GRE Subject Test: Biology : Chromatin and Chromosomes

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

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Example Questions

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Example Question #1 : Chromatin And Chromosomes

Which answer choice incorrectly matches the type of chromatin with the phase of mitosis?

Possible Answers:

Heterochromatin—interphase

Heterochromatin—prophase

Heterochromatin—metaphase

Euchromatin—interphase

Euchromatin—telophase

Correct answer:

Euchromatin—telophase

Explanation:

DNA is stored in loosely wound euchromatin before mitosis. During mitosis, the DNA condenses into chromosomes, which are made of heterochromatin. It becomes more dense during prophase, and stays that way until the end of mitosis. Euchromatin is more lightly packed than heterochromatin.

Mitosis follows the following sequence: prophase, metaphase, anaphase, telophase, cytokinesis. Interphase refers to the time period between mitotic divisions. During interphase, most DNA is euchromatin, but some regions remain as heterochromatin to prevent unwanted transcription; thus DNA exists as both types of chromatin during interphase, but only as heterochromatin during mitosis. Matching euchromatin to telophase is the answer, as this is a false statement.

Example Question #1 : Chromatin And Chromosomes

What is a nucleosome?

Possible Answers:

A complex of proteins that controls nuclear import 

Non-chromosomal DNA that is free floating in the cytosol

A membrane-bound organelle that contains epigenetic information

A repeating unit of chromatin

Correct answer:

A repeating unit of chromatin

Explanation:

Nucleosomes are the basic, repeating units of eukaryotic chromatin. They consist of chromosomal DNA wrapped around special DNA-binding proteins called histones. There are many examples of non-chromosomal DNA, such as plasmids, but they do not contain nucleosomes. Nuclear import is controlled by importin proteins.

Example Question #2 : Chromatin And Chromosomes

__________ are proteins used to package DNA. 

Possible Answers:

Nucleosomes

Chromatins

Telomerases

Histones

Correct answer:

Histones

Explanation:

Histones are proteins that bind and package DNA. The strand of DNA is wound around histone proteins, condensing it to fit in the nucleus and acting to moderate gene expression. Chromatin is the term given to the complex of DNA associated with histones. A nucleosome is the smallest repeating unit of chromatin, formed from eight histone proteins and two loops of coiled DNA. Telomerase is an enzyme responsible for maintaining the integrity of the telomeres.

Example Question #1 : Chromatin And Chromosomes

__________ is also known as "light" chromatin and represents DNA that can actively undergo __________.

Possible Answers:

Heterochromatin . . . transcription

Euchromatin . . . translation

Euchromatin . . . transcription

Heterochromatin . . . translation

Correct answer:

Euchromatin . . . transcription

Explanation:

Euchromatin is the name given to chromatin that appears lighter when viewed under a microscope. It is actually relatively decondensed chromatin that is available for active transcription. Because it is decondensed it is more accessible to RNA polymerase and, therefore, easier to transcribe. In contrast, heterochromatin is tightly wound, dense DNA that is inaccessible by RNA polymerase and is considered inactive.

Translation is the process of synthesizing proteins from mRNA transcripts and does not directly involve DNA or chromatin.

Example Question #1 : Chromatin And Chromosomes

Which of the following is true of chromatin?

Possible Answers:

Chromatin is present in all eukaryotic and prokaryotic DNA

Nucleosomes consist of DNA wrapped in proteins known as transcriptions factors

Chromatin never gets further compacted than the 10nm fiber

Patterns of methylation or acetylation of histones can regulate gene expression

Correct answer:

Patterns of methylation or acetylation of histones can regulate gene expression

Explanation:

Chromatin is not present in all eukaryotic and prokaryotic DNA; most prokaryotic DNA is circular and does not require the complex folding of eukaryotic chromatin. Chromatin exists in more compacted states than 10nm. In particular, the 30nm version is commonly recognized as heterochromatin (DNA that is not being actively transcribed). Packaging can also be more condensed during certain stages of mitosis. Nucleosomes are the smallest units of chromatin and are strands of DNA wrapped in proteins known as histones.

Patterns of methylation and acetylation of these histones have been shown to repress and activate gene expression, respectively, and are important factors in regulating gene expression and epigenetics.

Example Question #1 : Chromatin And Chromosomes

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

Possible Answers:

Humans have homologous chromosome pairs

Humans have both sex chromosomes and somatic chromosomes

Humans have two stages of cell division

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 #5 : Chromatin And Chromosomes

Which correctly describes eukaryotic histones in a nucleosome structure?

Possible Answers:

A core histone heptamer plus a linker histone

A core histone octamer plus a linker histone

A core histone plus a linker histone octamer

A core histone octamer plus 2 linker histones

A core histone nonamer

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 #1 : Chromatin And Chromosomes

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

Possible Answers:

Position of centromeres

Number of chromosomes

Gene loci

Differences in satellite chromosome position and number 

Size of chromosomes

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 #2 : Chromatin And Chromosomes

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

Possible Answers:

Nucleosome

Nucleolus

Chromatin

Histone

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 #1 : Chromatin And Chromosomes

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

Possible Answers:

chromosomes

chromatin

nucleosomes

histones

None of the other answers 

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. 

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