Biochemistry : Modifications and Epigenetics

Study concepts, example questions & explanations for Biochemistry

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Example Questions

Example Question #1 : Modifications And Epigenetics

Which statement concerning DNA methylation in mammals is false?

Possible Answers:

Deamination of unmethylated cytosine gives uracil

None of the other answers is false

CpG sites are mutation hot spots

Deamination of methylated cytosine gives thymine

DNA methyltransferase adds a methyl group to cytosines of the dinucleotide CpG

Correct answer:

None of the other answers is false

Explanation:

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?

Possible Answers:

Inversion

Null mutation

Translocation

Point mutation

Deletion

Correct answer:

Translocation

Explanation:

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 __________?

Possible Answers:

Phosphorylation

Acetylation 

Decarboxylation

Ligation

Methylation 

Correct answer:

Methylation 

Explanation:

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?

Possible Answers:

Methyl groups are added to the coding sequence in the DNA

Methyl groups are added to the promoter region of the DNA

Methyl groups are transferred from the coding sequence of the DNA to the promoter region

Methyl groups are removed from the coding sequence in the DNA

Methyl groups are removed from the promoter region of the DNA

Correct answer:

Methyl groups are added to the promoter region of the DNA

Explanation:

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?

Possible Answers:

Cytosine and adenine

Cytosine

Thymine

Guanine and adenine

Guanine and thymine

Correct answer:

Cytosine and adenine

Explanation:

The only two bases that can be methylated are cytosine and adenine.

Example Question #6 : 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 __________.

Possible Answers:

UV irradiation

defects in DNA topoisomerase

alkylating agents such as nitrogen mustard

Correct answer:

UV irradiation

Explanation:

Alkylating agents and  can also cause cancer, but they lead to methylation and mismatch mutations rather than the formation of pyrimidine dimers.

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