing DNA strand, always ensuring the complementary base is used.

The enzyme's dual role supports immediate error detection and correction, limiting the potential for genetic errors. This process is possible due to DNA polymerase's unique ability to correct mismatches during replication, ensuring high fidelity of the DNA replication process.
Exonuclease Activity
Exonuclease activity is an essential function of certain enzymes, including DNA polymerase, during DNA replication. This property allows the enzyme to remove incorrectly paired or unwanted nucleotides.

In the context of DNA replication proofreading, exonuclease activity occurs in the 3' to 5' direction. If DNA polymerase inserts the wrong nucleotide, the exonuclease activity enables the enzyme to backtrack and excise the mismatched base. Immediately after removal, a correct nucleotide is added.

This capability significantly contributes to the accuracy of DNA replication. By having exonuclease activity, DNA polymerase can correct errors on-the-go, thus playing a critical part in maintaining genetic fidelity.

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Most popular questions from this chapter

Distinguish between spontaneous and induced mutations. Give some examples of mutagens that cause induced mutations.

In this chapter, we focused on how gene mutations arise and how cells repair DNA damage. In particular, we discussed spontaneous and induced mutations, DNA repair methods, and transposable elements, Based on your knowledge of these topics, answer several fundamental questions: (a) How do we know that mutations occur spontaneously? (b) How do we know that certain chemicals and wavelengths of radiation induce mutations in DNA? (c) How do we know that DNA repair mechanisms detect and correct the majority of spontaneous and induced mutations?

In maize, a \(D s\) or \(A c\) transposon can cause mutations in genes at or near the site of transposon insertion. It is possible for these elements to transpose away from their original site, causing a reversion of the mutant phenotype. In some cases, however, even more severe phenotypes appear, due to events at or near the mutant allele. What might be happening to the transposon or the nearby gene to create more severe mutations?

Two related forms of muscular dystrophy-Duchenne muscular dystrophy (DMD) and Becker muscular dystrophy (BMD)-are both recessive, X-linked, single-gene conditions caused by point mutations, deletions, and insertion in the dystrophin gene. Bach mutated form of dystrophin is one allele. Of the two diseases, DMD is much more severe. Given your knowledge of mutations, the genetic code, and translation, propose an explanation for why the two disorders differ greatly in severity.

Blectrophilic oxidants are known to create the modified base named 7,8 -dihydro- 8 -axoguanine (oxoG) in DNA. Whereas guanine base-pairs with cytosine, oxoG base-pairs with either cytosine or adenine. (a) What are the sources of reactive oxidants within cells that cause this type of base alteration? (b) Drawing on your knowledge of nucleotide chemistry, draw the structure of \(0 \times 0 \mathrm{G},\) and, below it, draw guanine. Opposite guanine, draw cytosine, including the hydrogen bonds that allow these two molecules to base- pair, Does the structure of oxoG, in contrast to guanine, provide any hint as to why it basepairs with adenine? (c) Assume that an unrepaired oxoG lesion is present in the helix of DNA opposite cytosine. Predict the type of mutation that will occur following several rounds of replication. (d) Which DNA repair mechanisms might work to counteract an oxoG lesion? Which of these is likely to be most effective?
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