GRE Subject Test: Biology : Understanding Chromatin

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

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Example Questions

Example Question #11 : Mitosis And Meiosis

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

Possible Answers:

Heterochromatin—interphase

Euchromatin—interphase

Euchromatin—telophase

Heterochromatin—metaphase

Heterochromatin—prophase

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 repeating unit of chromatin

A complex of proteins that controls nuclear import 

A membrane-bound organelle that contains epigenetic information

Non-chromosomal DNA that is free floating in the cytosol

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:

Telomerases

Chromatins

Histones

Nucleosomes

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 #3 : Chromatin And Chromosomes

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

Possible Answers:

Heterochromatin . . . translation

Heterochromatin . . . transcription

Euchromatin . . . transcription

Euchromatin . . . 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 #4 : Chromatin And Chromosomes

Which of the following is true of chromatin?

Possible Answers:

Patterns of methylation or acetylation of histones can regulate gene expression

Nucleosomes consist of DNA wrapped in proteins known as transcriptions factors

Chromatin never gets further compacted than the 10nm fiber

Chromatin is present in all eukaryotic and prokaryotic DNA

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.

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