GRE Subject Test: Biology : Genetics, DNA, and Molecular Biology

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

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Example Questions

Example Question #176 : Gre Subject Test: Biology

Which of the following post-translational modifications has most typically been associated with protein degradation via activation of the cell's proteasome system? 

Possible Answers:

Palmitoylation

Ubiquitination

Acylation

Lipoylation

Gamma-carboxylation

Correct answer:

Ubiquitination

Explanation:

Conjugation of ubiquitin molecules to a protein activates the ubiquitin proteasome system, which is required by cells to break down proteins into their component amino acid residues to be reallocated in protein building as necessary. While the other modifications may contribute to some degradation pathway, ubiquitin is classically considered a marker of protein destruction via the proteasome system. 

Example Question #177 : Gre Subject Test: Biology

__________ is the addition of a long tail of adenine bases to an mRNA during mRNA processing, and this step is crucial for the stability of the mRNA as it gets exported to other parts of the cell.

Possible Answers:

Methylation

Ribonucleation

5' capping

Polyadenylation

mRNA splicing

Correct answer:

Polyadenylation

Explanation:

Polyadenylation is the process in which a "Poly-a tail," or a tail of adenine bases, is added to the 3' end of an mRNA. The other processes listed do not carry out this function.

Example Question #1 : Understanding Epigenetics

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

Possible Answers:

Imprinting

Histone acetytlation 

X-chromosome inactivation 

DNA methylation

Different exon sequences

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 #1 : Understanding Epigenetics

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

Possible Answers:

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

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

None of the other answers 

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

Modification of nucleosome 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 #1 : Understanding Epigenetics

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

Possible Answers:

Increase in DNA - lamina association

Decrease in transcription/ gene expression

Production and assembly of ribosomes

Increase in transcription/ gene expression

Decrease 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. 

Example Question #1 : Understanding Oncogenes

Which of the following choices could activate a proto-oncogene?

Possible Answers:

Exposure to mutagens

Chromosomal translocations

Gene duplication

Any of these could activate a proto-oncogene

Correct answer:

Any of these could activate a proto-oncogene

Explanation:

Proto-oncogenes are genes that have the ability to become oncogenes (genes that cause cancer). There are many ways to activate proto-oncogenes. Gene duplication can cause an increase in the expression of a particular protein, which can lead to cancer. Exposure to mutagens can cause a mutation on a proto-oncogene, which causes it to become activated. Chromosomal translocations can relocate proto-oncogenes to areas where they are expressed more rapidly. Most proto-oncogenes are involved in cell cycle regulation. Irregular expression of these genes can allow the cell to progress through the cell cycle too rapidly, resulting in unregulated cell division and tumor formation.

Example Question #1 : Understanding Oncogenes

Which of the following best describes an oncogene?

Possible Answers:

A gene that no longer makes a viable protein

A gene that causes uncontrollable growth

A gene that regulates cell growth

A gene that stimulates apoptosis in cells

Correct answer:

A gene that causes uncontrollable growth

Explanation:

Oncogenes can be thought of as cancerous genes, or rather a gene that has the potential to cause cancer. They typically occur when a normal proto-oncogene undergoes a mutation. Proto-oncogenes normally code for growth and development in cells, and tightly regulate these processes. If mutated, these newly cancerous genes can stimulate unregulated growth, a symptom characteristic of cancerous cells.

Example Question #1 : Understanding Dna Replication

Several enzymes are required for DNA replication. What is the class of enzymes that is required for unwinding the DNA at the replication fork?

Possible Answers:

Topoisomerase

DNA helicase

Telomerase

DNA polymerase

Correct answer:

DNA helicase

Explanation:

DNA helicases use ATP to break the hydrogen bonds that separate complementary strands of DNA. During DNA replication, DNA helicases move along the DNA backbone with the replication fork and are responsble for unwinding the DNA at the fork.

Example Question #1 : Understanding Dna Replication

Which of the following proteins is not necessary during DNA replication?

Possible Answers:

RNA polymerase

Single-strand binding proteins

DNA polymerase

Helicase

Correct answer:

RNA polymerase

Explanation:

RNA polymerase is an enzyme that transcribes RNA from DNA; it is not essential for DNA replication. This enzyme is easy to confuse with primase, whose primary function is to synthesize the RNA primers necessary for replication. DNA polymerase add nucleotides during replication, synthesizing the daughter strand from the parental template. Helicase is responsible for separating double-stranded DNA. Single-strand binding proteins are needed to keep DNA from reannealing after it has been denatured by helicase.

Example Question #2 : Understanding Dna Replication

Which statement correctly describes the process of DNA replication?

Possible Answers:

The original molecule is separated and each daughter molecule will have one old DNA strand and a new DNA strand

The old DNA strands stay together and are replicated to form an entirely new DNA molecule

Parent DNA strands are cut into pieces and the segments are replicated and combined to form two new DNA molecules

New DNA is replicated from parental template strands and an enzyme cuts the new strands away from the old strands

Correct answer:

The original molecule is separated and each daughter molecule will have one old DNA strand and a new DNA strand

Explanation:

DNA replicates in a semiconservative process. Parental strands are used as templates to synthesize daughter strands, which remain adhered to the parental template creating hybrid molecules of old and new DNA.

The original DNA molecules is "unzipped" by helicase to create the replication fork. DNA polymerase then begins to recruit nucleotides to bind to the exposed template, building the new DNA strand along the parental strand.

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