Biochemistry : Modifications and Epigenetics
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Example Questions
Example Question #1 : Modifications And Epigenetics
Which statement concerning DNA methylation in mammals is false?
DNA methyltransferase adds a methyl group to cytosines of the dinucleotide CpG
Deamination of methylated cytosine gives thymine
CpG sites are mutation hot spots
Deamination of unmethylated cytosine gives uracil
None of the other answers is false
None of the other answers is false
Unmethylated cytosine spontaneously deaminates to uracil. Over time, methylated cytosine is spontaneously deaminated to thymine. Random deamination of methylcytosines causes mutation, creating hot spots. The vast majority of DNA methylations in mammals occurs at CpG (cytosine-phospate-guanine) sites.
Example Question #2 : Modifications And Epigenetics
Which of the following describes a mutation in which a segment of DNA detaches from a chromosome and reattaches to another chromosome?
Translocation
Deletion
Inversion
Null mutation
Point mutation
Translocation
A null mutation is one that deactivates a gene entirely. Point mutations are those that occur within a single, small site in a gene. Inversion involves the reversal of orientation of a DNA segment. Deletion occurs when a whole part of a chromosome is removed, joining two ends that were far apart. Translocation involves the exchange of genetic material from two chromosomes that are not homologous.
Example Question #3 : Modifications And Epigenetics
Generally, silencing of a gene is accomplished by __________?
Phosphorylation
Acetylation
Methylation
Ligation
Decarboxylation
Methylation
The silencing of a gene is most often accomplished via methylation of the DNA. The methyl groups are added to the gene's promoter region and thus, the DNA is not read by transcriptional enzymes.
Example Question #4 : Modifications And Epigenetics
How does methylation cause the silencing of a gene?
Methyl groups are removed from the promoter region of the DNA
Methyl groups are transferred from the coding sequence of the DNA to the promoter region
Methyl groups are added to the promoter region of the DNA
Methyl groups are removed from the coding sequence in the DNA
Methyl groups are added to the coding sequence in the DNA
Methyl groups are added to the promoter region of the DNA
In order to silence a gene by methylation, methyl groups are added to the promoter region of DNA. This area is upstream of the coding sequence and is responsible for initiation of transcription. Thus, methylating the promoter region inhibits further transcription of the gene.
Example Question #5 : Modifications And Epigenetics
Which of the following DNA bases can be methylated in the promoter region to silence a gene?
Cytosine and adenine
Guanine and thymine
Thymine
Guanine and adenine
Cytosine
Cytosine and adenine
The only two bases that can be methylated are cytosine and adenine.
Example Question #5 : Modifications And Epigenetics
Formation of thymine dimers in DNA can lead to conditions such as melanoma when unrepaired. This DNA mutation is primarily caused by __________.
defects in DNA topoisomerase
alkylating agents such as nitrogen mustard
UV irradiation
UV irradiation
Alkylating agents and 
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