NHEJ vs HDR Explained: How Cells Repair DNA Edits

1. What is the primary characteristic of Non-Homologous End Joining (NHEJ)?

It joins broken DNA ends directly without a template

It uses a sister chromatid as a guide

It only happens during the S-phase

It requires the presence of an external donor DNA

It is 100% accurate every time

If the cell needs to fix a double-strand break quickly and lacks a matching DNA sequence to copy, then it uses NHEJ. If NHEJ simply ligates the broken ends back together, then it does not require a template.

Explanation

If the cell needs to fix a double-strand break quickly and lacks a matching DNA sequence to copy, then it uses NHEJ. If NHEJ simply ligates the broken ends back together, then it does not require a template.

2. In the comparison of NHEJ Vs HDR, NHEJ is considered the more error-prone pathway.

True

False

If NHEJ often results in the loss or gain of a few nucleotides at the repair site, then it frequently introduces mutations. Since HDR uses a template for precision, then NHEJ is the less accurate option in the NHEJ Vs HDR comparison.

Explanation

If NHEJ often results in the loss or gain of a few nucleotides at the repair site, then it frequently introduces mutations. Since HDR uses a template for precision, then NHEJ is the less accurate option in the NHEJ Vs HDR comparison.

3. The DNA repair pathway that requires a homologous sequence to guide the repair process is called ______-Directed Repair.

If the repair mechanism uses a complementary strand to ensure the original sequence is restored, then it is "directed" by that "homology."

Explanation

If the repair mechanism uses a complementary strand to ensure the original sequence is restored, then it is "directed" by that "homology."

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4. When analyzing NHEJ Vs HDR for gene editing, which pathway is typically used to "knock out" or disable a gene?

NHEJ

HDR

Mitosis

Translation

Glycolysis

If a researcher wants to break a gene's function, then they need to cause a frameshift mutation. If NHEJ naturally creates messy "indels" (insertions/deletions) when fixing a Cas9 cut, then it is the ideal pathway for gene knockouts.

Explanation

If a researcher wants to break a gene's function, then they need to cause a frameshift mutation. If NHEJ naturally creates messy "indels" (insertions/deletions) when fixing a Cas9 cut, then it is the ideal pathway for gene knockouts.

5. Which of the following are potential outcomes of the NHEJ repair process?

Small insertions of DNA

Small deletions of DNA

Perfect restoration of the sequence

Successful gene knockout

Replacement of a gene with a new sequence

If the cell jams DNA ends together, then it might lose bases (deletion) or add random ones (insertion). While it occasionally fixes the break perfectly, it cannot insert a specific new gene without a template, making Answer E incorrect.

Explanation

If the cell jams DNA ends together, then it might lose bases (deletion) or add random ones (insertion). While it occasionally fixes the break perfectly, it cannot insert a specific new gene without a template, making Answer E incorrect.

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6. In the study of NHEJ Vs HDR, the pathway that allows scientists to "knock in" a specific new sequence is ______.

If a scientist provides a donor DNA strand with the desired change, then the cell can use that strand as a blueprint. If this blueprint is used to edit the genome precisely, then the process is HDR.

Explanation

If a scientist provides a donor DNA strand with the desired change, then the cell can use that strand as a blueprint. If this blueprint is used to edit the genome precisely, then the process is HDR.

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7. HDR occurs most frequently during the G1 phase of the cell cycle when DNA is not being replicated.

True

False

If HDR requires a sister chromatid or a homologous template to function, then it must happen when those templates are available. Since templates are present after replication in the S and G2 phases, then HDR is restricted to those stages, making it inactive in G1.

Explanation

If HDR requires a sister chromatid or a homologous template to function, then it must happen when those templates are available. Since templates are present after replication in the S and G2 phases, then HDR is restricted to those stages, making it inactive in G1.

8. What is the main structural difference in the NHEJ Vs HDR decision made by a cell?

NHEJ works throughout the cell cycle; HDR is limited to S and G2 phases

NHEJ requires sugar; HDR requires fat

NHEJ only works in plants; HDR only works in animals

NHEJ is performed by ribosomes; HDR by mitochondria

There is no difference in timing

If NHEJ is the "default" emergency repair system, then it is available at all times. If HDR relies on having a second copy of the DNA (sister chromatid), then it can only occur after the DNA has been copied during the S-phase.

Explanation

If NHEJ is the "default" emergency repair system, then it is available at all times. If HDR relies on having a second copy of the DNA (sister chromatid), then it can only occur after the DNA has been copied during the S-phase.

9. The term "indels" refers to the random and that often occur during NHEJ repair.

If the repair process is messy, then bases may be removed (deleted) or extra bases may be added (inserted). Combining these two terms creates the word "indels."

Explanation

If the repair process is messy, then bases may be removed (deleted) or extra bases may be added (inserted). Combining these two terms creates the word "indels."

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10. When comparing NHEJ Vs HDR, which factors would encourage a cell to choose the HDR pathway?

The presence of a high concentration of donor DNA

The cell being in the G2 phase

The presence of homologous sister chromatids

The absence of all other enzymes

The cell being in the G0 (resting) phase

If the "tools" for HDR (template and specific enzymes) are present and the cell is actively dividing, then the likelihood of HDR increases. If the cell is resting (G0), it lacks these components and will favor NHEJ.

Explanation

If the "tools" for HDR (template and specific enzymes) are present and the cell is actively dividing, then the likelihood of HDR increases. If the cell is resting (G0), it lacks these components and will favor NHEJ.

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11. Why does Cas9 create a "Double-Strand Break" (DSB) to trigger these repair mechanisms?

To force the cell to use its natural repair pathways (NHEJ or HDR)

To kill the cell immediately

To turn the DNA into RNA

To make the DNA longer

To prevent the DNA from replicating

If Cas9 acts as a pair of molecular scissors, then it leaves a gap in the chromosome. If the cell detects a gap, then it must activate its internal repair systems (NHEJ or HDR) to survive, which allows scientists to hijack the process for editing.

Explanation

If Cas9 acts as a pair of molecular scissors, then it leaves a gap in the chromosome. If the cell detects a gap, then it must activate its internal repair systems (NHEJ or HDR) to survive, which allows scientists to hijack the process for editing.

12. In the context of NHEJ Vs HDR, a "silent" repair is more likely to happen with HDR than with NHEJ.

True

False

If "silent" means fixing the break without changing the genetic code, then precision is required. Since HDR uses a template to ensure the repair matches the original code exactly, then it is far more likely to result in a silent (perfect) fix.

Explanation

If "silent" means fixing the break without changing the genetic code, then precision is required. Since HDR uses a template to ensure the repair matches the original code exactly, then it is far more likely to result in a silent (perfect) fix.

13. In a CRISPR experiment, if you do not provide a donor template, the cell will be forced to use ______ to fix the break.

If the "blueprint" for HDR is missing from the test tube, then the cell has no guide to follow. If it has no guide, then it must use the template-free NHEJ pathway to close the DNA gap.

Explanation

If the "blueprint" for HDR is missing from the test tube, then the cell has no guide to follow. If it has no guide, then it must use the template-free NHEJ pathway to close the DNA gap.

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14. Which of the following best summarizes the trade-off in NHEJ Vs HDR?

NHEJ is fast but messy; HDR is slow but precise

NHEJ is for proteins; HDR is for lipids

NHEJ is only for viruses; HDR is for bacteria

NHEJ requires light; HDR requires darkness

There is no trade-off

If speed is the priority for cell survival, then the quick "glue" of NHEJ is used. If accuracy is the priority for fixing a specific mutation, then the slower, more complex "copy-paste" of HDR is required.

Explanation

If speed is the priority for cell survival, then the quick "glue" of NHEJ is used. If accuracy is the priority for fixing a specific mutation, then the slower, more complex "copy-paste" of HDR is required.

15. Which components are necessary to successfully perform an HDR-mediated gene correction?

Cas9 protein

Guide RNA (gRNA)

A donor DNA template

Microtubules

Ribosomes

If the goal is precision editing, then you need the cutter (Cas9), the map (gRNA), and the new blueprint (donor DNA). Microtubules and ribosomes are not involved in the direct chemistry of DNA repair.

Explanation

If the goal is precision editing, then you need the cutter (Cas9), the map (gRNA), and the new blueprint (donor DNA). Microtubules and ribosomes are not involved in the direct chemistry of DNA repair.

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16. The efficiency of ______ is generally lower than the efficiency of NHEJ in most human cell types.

If we compare the success rates of the two pathways, then we see that cells "prefer" the simpler NHEJ. If HDR requires more specific conditions and parts, then it is harder to achieve in a laboratory setting.

Explanation

If we compare the success rates of the two pathways, then we see that cells "prefer" the simpler NHEJ. If HDR requires more specific conditions and parts, then it is harder to achieve in a laboratory setting.

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17. Non-dividing cells, such as neurons, almost exclusively use NHEJ for DNA repair.

True

False

If HDR requires the cell to be in the S or G2 phase of the cell cycle, then cells that never divide cannot use it. If neurons are permanently in the G0 phase, then they must rely on NHEJ to fix double-strand breaks.

Explanation

If HDR requires the cell to be in the S or G2 phase of the cell cycle, then cells that never divide cannot use it. If neurons are permanently in the G0 phase, then they must rely on NHEJ to fix double-strand breaks.

18. How does the "Homology" in NHEJ Vs HDR work?

The donor DNA has ends that match the sequences on either side of the break

The DNA has the same color

The DNA is the same weight

The DNA comes from the same person

The DNA is shaped like a circle

If the repair machinery needs to "know" where to put the new DNA, then it looks for matching sequences. If the donor DNA has "homology arms" that pair with the target site, then the cell recognizes it as the correct template for repair.

Explanation

If the repair machinery needs to "know" where to put the new DNA, then it looks for matching sequences. If the donor DNA has "homology arms" that pair with the target site, then the cell recognizes it as the correct template for repair.

19. What could happen if NHEJ repairs a break in the middle of an "Exon" (coding region)?

A frameshift mutation might occur

The resulting protein might be non-functional

The gene might be permanently turned off

The gene might turn into a different species

The cell might stop needing oxygen

If NHEJ adds or removes a base in a coding region, then the entire triplet reading frame is shifted. If the frame shifts, then the protein will be garbled and the gene is effectively "knocked out."

Explanation

If NHEJ adds or removes a base in a coding region, then the entire triplet reading frame is shifted. If the frame shifts, then the protein will be garbled and the gene is effectively "knocked out."

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20. Finalizing the logic of ______, scientists are developing drugs that block NHEJ to force the cell to use the more precise HDR pathway.

If the cell naturally prefers the messy NHEJ path, then precision editing is difficult. If scientists can chemically "turn off" the NHEJ enzymes, t

Explanation

If the cell naturally prefers the messy NHEJ path, then precision editing is difficult. If scientists can chemically "turn off" the NHEJ enzymes, t

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Fixing the Break: NHEJ vs HDR Explained

1. What is the primary characteristic of Non-Homologous End Joining (NHEJ)?

2.

What first name or nickname would you like us to use?

2. In the comparison of NHEJ Vs HDR, NHEJ is considered the more error-prone pathway.

3. The DNA repair pathway that requires a homologous sequence to guide the repair process is called ______-Directed Repair.

4. When analyzing NHEJ Vs HDR for gene editing, which pathway is typically used to "knock out" or disable a gene?

5. Which of the following are potential outcomes of the NHEJ repair process?

6. In the study of NHEJ Vs HDR, the pathway that allows scientists to "knock in" a specific new sequence is ______.

7. HDR occurs most frequently during the G1 phase of the cell cycle when DNA is not being replicated.

8. What is the main structural difference in the NHEJ Vs HDR decision made by a cell?

9. The term "indels" refers to the random and that often occur during NHEJ repair.

10. When comparing NHEJ Vs HDR, which factors would encourage a cell to choose the HDR pathway?

11. Why does Cas9 create a "Double-Strand Break" (DSB) to trigger these repair mechanisms?

12. In the context of NHEJ Vs HDR, a "silent" repair is more likely to happen with HDR than with NHEJ.

13. In a CRISPR experiment, if you do not provide a donor template, the cell will be forced to use ______ to fix the break.

14. Which of the following best summarizes the trade-off in NHEJ Vs HDR?

15. Which components are necessary to successfully perform an HDR-mediated gene correction?

16. The efficiency of ______ is generally lower than the efficiency of NHEJ in most human cell types.

17. Non-dividing cells, such as neurons, almost exclusively use NHEJ for DNA repair.

18. How does the "Homology" in NHEJ Vs HDR work?

19. What could happen if NHEJ repairs a break in the middle of an "Exon" (coding region)?

20. Finalizing the logic of ______, scientists are developing drugs that block NHEJ to force the cell to use the more precise HDR pathway.

Fixing the Break: NHEJ vs HDR Explained

1. What is the primary characteristic of Non-Homologous End Joining (NHEJ)?

2.

What first name or nickname would you like us to use?

2. In the comparison of NHEJ Vs HDR, NHEJ is considered the more error-prone pathway.

3. The DNA repair pathway that requires a homologous sequence to guide the repair process is called ______-Directed Repair.

4. When analyzing NHEJ Vs HDR for gene editing, which pathway is typically used to "knock out" or disable a gene?

5. Which of the following are potential outcomes of the NHEJ repair process?

6. In the study of NHEJ Vs HDR, the pathway that allows scientists to "knock in" a specific new sequence is ______.

7. HDR occurs most frequently during the G1 phase of the cell cycle when DNA is not being replicated.

8. What is the main structural difference in the NHEJ Vs HDR decision made by a cell?

9. The term "indels" refers to the random ______ and ______ that often occur during NHEJ repair.

10. When comparing NHEJ Vs HDR, which factors would encourage a cell to choose the HDR pathway?

11. Why does Cas9 create a "Double-Strand Break" (DSB) to trigger these repair mechanisms?

12. In the context of NHEJ Vs HDR, a "silent" repair is more likely to happen with HDR than with NHEJ.

13. In a CRISPR experiment, if you do not provide a donor template, the cell will be forced to use ______ to fix the break.

14. Which of the following best summarizes the trade-off in NHEJ Vs HDR?

15. Which components are necessary to successfully perform an HDR-mediated gene correction?

16. The efficiency of ______ is generally lower than the efficiency of NHEJ in most human cell types.

17. Non-dividing cells, such as neurons, almost exclusively use NHEJ for DNA repair.

18. How does the "Homology" in NHEJ Vs HDR work?

19. What could happen if NHEJ repairs a break in the middle of an "Exon" (coding region)?

20. Finalizing the logic of ______, scientists are developing drugs that block NHEJ to force the cell to use the more precise HDR pathway.

9. The term "indels" refers to the random and that often occur during NHEJ repair.