T.12 DNA MUTATIONS

Question 1

What is a mutation?

Answer 1

A mutation is any type of alteration in DNA sequence that is inherited and may be transmitted to the offspring.

Question 2

Why are mutations significant?

Answer 2

Because DNA stores all genetic information encoding proteins, mutations in its sequence can cause severe consequences.

Question 3

Why is it fundamental to repair DNA alterations?

Answer 3

Because uncorrected mutations can be passed to offspring and may disrupt essential protein functions.

Question 4

What is a mutagen?

Answer 4

A mutagen is any agent capable of provoking a mutation.

Question 5

What are some mutation-inducing factors?

Answer 5

UV radiation, X-rays, chemicals (like methyl donors, benzopyrene), oxidative stress (ROS or detoxification derivatives).

Question 6

What biological processes can induce mutations?

Answer 6

Replication, recombination, transduction.

Question 7

Can DNA mutations arise spontaneously?

Answer 7

Yes, mutations may result from spontaneously-induced reactions of DNA.

Question 8

How are mutations classified based on magnitude?

Answer 8

High scale (chromosomal mutation) and low scale (punctual mutations).

Question 9

What is a chromosomal mutation?

Answer 9

A chromosomal mutation alters millions of base pair regions.

Question 10

What is a punctual mutation?

Answer 10

A punctual mutation affects a single nucleotide and its complementary base.

Question 11

How are mutations classified based on cell type?

Answer 11

They are classified into germinal cell mutations and somatic cell mutations.

Question 12

What is the effect of mutations in germinal cells?

Answer 12

They affect the whole organism.

Question 13

What is the effect of mutations in somatic cells?

Answer 13

They affect a percentage of cells, depending on the timepoint of mutation acquisition.

Question 14

How are mutations classified by their effect on proteins?

Answer 14

They can be silent or non-silent mutations.

Question 15

What is a silent mutation?

Answer 15

It causes no amino acid change because the new codon codes for the same amino acid.

Question 16

What is a conservative non-silent mutation?

Answer 16

It replaces the amino acid with another that has similar chemical properties.

Question 17

What is a non-conservative non-silent mutation?

Answer 17

It replaces the amino acid with a completely different one.

Question 18

What are some outcomes of substitution mutations?

Answer 18

They may produce an active/inactive protein, a longer protein (stop codon removal), or a shorter protein (new stop codon).

Question 19

What is the effect of a 3bp insertion or deletion?

Answer 19

It does not affect the reading frame.

Question 20

What is a frameshift mutation?

Answer 20

An insertion or deletion of base pairs not in multiples of 3 that alters the reading frame.

Question 21

What type of alterations are considered mutations?

Answer 21

Only inheritable alterations in DNA sequences that are not repaired and passed to daughter cells.

Question 22

What are the mechanisms to prevent DNA mutations?

Answer 22

Prevention of DNA oxidation, direct repair, nucleotide excision repair, mismatch repair, base excision repair, HR and NHEJ.

Question 23

What prevents DNA oxidation?

Answer 23

Enzymes such as superoxide dismutase (SOD) and catalase.

Question 24

How does superoxide dismutase (SOD) help?

Answer 24

It eliminates superoxide radicals by converting them into H₂O₂.

Question 25

What does catalase do?

Answer 25

It decomposes toxic H₂O₂ into oxygen and water.

Question 26

How do SOD and catalase together protect DNA?

Answer 26

They prevent formation of free oxidative radicals (ROS) that can damage DNA sequences.

Question 27

When is direct repair used?

Answer 27

When mutation is caused by chemical agents like methylation of base pairs.

Question 28

How does methylation affect guanine?

Answer 28

Methylation can cause guanine to bind with thymine instead of cytosine, altering the DNA sequence.

Question 29

What enzyme is involved in direct repair of methylated guanine?

Answer 29

Methyltransferase, which has cysteine residues that remove the methyl group.

Question 30

What happens if methylation is not repaired?

Answer 30

After further replication, thymine will pair with adenine, changing the G-C pair into a T-A mutation.

Question 31

What is the result of an unrepaired methylation mutation?

Answer 31

All daughter cells derived from that cell will carry the mutation.

Question 32

What damage does UV light cause to DNA?

Answer 32

It produces thymine dimers between adjacent thymines in the same DNA chain.

Question 33

What can thymine dimers result in?

Answer 33

They can cause separation of DNA strands.

Question 34

What repair mechanism addresses thymine dimers?

Answer 34

Nucleotide excision repair (NER).

Question 35

How does NER detect DNA damage?

Answer 35

Proteins scan the DNA sequence for possible damage.

Question 36

What happens after damage is recognized in NER?

Answer 36

Enzymes cut off the DNA strand a few nucleotides around the damage (around 29 nucleotides).

Question 37

What replaces the cut strand in NER?

Answer 37

Helicases, DNA polymerase, and ligases work together to replace the fragment.

Question 38

What is Transcription-Coupled NER (TC-NER)?

Answer 38

A sub-pathway of NER that stops transcription when RNA polymerase encounters damage.

Question 39

What activates NER in TC-NER?

Answer 39

RNA polymerase recognizing the damage activates the repair system.

Question 40

What happens after damage is repaired in TC-NER?

Answer 40

Transcription resumes.

Question 41

Why is TC-NER important?

Answer 41

It is vital for preventing the accumulation of mutations, especially in cancer cases.

Question 42

What causes mismatch mutations?

Answer 42

Incorrect base pairing during replication or deamination of nitrogenous bases.

Question 43

What is an example of base deamination?

Answer 43

If guanine loses its amino group, it converts into uracil.

Question 44

What proteins are involved in mismatch repair (MMR)?

Answer 44

Proteins called MUT that detect mismatches and recruit enzymes to correct them.

Question 45

How does MMR work?

Answer 45

MUT proteins cut both sides of the incorrect nucleotide; helicases, DNA pol, and ligases are not necessary.

Question 46

How is the error strand recognized in prokaryotic MMR?

Answer 46

By methylation of the template strand.

Question 47

How is the error strand recognized in eukaryotic MMR?

Answer 47

Through the primers present in the new strand.

Question 48

When is base excision repair (BER) activated?

Answer 48

When nitrogenous bases are modified, oxidized, damaged, or deleted.

Question 49

What enzyme starts BER?

Answer 49

N-glycosylase, which cuts the N-glycosidic bond between the base and deoxyribose.

Question 50

What is formed after N-glycosylase acts?

Answer 50

An abasic (AP) site.

Question 51

What enzyme cuts the phosphodiester bond in BER?

Answer 51

An endonuclease.

Question 52

What does DNA polymerase β do in BER?

Answer 52

It hydrolyzes the phosphodiester bond and introduces a nucleotide.

Question 53

What joins the new nucleotide to the strand in BER?

Answer 53

DNA ligase.

Question 54

When do HR and NHEJ repair mechanisms act?

Answer 54

When there is a DNA strand break.

Question 55

What repair is used during S phase?

Answer 55

Homologous recombination (HR).

Question 56

How does HR repair DNA?

Answer 56

It uses the non-mutated homologous chromosome as a template, preventing errors.

Question 57

When is NHEJ used?

Answer 57

During G1 phase when no homologous chromosomes are present.

Question 58

What is a disadvantage of NHEJ?

Answer 58

It can result in insertions or deletions due to incorrect joining.

Question 59

What mutation types are caused by spontaneous reactions?

Answer 59

Failures in replication and base deamination.

Question 60

What prevents spontaneous reaction mutations?

Answer 60

DNA polymerase and mismatch repair mechanisms.

Question 61

What mutations are caused by chemical agents?

Answer 61

Base modifications.

Question 62

What prevents or repairs chemical agent mutations?

Answer 62

SOD, catalase, methyltransferase, and base excision repair.

Question 63

What damage is caused by physical agents?

Answer 63

DNA breakage and thymine dimer formation.

Question 64

What repairs physical agent damage?

Answer 64

Homologous recombination (HR), non-homologous end joining (NHEJ), and nucleotide excision repair (NER).