Biology 1.3 -1.4 DNA Proofreading and Mismatch Repair

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Questions and Answers

What is a common cause of DNA mismatches during replication?

Radiation exposure

Chemical exposure

Errors in replication (correct)

Exposure to UV light

Which type of DNA damage can the base excision repair system correct?

Double-strand breaks

Thymine dimers

Bulky adducts

Alkylation of bases (correct)

What enzyme is responsible for recognizing and excising damaged bases in the base excision repair system?

DNA glycosylase (correct)

DNA ligase

DNA polymerase

Endonuclease

Which enzyme is involved in filling gaps during DNA repair?

DNA polymerase Signup and view all the answers

What type of DNA damage is addressed by the nucleotide excision repair pathway?

Thymine dimers Signup and view all the answers

Which of the following accurately describes the action of NER endonuclease during DNA repair?

It cleaves the phosphodiester bond Signup and view all the answers

What condition can cause DNA damage leading to the activation of repair mechanisms?

Exposure to high energy radiation Signup and view all the answers

What happens after a DNA glycosylase excises a damaged base?

DNA polymerase fills the gap Signup and view all the answers

What is the estimated error rate for DNA polymerases during replication?

Once every 100,000 nucleotides Signup and view all the answers

Which function allows DNA polymerases to improve accuracy during replication?

Proofreading Signup and view all the answers

What mechanism is activated when a mismatch is detected immediately after DNA replication?

DNA mismatch repair (MMR) Signup and view all the answers

What is the role of 3' to 5' exonuclease activity in DNA polymerases?

To excise incorrectly paired nucleotides Signup and view all the answers

How do eukaryotes distinguish the daughter strand from the template strand in DNA mismatch repair?

By the state of strand methylation Signup and view all the answers

What happens to a mismatched base during replication if it is detected?

It is excised by an endonuclease Signup and view all the answers

What base pairing occurs in a mismatch as mentioned in the content?

G-T and A-C Signup and view all the answers

Which statement about DNA polymerases is NOT correct?

They can initiate their own replication. Signup and view all the answers

What type of DNA damage is specifically caused by UV exposure?

Thymine dimers Signup and view all the answers

Which enzyme functions to cleave the DNA strand during nucleotide excision repair?

Endonuclease Signup and view all the answers

What is the main function of DNA ligase in the repair process?

To seal nicks in the DNA backbone Signup and view all the answers

When is homologous recombination typically utilized?

When a sister chromatid is present Signup and view all the answers

What is nonhomologous end joining primarily responsible for?

Rejoining broken DNA strands without a template Signup and view all the answers

What distinguishes the template strand from the newly synthesized daughter strand during DNA replication?

The daughter strand lacks any methyl groups. Signup and view all the answers

Which phase of the cell cycle is most relevant for performing homologous recombination?

S phase Signup and view all the answers

Why are endonucleases essential in the mismatch repair process?

They cleave nucleotides from within an existing DNA strand. Signup and view all the answers

What might happen if double-stranded breaks in DNA are not repaired?

Large portions of the chromosome can be lost Signup and view all the answers

Which situation may lead to the failure of homologous recombination?

Absence of any homologous chromosome Signup and view all the answers

What role does DNA ligase play in the mismatch repair process?

It catalyzes the formation of new phosphodiester bonds. Signup and view all the answers

What is the approximate error rate of DNA replication when accounting for proofreading and mismatch repair systems?

One in every 10-10 to 10-11 nucleotides. Signup and view all the answers

What happens to mismatched nucleotides during the mismatch repair process?

They are excised and replaced with correct nucleotides. Signup and view all the answers

What is primarily recognized by mismatch repair enzymes when identifying mismatched bases?

Single-stranded breaks in newly synthesized DNA Signup and view all the answers

Which factor is least likely to contribute to the error rate in DNA replication?

Use of exonuclease activity during repair Signup and view all the answers

Which statement accurately describes the mismatch repair (MMR) system?

It identifies and corrects mismatches using special enzymes. Signup and view all the answers

What is the structural subunit formed when DNA wraps around a histone octamer?

Nucleosome Signup and view all the answers

Which histone type is referred to as the linker histone?

H1 Signup and view all the answers

How do histone proteins facilitate the binding of DNA?

By having a net positive charge Signup and view all the answers

What characterizes euchromatin as opposed to heterochromatin?

Euchromatin is accessible for gene expression Signup and view all the answers

During which process do histones temporarily leave the DNA?

Transcription and replication Signup and view all the answers

What impact does the configuration of chromatin have on gene expression?

The configuration influences accessibility to genes Signup and view all the answers

What occurs to chromatin during cell division?

It becomes more condensed Signup and view all the answers

What are the major types of histones involved in nucleosome formation?

H1, H2A, H2B, H3, H4 Signup and view all the answers

Study Notes

DNA Proofreading and Mismatch Repair

DNA polymerases possess a 3' to 5' exonuclease activity that acts as a proofreading function, enabling them to remove incorrectly paired nucleotides during replication.
The mismatch repair (MMR) system detects base pair mismatches that were not caught during proofreading.
During MMR, an endonuclease removes the mismatched base and several surrounding nucleotides from the daughter strand.
In prokaryotes, the template strand is distinguished from the daughter strand through methylation patterns.
In eukaryotes, the daughter strand is recognized by single-stranded breaks present only in newly synthesized DNA.
After excision, DNA polymerase incorporates correct nucleotides and DNA ligase seals the gaps.
The combined accuracy of DNA polymerase, its proofreading ability, and the MMR system results in an error rate of approximately one in every 1010-1011 nucleotides.

DNA Damage and Repair

DNA damage can occur due to spontaneous changes, exposure to harmful conditions (e.g., UV light, radiation), or errors during replication.
The base excision repair (BER) system addresses damage that doesn't significantly distort the DNA double helix (e.g., oxidation, deamination, alkylation).
BER involves a DNA glycosylase excising the damaged base, followed by an endonuclease cleaving the phosphodiester bond.
DNA polymerase then fills the gap, and DNA ligase rejoins the DNA strands.
Nucleotide excision repair (NER) is activated for more extensive or bulky DNA damage, like thymine dimers caused by UV radiation.
NER utilizes endonucleases to remove the damaged region, and DNA polymerase and DNA ligase subsequently fill in the gaps and repair the DNA.
Double-stranded breaks in DNA can lead to the loss of large portions of chromosomes if not repaired.
Homologous recombination uses a homologous chromosome as a template to repair the break, typically during S and G2 phases of the cell cycle.
Nonhomologous end joining repairs broken strands by directly rejoining the ends without requiring a homologous template.

Histones and Nucleosomes

DNA wraps around a complex of eight histone proteins (an octamer) to form a nucleosome.
There are five major histone types: H1, H2A, H2B, H3, and H4.
The nucleosome core is composed of two each of H2A, H2B, H3, and H4, with a linker histone (H1) present outside the core.
The positive charge of histone proteins, due to the abundance of arginine and lysine, facilitates their binding to negatively charged DNA.
In unwound chromatin, nucleosomes resemble beads on a string, with linker DNA connecting them.
During replication and transcription, histones temporarily leave the DNA to allow access to the helix.

Euchromatin and Heterochromatin

Chromatin can be configured either loosely (euchromatin) or densely (heterochromatin), impacting gene accessibility and expression.
Euchromatin is more accessible and associated with active gene expression.
Heterochromatin is more tightly packed and associated with repressed genes.
During cell division, chromatin becomes even more condensed in preparation for mitosis.

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Description

Explore the mechanisms of DNA proofreading and mismatch repair in this quiz. Understand how DNA polymerases correct errors during replication and learn about the roles of MMR in prokaryotes and eukaryotes. Test your knowledge on these essential processes for maintaining genetic integrity.