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 #5 : Chromatin And Chromosomes

Human beings have diploid cells. What is indicated by this statement?

Possible Answers:

Humans have both sex chromosomes and somatic chromosomes

Humans have two stages of cell division

Humans have homologous chromosome pairs

Humans have two chromosomes in each cell nucleus

Correct answer:

Humans have homologous chromosome pairs

Explanation:

Human beings have somatic (body) cells that are diploid. This means that each cell has two copies of each of the 23 chromosomes: one from the father and one from the mother. As a result, the karyotype of a human being would show 23 pairs of chromosomes, for a total of 46. Diploid cells contain two non-identical copies of the same genes. All diploid cells will contain two separate alleles for each gene in the genome, represented by the two homologous chromosomes.

An important note to make is that human germ (sex) cells are haploid, meaning that the chromosomes are not paired in sperm cells and egg cells.

Example Question #1 : Chromatin And Chromosomes

Which correctly describes eukaryotic histones in a nucleosome structure?

Possible Answers:

A core histone octamer plus a linker histone

A core histone nonamer

A core histone heptamer plus a linker histone

A core histone octamer plus 2 linker histones

A core histone plus a linker histone octamer

Correct answer:

A core histone octamer plus a linker histone

Explanation:

A nucleosome is defined as a core region of histones plus one stretch of linker DNA. This gives a "beads on a string" shape, which can be further packaged into chromatin. These nucleosomes contain a DNA wrapped histone octamer in the core region, and a linker histone in the linker DNA region. The histone octamer has 2 each of H2A, H2B, H3, and H4 histones. The linker DNA has an H1 histone. 

Example Question #7 : Chromatin And Chromosomes

Which of the following are not observed when performing a karyotype?

Possible Answers:

Number of chromosomes

Gene loci

Differences in satellite chromosome position and number 

Size of chromosomes

Position of centromeres

Correct answer:

Gene loci

Explanation:

Karyotypes describe whole chromosome structure, including the number and size of chromosomes, position of centromeres, distribution of heterochromatin versus euchromatin, and the presence of satellite chromosomes that are found near the centromeres. However, a karyotype is unable to label specific gene sequences and determine their chromosomal locations. Most karyotypes depict chromosomes of a cell in metaphase.

Example Question #8 : Chromatin And Chromosomes

What is the name of the globular protein around which DNA is wrapped in a chromosome?

Possible Answers:

Nucleosome

Chromatin

Histone

Nucleolus

Correct answer:

Histone

Explanation:

Chromosomes have a great deal of protein involved in their structure so that DNA can be tightly coiled in order to fit into the nucleus. At the smallest level of organization, the DNA wraps itself around small globular proteins called histones. Complexes of histones and DNA form nucleosomes, which appear as "beads" on the DNA strand.

Chromatin refers to the decondensed DNA that has not formed separate chromosomes. The nucleolus is a nuclear structure where ribosomal subunits are synthesized.

Example Question #9 : Chromatin And Chromosomes

Proteins that DNA wraps around to form tightly packaged and organized structural units are referred to as __________.

Possible Answers:

None of the other answers 

chromatin

chromosomes

nucleosomes

histones

Correct answer:

histones

Explanation:

The correct answer is histones. Histones are alkaline proteins in the nucleus that organize DNA into structural units called nucleosomes. Chromatin and chromosomes are more complex structural units composed of DNA, histones, nucleosomes, and other proteins. 

Example Question #11 : Chromatin And Chromosomes

Which of the following proteins is necessary for nucleosome formation?

Possible Answers:

Nuclear lamin

Chromatin

DNA ligase

Histones

Histone methyltransferases

Correct answer:

Histones

Explanation:

The correct answer is histones. DNA wraps around histones to form the nucleosomes. Further condensation of many nucleosomes results in chromatin (DNA state). Histone methyltransferases are important enzymes that modify histones during epigenetic gene regulation; however, they are not necessary for nucleosome formation. DNA ligase is responsible for catalyzing the formation of phosphodiester bonds, but is unrelated to nucleosome formation.

Example Question #1 : Understanding Introns And Exons

In a eukaryotic cell, a molecule of pre-mRNA is found to have four exons and three introns. Which of the following are possible combinations of the exons, if the order in which they are written is the order in which they will be translated?

I. Exon 1, Exon 2, Exon 3, Exon 4

II. Exon 1, Exon 3, Exon 4

III. Exon 4, Exon 1, Exon 2, Exon 3

Possible Answers:

I and II

II only

I only

I, II, and III

Correct answer:

I and II

Explanation:

This question is asking about alternative splicing. Alternative splicing is a means by which several different proteins can arise from the same pre-mRNA due to the order in which the exons are organized. This typically takes the form of exon skipping. Therefore, both 1 and 2 are potential mature mRNAs that could arise from this pre-mRNA. Option 3 is not an acceptable transcript, however, because alternative splicing maintains the integrity of the genomic order of the exons (i.e. exon 4 will not come before exon 1, 2, or 3).

Example Question #2 : Understanding Introns And Exons

__________ are parts of __________ molecules that do not contain information about a protein's primary structure.

Possible Answers:

Exons . . . pre-mRNA

Exons . . . mRNA

Introns . . . pre-mRNA

Introns . . . mRNA

Correct answer:

Introns . . . pre-mRNA

Explanation:

After transcription, the resulting RNA molecule must undergo post-transcriptional modification before it becomes mature mRNA. Before these modifications, it is known as heteronuclear RNA (htRNA) or pre-mRNA.

Introns are portions of pre-mRNA molecules that are spliced prior to translation. Unlike exons, introns do not contain information about the structure of the protein. Only after intron splicing is the molecule considered mRNA.

Example Question #3 : Understanding Introns And Exons

The primary transcript is much longer than the mRNA that will eventually be translated. This can be explained by which of the following?

Possible Answers:

The poly-A tail is still on the primary transcript

Introns have not yet been removed from the transcript

The 5' cap has not yet been added to the transcript

Exons have not yet been added to the transcript

Correct answer:

Introns have not yet been removed from the transcript

Explanation:

Immediately following transcription, the primary transcript will undergo a variety of changes before being translated. One of the largest changes is that a spliceosome complex will remove introns from the primary transcript. Introns are not involved in protein creation, and their removal makes the transcript much shorter. The final mRNA transcript consists of a string of exons, a 5' cap, and a 3' poly-A tail.

Example Question #4 : Understanding Introns And Exons

In most cases, introns are spliced out of mature messenger RNA (mRNA) and are not a part of the final translated protein product of a gene. Even though they are not included in the final protein, why are introns important?

Possible Answers:

Introns can generate non coding RNAs that influence gene expression

None of these

Introns are involved in some special regulatory functions like mRNA export and non-sense mediated decay

Introns allow for alternative splicing of exons to create multiple proteins from one gene sequence

All of these

Correct answer:

All of these

Explanation:

These are all reasons that introns are important, despite the fact that they are not included in final proteins. Introns can allow for alternative splicing of exons, in which exons are placed in different orders to create different proteins from one gene. In the gene Dscam in Drosophila, alternative splicing allows for around 38,000 different proteins from one gene sequence. Some introns become non-coding RNAs that control expression of genes. Lastly, it has recently been shown that introns are involved in some special functions like mRNA export - in which mRNA's are moved between the nucleus and other cellular compartments.

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