BCH 5413 - Exam 3 Study Guide | 471 Questions and
Correct Answers (Verified Answers) | Already Graded
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What is DNA damage - ANSWER a chemical alteration to DNA
Is DNA continually damaged? - ANSWER YES
- there are 10^4 to 10^6 damaging events in a single human cell every day
DNA damage can be caused by.. - ANSWER - spontaneous cleavage of
inherently unstable chemical bonds in DNA (eg. deamination and base loss)
- environmental/external agents (eg. UV, ionizing radiation)
- endogenous genotoxic chemicals (eg. ROS generated from metabolism)
If left unrepaired, DNA damage can lead to.. - ANSWER mutation
A mutation is.. - ANSWER a change in the normal sequence of DNA
If a particular kind of DNA damage is likely to cause a mutation, we call it.. -
ANSWER genotoxic
Mutations mostly occur during.. - ANSWER replication
- when a DNA Pol inserts wrong nucleotides as it reads a damaged template
,*on normal templates, the frequency of copying errors by DNA pol is low
Mutation types - ANSWER - insertion/deletion
- point mutations (changes in single base pair)
Point mutations
- in protein coding regions
- outside coding regions - ANSWER In protein coding regions
- nonsense mutation (due to stop codon)
- missense mutation (AA substitution)
- silent mutation
Outside coding regions
- mostly without a consequence
- exceptions: intronic regions affecting splicing, regulatory elements
Mutations may alter gene function, and can cause diseases such as ____ (in
oncogenes/tumor suppressor genes) - ANSWER cancer
Damage caused by alkylation of bases
- alkylation is a process where electrophilic substances... - ANSWER - seek
negative centers in DNA (and other molecules)
- attack them
- add carbon-containing groups (alkyl groups)
,Alkylating agents add alkyl groups to... - ANSWER bases
Many ____ are electrophiles that alkylate DNA - ANSWER carcinogens
The most commonly attached targets of alkylation in DNA are... - ANSWER
phosphodiester bonds
Do alkylation events change base pairing? - ANSWER SOMETIMES
- some do not and are innocuous (alkylation of guanine N7)
- some disrupt base pairing (alkylation of adenine N3) and may stall DNA
replication and thus are cytotoxic (cause mutation if cell attempts to replicate
without repair)
- others alter base pairing properties of a base and are mutagenic/genotoxic
(alkylation of guanine O6)
Damage caused by radiation: UV rays (non-ionizing radiation)
- energy
- what it causes - ANSWER - comparatively low energetic
- cross-link adjacent pyrimidines, resulting in formation of pyrimidine dimers
(CPD's) which distort DNA, disrupt base pairing, and block replication or cause
replication errors if bypassed
Gamma and X-rays
- energy
- what it does
- what it forms - ANSWER - much more energetic
, - ionize molecules around the DNA (eg. water)
- form highly reactive free radicals that attach DNA and can break single or double
strands and alter bases (oxidative damage)
What are highly reactive free radicals - ANSWER chemical substances with
unpaired electron
Reversing DNA damage
- O6-alkylguanine - ANSWER O6-alkylguanine (caused by EMS) can be directly
reversed by the "suicide enzyme" O6-methylguanine methyltransferase
- this enzyme accepts the alkyl group onto the sulfur group of one of its cysteines
and becomes irreversibly inactivated
Eukaryotic base excision repair
- acts on... - ANSWER acts on subtle damage to DNA bases
- 5 methyl C deamination
- loss of a base
- oxoG
- base alkylation
Deamination leads to mutations
- most common point mutation - ANSWER Spontaneous C to T mutation is
caused by deamination of 5-methyl C to T.
- most common because 5% of all C residues in DNA are methylated
Deamination leads to mutations
Correct Answers (Verified Answers) | Already Graded
A+ | Latest Update 2025-2026
What is DNA damage - ANSWER a chemical alteration to DNA
Is DNA continually damaged? - ANSWER YES
- there are 10^4 to 10^6 damaging events in a single human cell every day
DNA damage can be caused by.. - ANSWER - spontaneous cleavage of
inherently unstable chemical bonds in DNA (eg. deamination and base loss)
- environmental/external agents (eg. UV, ionizing radiation)
- endogenous genotoxic chemicals (eg. ROS generated from metabolism)
If left unrepaired, DNA damage can lead to.. - ANSWER mutation
A mutation is.. - ANSWER a change in the normal sequence of DNA
If a particular kind of DNA damage is likely to cause a mutation, we call it.. -
ANSWER genotoxic
Mutations mostly occur during.. - ANSWER replication
- when a DNA Pol inserts wrong nucleotides as it reads a damaged template
,*on normal templates, the frequency of copying errors by DNA pol is low
Mutation types - ANSWER - insertion/deletion
- point mutations (changes in single base pair)
Point mutations
- in protein coding regions
- outside coding regions - ANSWER In protein coding regions
- nonsense mutation (due to stop codon)
- missense mutation (AA substitution)
- silent mutation
Outside coding regions
- mostly without a consequence
- exceptions: intronic regions affecting splicing, regulatory elements
Mutations may alter gene function, and can cause diseases such as ____ (in
oncogenes/tumor suppressor genes) - ANSWER cancer
Damage caused by alkylation of bases
- alkylation is a process where electrophilic substances... - ANSWER - seek
negative centers in DNA (and other molecules)
- attack them
- add carbon-containing groups (alkyl groups)
,Alkylating agents add alkyl groups to... - ANSWER bases
Many ____ are electrophiles that alkylate DNA - ANSWER carcinogens
The most commonly attached targets of alkylation in DNA are... - ANSWER
phosphodiester bonds
Do alkylation events change base pairing? - ANSWER SOMETIMES
- some do not and are innocuous (alkylation of guanine N7)
- some disrupt base pairing (alkylation of adenine N3) and may stall DNA
replication and thus are cytotoxic (cause mutation if cell attempts to replicate
without repair)
- others alter base pairing properties of a base and are mutagenic/genotoxic
(alkylation of guanine O6)
Damage caused by radiation: UV rays (non-ionizing radiation)
- energy
- what it causes - ANSWER - comparatively low energetic
- cross-link adjacent pyrimidines, resulting in formation of pyrimidine dimers
(CPD's) which distort DNA, disrupt base pairing, and block replication or cause
replication errors if bypassed
Gamma and X-rays
- energy
- what it does
- what it forms - ANSWER - much more energetic
, - ionize molecules around the DNA (eg. water)
- form highly reactive free radicals that attach DNA and can break single or double
strands and alter bases (oxidative damage)
What are highly reactive free radicals - ANSWER chemical substances with
unpaired electron
Reversing DNA damage
- O6-alkylguanine - ANSWER O6-alkylguanine (caused by EMS) can be directly
reversed by the "suicide enzyme" O6-methylguanine methyltransferase
- this enzyme accepts the alkyl group onto the sulfur group of one of its cysteines
and becomes irreversibly inactivated
Eukaryotic base excision repair
- acts on... - ANSWER acts on subtle damage to DNA bases
- 5 methyl C deamination
- loss of a base
- oxoG
- base alkylation
Deamination leads to mutations
- most common point mutation - ANSWER Spontaneous C to T mutation is
caused by deamination of 5-methyl C to T.
- most common because 5% of all C residues in DNA are methylated
Deamination leads to mutations
