GRE Subject Test: Biology : Understanding Epigenetics

Study concepts, example questions & explanations for GRE Subject Test: Biology

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Example Questions

Example Question #1 : Understanding Epigenetics

Which of the following is not an example of an epigenetic cause of trait variation? 

Possible Answers:

Imprinting

DNA methylation

Different exon sequences

Histone acetytlation 

X-chromosome inactivation 

Correct answer:

Different exon sequences

Explanation:

Epigenetics are changes to the genome that result in phenotypic variation that have nothing to do with changes in the actual DNA sequence. All listed answers occur independently of DNA sequence, except for "different exon sequences," which is the actual sequence of an exon. This referces to alternative splicing, an is not related to the modification of DNA or histones.

Example Question #2 : Understanding Epigenetics

Which of the following is true about the effects of epigenetics on gene expression? 

Possible Answers:

None of the other answers 

Modification of chromatin lysine residues alters gene expression by changing the affinity between DNA and chromatin

Modification of nucleosome lysine residues alters gene expression by changing the affinity between DNA and histones

Modification of histone arginine residues alters gene expression by changing the affinity between DNA and histones

Modification of histone lysine residues alters gene expression by changing the affinity between DNA and histones

Correct answer:

Modification of histone lysine residues alters gene expression by changing the affinity between DNA and histones

Explanation:

Methylation and acetylation of histones occurs on lysine residues, thereby decreasing or increasing gene expression, respectively. Methylation increases the affinity for histones and DNA, where acetylation decreases the affinity for histones and DNA. Gene expression is in part controlled by modification of histone proteins, rather non-histone chromosomal proteins. 

Example Question #3 : Understanding Epigenetics

In terms of epigenetics, acetylated histones usually result in which of the following?

Possible Answers:

Decrease in transcription/ gene expression

Production and assembly of ribosomes

Increase in transcription/ gene expression

Decrease in DNA - lamina association 

Increase in DNA - lamina association

Correct answer:

Increase in transcription/ gene expression

Explanation:

The correct answer is an increase in gene expression. Histone acetylation removes positive charges on the histones, reducing the affinity of DNA for histones. Remember that DNA is negatively charged due to the phosphate groups on its backbone. DNA and histones are attracted to each other because histones are positively charged due to being rich in basic amino acid residues. Acetylation relaxes the tightly bound DNA allowing transcription factors to bind promoter regions. DNA deacetylation and methylation supress gene transcription by making DNA and histones associate more tightly together, decreasing the ability of transcription factors and/or RNA polymerase to bind the DNA. Histone modifications such as acetylation, deacetylation, and methylation do not directly affect the amount of DNA. If a histone is acetylated on a part of the DNA which codes for the genes for ribosome production, then an increase in ribosomal production and assembly could occur, but genes coding for ribosomes are greatly outnumbered by other genes, and thus, this is not the usual result of acetylating histones. 

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