DNA Repair and Mutations Overview

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Questions and Answers

What is the mechanism by which base analogues cause DNA damage?

They resemble DNA bases and are incorporated during replication. (correct)

They react with proteins to induce mutations.

They intercalate into the DNA double helix.

They create thymine dimers through UV exposure.

Which statement accurately describes intercalating agents?

They mimic the structure of DNA bases.

They directly modify the chemical structure of DNA bases.

They create cross-links between two DNA strands.

They distort the DNA double helix during replication. (correct)

Which of the following best summarizes the action of base modifying agents?

They create covalent bonds with DNA to stabilize it.

They alter the structure of DNA bases to prevent replication. (correct)

They increase the overall size of DNA strands.

They cause breakage in the double strands of DNA.

What is the consequence of thymine dimers in DNA?

They form covalent linkages between adjacent thymine bases. Signup and view all the answers

What effect do base analogues like 5-bromouracil have on DNA replication?

They lead to the incorporation of incorrect bases. Signup and view all the answers

Which type of mutagen causes distortions within the DNA structure?

Chemical intercalating agents. Signup and view all the answers

What is NOT a mechanism by which mutagenic chemicals can alter DNA?

Enhancing DNA repair mechanisms. Signup and view all the answers

How do chemical mutagens typically interact with DNA?

By modifying or mimicking DNA bases. Signup and view all the answers

What is the primary function of direct reversal systems in DNA repair?

To reverse the damage previously inflicted on DNA Signup and view all the answers

Which role does DNA polymerase play in excision repair systems?

It synthesizes new DNA to fill in the gaps Signup and view all the answers

What triggers the action of DNA helicase in nucleotide excision repair?

The detection of pyrimidine dimers Signup and view all the answers

What characterizes the mechanism of Non-Homologous End Joining (NHEJ)?

It is a quick method resulting in potential gene loss Signup and view all the answers

In which DNA repair process is undamaged DNA used as a template for repair?

Homologous Recombination (HR) Signup and view all the answers

Which type of cancer is associated with mutations in mismatch repair genes?

Hereditary nonpolyposis colorectal cancer Signup and view all the answers

Which of the following is a characteristic of base excision repair (BER)?

It involves removal of the sugar phosphate backbone and filling in gaps Signup and view all the answers

What type of DNA damage does nucleotide excision repair (NER) specifically address?

Pyrimidine dimers caused by UV light Signup and view all the answers

What characterizes gene mutations compared to other mutation types?

They involve small changes in DNA structure affecting a single gene. Signup and view all the answers

Which type of mutation results in a change that does not affect the amino acid sequence?

Silent mutation Signup and view all the answers

What role does DNA mismatched repair (MMR) play in preventing mutations?

It corrects incorrect base pairings introduced during DNA replication. Signup and view all the answers

Which type of mutation introduces a premature stop codon in the mRNA sequence?

Nonsense mutation Signup and view all the answers

What kind of mutations are caused by environmental factors such as radiation or chemicals?

Induced mutations Signup and view all the answers

Which mutation mechanism can result in a new reading frame after the insertion of a nucleotide?

Frameshift mutation Signup and view all the answers

What is the consequence of deamination of DNA bases?

It changes the base identity, potentially leading to mutations. Signup and view all the answers

What does a point mutation typically affect?

One specific base in the DNA sequence. Signup and view all the answers

What is the role of DNA ligase in DNA repair mechanisms?

It joins together new DNA segments after repair. Signup and view all the answers

Which mutation is classified as a 'larger mutation' affecting multiple base pairs?

Insertion Signup and view all the answers

How does proofreading by DNA polymerase function in relation to mutations?

It detects and removes incorrect bases during replication. Signup and view all the answers

Which of the following statements about mutations is true?

Mutations contribute to genetic diversity. Signup and view all the answers

What happens during the process of depurination?

A purine base is lost from the DNA strand. Signup and view all the answers

Study Notes

Learning Objectives and Exam Revision Advice

Students should be able to discuss and give examples of multiple DNA repair mechanisms.
Students should be able to discuss different types of mutations and describe the methods used to repair DNA replication errors.

Introduction: Mutations

A heritable change in the genetic material.
Mutations provide allelic variations, which are important for evolutionary change.
Mutations can be harmful and often cause diseases.
Organisms have developed ways to repair damaged DNA.

Types of Mutations

There are three main types of mutations:
- Chromosome mutations: Changes in chromosome structure.
- Genome mutations: Changes in chromosome number.
- Gene mutations: Small changes in DNA structure that affect a single gene.

Gene Mutations

Mutations can be point mutations or larger mutations.
- Point mutations: A change in one base.
  - Insertion: Addition of one or more nucleotides.
  - Deletion: Removal of one or more nucleotides.
  - Inversion: Flipping a fragment of DNA.

Point Mutations: Silent

A change in a base pair (bp) can cause a change in mRNA, but does not change the amino acid sequence of the protein.

Point Mutations: Neutral

A change in bp causes change in mRNA and amino acid substitution, but no detectable change in the properties of the amino acid.

Point Mutations: Missense

A change in bp causes change in mRNA and a different amino acid.

Point Mutations: Nonsense

A change in bp causes a change in mRNA to a STOP codon (UAG, UAA, or UGA). No amino acid is produced.

Larger Mutations: Insertion/Deletion

Insertion or deletion of base pairs (bp).
The change in bp will alter the reading frame downstream.
This can create new stop codons and change the length of the polypeptide.

Mutations Can Occur From DNA Replication Errors

Replication machinery makes approximately one mistake per 10 million nucleotides copied.
A mismatch error happens when the wrong base is incorporated; this needs to be corrected to avoid mutations. Errors need a backup system to recognise and correct mistakes, such as DNA mismatch repair (MMR).

DNA Mismatch Repair (MMR)

MMR protein recognises mismatch errors.
A section of the new strand with the error is cut out and replaced.
DNA polymerase replaces the wrong DNA.
DNA ligase seals the gap.

DNA Damage Can Occur Spontaneously

Rare mistakes in DNA replication are corrected.
DNA can be damaged in other ways, requiring different repair mechanisms, like chemical reactions.
Random interactions between DNA and surrounding molecules can cause damage to the DNA sequence.
Depurination and deamination are examples of these chemical reactions.
If chemical modification of nucleotides is not repaired, it results in mutations.

DNA Damage Can Occur in Response to Mutagens

Mutations can be caused by environmental mutagens—agents that cause DNA changes.
- Radiation (UV and X-rays)
- Chemicals (base analogues, base modifying agents, and intercalating agents).

Radiation as Mutagens

Ionising radiation (X-rays) causes single-strand and double-strand DNA breaks.
Non-ionising radiation (UV light) causes thymine dimers.

Chemicals as Mutagens

Base analogues resemble DNA bases and are incorporated during replication, potentially causing mismatches (e.g., 5-Bromouracil).
Base-modifying agents react with DNA bases, changing their structure.
Intercalating agents, such as ethidium bromide, insert themselves between DNA base pairs, distorting the double helix and causing problems during replication.

Repair of DNA Damage

Two general types of repair systems:
- Direct reversal systems correct damaged areas by reversing the damage directly.
- Excision repair systems cut out damaged areas, repair the gap with new DNA synthesis, and seal those gaps.
  - Base Excision Repair (BER)
  - Nucleotide Excision Repair (NER)
- Non-Homologous End Joining (NHEJ)
- Homologous Recombination (HR)

DNA Repair and Disease

Mutations in genes encoding DNA repair proteins are associated with hereditary cancers.
- Hereditary nonpolyposis colorectal cancer (mismatch repair)
- Xeroderma pigmentosum (nucleotide excision repair)

Summary

DNA mutations can occur spontaneously or due to environmental damage.
DNA repair mechanisms exist to maintain DNA fidelity.
Many different DNA repair mechanisms exist to correct replication errors and DNA damage.

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Description

This quiz covers the fundamental concepts of DNA repair mechanisms and various types of mutations. Students will explore the significance of mutations in genetics and their implications for disease. The quiz also discusses methods for correcting DNA replication errors.