MCAT Biology : Regulation Mechanisms and Epigenetics

Study concepts, example questions & explanations for MCAT Biology

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Example Questions

Example Question #1 : Regulation Mechanisms And Epigenetics

Human chromosomes are divided into two arms, a long q arm and a short p arm.  A karyotype is the organization of a human cell’s total genetic complement.  A typical karyotype is generated by ordering chromosome 1 to chromosome 23 in order of decreasing size. 

When viewing a karyotype, it can often become apparent that changes in chromosome number, arrangement, or structure are present.  Among the most common genetic changes are Robertsonian translocations, involving transposition of chromosomal material between long arms of certain chromosomes to form one derivative chromosome.  Chromosomes 14 and 21, for example, often undergo a Robertsonian translocation, as below.

1

A karyotype of this individual for chromosomes 14 and 21 would thus appear as follows:

Pic2

Though an individual with aberrations such as a Robertsonian translocation may be phenotypically normal, they can generate gametes through meiosis that have atypical organizations of chromosomes, resulting in recurrent fetal abnormalities or miscarriages.

 

In the DNA component of a chromosome, changes such as methylation result in alterations in how DNA is processed without changing its sequence. This is known as __________.

Possible Answers:

epigenetic modification

translation

transcription

alternative splicing

RNA interference

Correct answer:

epigenetic modification

Explanation:

Epigenetic modification is defined as the modifications that lead to alternative gene expression without changes in the sequence of the nucleic acid itself. The other choices are incorrect, though "alternative splicing" is a tempting option. This also leads to changes in gene production, but derives from splicing out of different regions of the DNA sequence itself and rejoining it to other sections. Epigenetics does not rely on any sequence changes at all.

Example Question #2 : Regulation Mechanisms And Epigenetics

The concept of genomic imprinting is important in human genetics. In genomic imprinting, a certain region of DNA is only expressed by one of the two chromosomes that make up a typical homologous pair. In healthy individuals, genomic imprinting results in the silencing of genes in a certain section of the maternal chromosome 15. The DNA in this part of the chromosome is "turned off" by the addition of methyl groups to the DNA molecule. Healthy people will thus only have expression of this section of chromosome 15 from paternally-derived DNA.

The two classic human diseases that illustrate defects in genomic imprinting are Prader-Willi and Angelman Syndromes. In Prader-Willi Syndrome, the section of paternal chromosome 15 that is usually expressed is disrupted, such as by a chromosomal deletion. In Angelman Syndrome, maternal genes in this section are deleted, while paternal genes are silenced. Prader-Willi Syndrome is thus closely linked to paternal inheritance, while Angelman Syndrome is linked to maternal inheritance.

Figure 1 shows the chromosome 15 homologous pair for a child with Prader-Willi Syndrome. The parental chromosomes are also shown. The genes on the mother’s chromosomes are silenced normally, as represented by the black boxes. At once, there is also a chromosomal deletion on one of the paternal chromosomes. The result is that the child does not have any genes expressed that are normally found on that region of this chromosome.

 

 

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The passage indicates that genomic imprinting can be the result of silencing genes by adding methyl groups to DNA sequences. Which of the following is true of methyl groups?

Possible Answers:

They directly inhibit DNA transcription

They reduce protein stability

They directly inhibit protein translation

They directly inhibit nuclear pore transport

They directly inhibit ribosome assembly

Correct answer:

They directly inhibit DNA transcription

Explanation:

The best answer here is the inhibition of transcription. The question indicates that methyl groups are added directly to the DNA structure. The only step to get from DNA to protein that makes direct use of the DNA structure is transcription.

If methyl groups were added to mRNA, then inhibition of protein translation would be a better answer.

Example Question #3 : Regulation Mechanisms And Epigenetics

Human chromosomes are divided into two arms, a long q arm and a short p arm.  A karyotype is the organization of a human cell’s total genetic complement.  A typical karyotype is generated by ordering chromosome 1 to chromosome 23 in order of decreasing size. 

When viewing a karyotype, it can often become apparent that changes in chromosome number, arrangement, or structure are present.  Among the most common genetic changes are Robertsonian translocations, involving transposition of chromosomal material between long arms of certain chromosomes to form one derivative chromosome.  Chromosomes 14 and 21, for example, often undergo a Robertsonian translocation, as below.

1

A karyotype of this individual for chromosomes 14 and 21 would thus appear as follows:

Pic2

Though an individual with aberrations such as a Robertsonian translocation may be phenotypically normal, they can generate gametes through meiosis that have atypical organizations of chromosomes, resulting in recurrent fetal abnormalities or miscarriages.

 

Histones are important components of chromosomes that help to form the scaffolding around which DNA wraps while organizing. Considering the structure of DNA, what is most likely true of histones?

Possible Answers:

Histones interact with DNA mainly through covalent interactions

They are acidic proteins because DNA is generally negatively charged

They are neutral proteins and interact with DNA via van der Walls forces

They are basic proteins and undergo acetylation to vary DNA binding

Histones are fixed structures, and associated with DNA in every phase of the cell cycle

Correct answer:

They are basic proteins and undergo acetylation to vary DNA binding

Explanation:

Histones are basic proteins; DNA is acidic, allowing them to interact via dipole intermolecular bonds. One must understand that the transition from heterochromatin to euchromatin involves the tightening of DNA into chromosomes, a process driven in part by modification of the histones via acetylation. While you can answer this question by just deducing the choice from the acidic nature of DNA, it would not make sense for histones to be inert, as DNA morphology must change so dramatically through the cell cycle. Additionally, we would not expect covalent interactions due to this need to vary structure. Covalent bonds are more permanent.

Example Question #4 : Regulation Mechanisms And Epigenetics

The cell is the most basic functional unit of life.  Everything that we consider to be living is made up of cells, and while there are different kinds of cells, they all have some essential features that link them all together under the category of "life."  One of the most important parts of a cell is the membrane that surrounds it, seperating it from the rest of the environment.

While organisms from the three main domains live in incredibly different environments, they all possess similar cell membranes.  This phospholipid bilayer protects the cell, giving it a way to allow certain things in while keeping other things out.  Though organisms from different domains have different kinds of fatty linkages in their membranes, they all serve this essential purpose.

Membranes contain all kinds of essential proteins and signal molecules that allow the inside of the cell to respond to the outside of the cell.  In a multicellular eukaryote, this ability can be used to allow cells to communicate.  In a bacterial colony, an extracellular signal could be used to signal other bacteria.  Signals cascade through a series of molecular pathways that go from the outside of the cell all the way to the nucleus and back out again, giving the cell control on a genetic level.  This allows cellular responses to be quick and effective, and it also allows the cell to control how long it stays in that state.

What is an important aspect of the control a cell has over its molecular responses?

Possible Answers:

RNA produced from DNA is very stable and lasts a long time.

DNA produced from RNA is very stable and lasts a long time.

DNA produced from RNA degrades quickly.

Both RNA and DNA can be created or degraded at will.

RNA produced from DNA degrades quickly.

Correct answer:

RNA produced from DNA degrades quickly.

Explanation:

Whenever a cell responds to a signal, it transcribes DNA into RNA which can then be translated into protein.  That protein is the molecule that will affect the cell somehow and cause it to react.  RNA is very unstable on its own, and will only last a short amount of time in the cell.  Therefore, when a cell wants to stop having that particular reaction, it can simply stop making RNA, and the signal will be stopped quickly.

Example Question #5 : Regulation Mechanisms And Epigenetics

One component of the immune system is the neutrophil, a professional phagocyte that consumes invading cells. The neutrophil is ferried to the site of infection via the blood as pre-neutrophils, or monocytes, ready to differentiate as needed to defend their host.

In order to leave the blood and migrate to the tissues, where infection is active, the monocyte undergoes a process called diapedesis. Diapedesis is a process of extravasation, where the monocyte leaves the circulation by moving in between endothelial cells, enters the tissue, and matures into a neutrophil.

Diapedesis is mediated by a class of proteins called selectins, present on the monocyte membrane and the endothelium. These selectins interact, attract the monocyte to the endothelium, and allow the monocytes to roll along the endothelium until they are able to complete diapedesis by leaving the vasculature and entering the tissues.

The image below shows monocytes moving in the blood vessel, "rolling" along the vessel wall, and eventually leaving the vessel to migrate to the site of infection.

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The maturation of monocytes into neutrophils requires the expression of new segments of DNA. The expression of these genes is mediated by demethylation of the needed DNA sequences. This is an example of __________.

Possible Answers:

DNA splicing

RNA splicing

translational modification

epigenetic modification

transcriptional modification

Correct answer:

epigenetic modification

Explanation:

Any change to the DNA itself that modifies expression without changing the base sequence can be thought of as an epigenetic change. Methylation and demethylation are common types of epigenetic modification.

Example Question #6 : Regulation Mechanisms And Epigenetics

The concept of genomic imprinting is important in human genetics. In genomic imprinting, a certain region of DNA is only expressed by one of the two chromosomes that make up a typical homologous pair. In healthy individuals, genomic imprinting results in the silencing of genes in a certain section of the maternal chromosome 15. The DNA in this part of the chromosome is "turned off" by the addition of methyl groups to the DNA molecule. Healthy people will thus only have expression of this section of chromosome 15 from paternally-derived DNA.

The two classic human diseases that illustrate defects in genomic imprinting are Prader-Willi and Angelman Syndromes. In Prader-Willi Syndrome, the section of paternal chromosome 15 that is usually expressed is disrupted, such as by a chromosomal deletion. In Angelman Syndrome, maternal genes in this section are deleted, while paternal genes are silenced. Prader-Willi Syndrome is thus closely linked to paternal inheritance, while Angelman Syndrome is linked to maternal inheritance.

Figure 1 shows the chromosome 15 homologous pair for a child with Prader-Willi Syndrome. The parental chromosomes are also shown. The genes on the mother’s chromosomes are silenced normally, as represented by the black boxes. At once, there is also a chromosomal deletion on one of the paternal chromosomes. The result is that the child does not have any genes expressed that are normally found on that region of this chromosome.

 

 

Untitled

Histones are proteins that can interact with some sequences of DNA to help it coil into a more manageable arrangement within the nucleus. If the DNA-histone interaction is mediated primarily by intermolecular bonds, which of the following is likely true of histones?

Possible Answers:

They are acidic and depend on covalent interactions

They are neutral and depend on van der Waals interactions

They are acidic and depend on dipole interactions

They are basic and depend on covalent interactions

They are basic and depend on dipole interactions

Correct answer:

They are basic and depend on dipole interactions

Explanation:

DNA is acidic, and thus has a generally negative charge in aqueous conditions (consider what this means for the electrophoresis pattern of DNA in an agarose gel). Because the interaction has to be tight to coil DNA effectively, it must be a dipole interaction. Dipole interactions are relatively strong intermolecular forces. Covalent forces, however, are intramolecular and much more permanent than dipole interactions.

Acidic DNA has a negativecharge, which will be drawn to a basic histone with a positive charge.

Example Question #7 : Regulation Mechanisms And Epigenetics

A competitive inhibitor for RNA polymerase III would have the most significant direct effect on which of the following?

Possible Answers:

Replication

Translation

Replication, transcription and translation

None of these

Transcription

Correct answer:

Translation

Explanation:

RNA polymerase III is used to transcribe tRNA. tRNA is used to carry amino acids to ribosomes where they can be used for translation. 

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