EBIO 2070 EXAM 3 2025/2026 QUESTIONS
AND ANSWERS 100% PASS
Substitution - ANS One nucleotide replaced by another
Causes of substitutions - ANS DNA polymerase errors, spontaneous deamination, alkylating
chemicals, UV light
Spontaneous deamination - ANS Turns cytosine into uracil
Alkylating chemicals - ANS Alkylating chemicals like ENU and EMS change the structure of the
N-containing bases; used to introduce substitutions in genetic model systems
How does UV light create substitutions? - ANS Indirectly through DNA repair mechanisms
1. UVB light creates pyrimidine dimers
2. Normal DNA polymerase cannot replicate through them
3. Usually get "undone" by photolyases
4. Second line of defense is nucleotide excision repair - region around the dimers is removed
and replaced
5. When 3 and 4 don't work, translesion synthesis with special DNA polymerase can occur - this
is error prone and can cause substitutions
1 @COPYRIGHT 2025/2026 ALLRIGHTS RESERVED
, Consequences of substitutions - ANS 1. There are few consequences for substitutions in non-
protein coding sequences (most of the genome)
2. Substitutions in protein coding exons and important non-protein coding sequences
(enhancers, promoters, etc.) can cause changes in protein structure and gene expression
3. Loss of splice donor/acceptor sites can interfere with splicing, often leading to introns being
left in
4. Loss of polyadenylation signal can prevent poly-A tail formation
Tyrosinase is an enzyme needed to make the pigment melanin encoded by two exons, exon 1
and exon 2. A goat has a mutation in its tyrosinase gene that removes a splice donor site in the
tyrosinase transcript. As a result, the transcript cannot be properly spliced and a 2,000
nucleotide-long intron remains between exon 1 and exon 2 of the mature tyrosinase mRNA.
What would you predict about the goat's tyrosinase protein? - ANS It is shorter than normal -
the ribosome will hit a stop codon within the intron and not finish the protein
In a random sequence of non-protein coding DNA or RNA on average, how often would you
come across a stop codon? - ANS One stop codon for every 64 nucleotides
Why would you expect substitution mutations in CREs to have little, or no, effect on gene
expression?
A. Because transcription factors don't need exact binding site sequences to bind enhancers
B. Because each type of transcription factor binds to several sites in an given enhancer
C. Because enhancers don't use the genetic code
D. Two of the above are correct
E. a, b, and c - ANS E
Which type of repair mechanism will fix UV induced mutations?
A. DNA polymerase proofreading
B. Deamination
C. Nucleotide excision repair
2 @COPYRIGHT 2025/2026 ALLRIGHTS RESERVED
AND ANSWERS 100% PASS
Substitution - ANS One nucleotide replaced by another
Causes of substitutions - ANS DNA polymerase errors, spontaneous deamination, alkylating
chemicals, UV light
Spontaneous deamination - ANS Turns cytosine into uracil
Alkylating chemicals - ANS Alkylating chemicals like ENU and EMS change the structure of the
N-containing bases; used to introduce substitutions in genetic model systems
How does UV light create substitutions? - ANS Indirectly through DNA repair mechanisms
1. UVB light creates pyrimidine dimers
2. Normal DNA polymerase cannot replicate through them
3. Usually get "undone" by photolyases
4. Second line of defense is nucleotide excision repair - region around the dimers is removed
and replaced
5. When 3 and 4 don't work, translesion synthesis with special DNA polymerase can occur - this
is error prone and can cause substitutions
1 @COPYRIGHT 2025/2026 ALLRIGHTS RESERVED
, Consequences of substitutions - ANS 1. There are few consequences for substitutions in non-
protein coding sequences (most of the genome)
2. Substitutions in protein coding exons and important non-protein coding sequences
(enhancers, promoters, etc.) can cause changes in protein structure and gene expression
3. Loss of splice donor/acceptor sites can interfere with splicing, often leading to introns being
left in
4. Loss of polyadenylation signal can prevent poly-A tail formation
Tyrosinase is an enzyme needed to make the pigment melanin encoded by two exons, exon 1
and exon 2. A goat has a mutation in its tyrosinase gene that removes a splice donor site in the
tyrosinase transcript. As a result, the transcript cannot be properly spliced and a 2,000
nucleotide-long intron remains between exon 1 and exon 2 of the mature tyrosinase mRNA.
What would you predict about the goat's tyrosinase protein? - ANS It is shorter than normal -
the ribosome will hit a stop codon within the intron and not finish the protein
In a random sequence of non-protein coding DNA or RNA on average, how often would you
come across a stop codon? - ANS One stop codon for every 64 nucleotides
Why would you expect substitution mutations in CREs to have little, or no, effect on gene
expression?
A. Because transcription factors don't need exact binding site sequences to bind enhancers
B. Because each type of transcription factor binds to several sites in an given enhancer
C. Because enhancers don't use the genetic code
D. Two of the above are correct
E. a, b, and c - ANS E
Which type of repair mechanism will fix UV induced mutations?
A. DNA polymerase proofreading
B. Deamination
C. Nucleotide excision repair
2 @COPYRIGHT 2025/2026 ALLRIGHTS RESERVED
