MCAT Biology : Transcription

Study concepts, example questions & explanations for MCAT Biology

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Example Questions

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Example Question #1 : Transcription

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.

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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 chromosome 21, parts of the DNA are converted to protein, while other parts are interspersed, but do not correlate to the final protein sequence. The portions of the DNA that code for final amino acid sequence are called __________.

Possible Answers:

introns

coding strands

Okazaki fragments

template strands

exons

Correct answer:

exons

Explanation:

In the splicing model of DNA expression, certain regions of DNA are converted to proteins while intervening portions are cut out.  The portions of "junk DNA" are known as introns, while exons are the sequences actually converted to protein. Okazaki fragments may appear tempting, but actually refers to fragments of DNA synthesized during replication of the lagging strand.

Example Question #1 : Transcription

Which of the following is not true about RNA?

Possible Answers:

RNA contains adenine

RNA contains uracil

The sole function of RNA is translation to a protein product

RNA can be double stranded

RNA contains a five-carbon sugar

Correct answer:

The sole function of RNA is translation to a protein product

Explanation:

There are a few different types of RNA, each serving different purposes. Messenger RNA (mRNA) is transcribed to a protein product, but transfer RNA (tRNA) acts as a carrier for amino acids while ribosomal RNA (rRNA) forms some structures of the ribosome. Micro RNA (miRNA) can be used to regulate transcription.

RNA can be single or double stranded, leading to both ssRNA and dsRNA viruses. The ribose sugar in RNA forms a five-carbon ring, much like deoxyribose in DNA. RNA contains both adenine and uracil, though thymine is not found in RNA.

Example Question #2 : Transcription

What enzyme is required for transcription of mRNA in eukaryotes?

Possible Answers:

DNA polymerase

RNA polymerase III

RNA polymerase II

RNA polymerase I

Correct answer:

RNA polymerase II

Explanation:

RNA polymerase II is required for transcription of mRNA, snRNA, and miRNA. Alternatively, RNA polymerase I transcribes some rRNA and RNA polymerase III transcribes tRNA, some rRNA, and other small RNAs. DNA polymerase is required for DNA replication but does not play a role in transcription.

Example Question #2 : Transcription

Which of the following statements is FALSE regarding transcription in eukaryotic cells?

Possible Answers:

Transcription results in strands that contain uracil instead of thymine.

RNA polymerase only transcribes one side of the DNA strand.

RNA polymerase requires a sequence on DNA to show it where to bind.

Transcription takes place in the cytoplasm.

Correct answer:

Transcription takes place in the cytoplasm.

Explanation:

Remember that DNA is found in the nucleus of cells. As a result, it would not make sense that transcription would take place in the cytoplasm.

RNA polymerase replaces thymine with uracil, and a specific DNA sequence (the promoter) shows RNA polymerase where to begin transcription on the template strand. RNA polymerase will only transcribe one DNA strand (the template strand) because the complementary strand would result in a different mRNA product.

Example Question #1 : Transcription

Which of the following statements is true concerning DNA replication and transcription?

Possible Answers:

Both strands act as templates in replication and transcription.

Both require a primer to begin.

Both use DNA as the template.

Both are performed at the same rate.

Correct answer:

Both use DNA as the template.

Explanation:

In both replication and transcription, DNA is the template used to create the desired product. RNA polymerase uses a promoter, and DNA polymerase uses an RNA primer, in order to determine where to begin. DNA polymerase moves much more quickly than RNA polymerase, and RNA polymerase only uses one of the two strands in order to make the desired product.

Example Question #1 : Transcription

An operon is a section of DNA that is composed of a promoter, an operator, and all of the structural genes that will be transcribed in one mRNA. The lac operon is perhaps the most famous example of how prokaryotic organisms will simultaneously transcribe all of the structural genes necessary to accomplish a certain function in the cell. In the absence of glucose, the lac operon will be transcribed so that lactose can be metabolized in the cell for energy.

Which of the following conditions would result in an increase in transcription of the structural genes found on the lac operon?

Possible Answers:

A sufficient amount of glucose available.

Low CAP levels.

An increase in cyclic AMP.

A very low amount of lactose available.

Correct answer:

An increase in cyclic AMP.

Explanation:

When glucose levels are low, there is an increase in cyclic AMP. The cAMP wil then activate CAP, which will attach upstream to the promoter of the lac operon and promote transcription.

Example Question #3 : Transcription

In 2013, scientists linked a cellular response called the unfolded protein response (UPR) to a series of neurodegenerative diseases, including such major health issues as Parkinson’s and Alzheimer’s Disease. According to their work, the unfolded protein response is a reduction in translation as a result of a series of enzymes that modify a translation initiation factor, eIF2, as below:

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In the above sequence, the unfolded protein sensor binds to unfolded protein, such as the pathogenic amyloid-beta found in the brains of Alzheimer’s Disease patients. This sensor then phosphorylates PERK, or protein kinase RNA-like endoplasmic reticulum kinase. This leads to downstream effects on eIF2, inhibition of which represses translation. It is thought that symptoms of neurodegenerative disease may be a result of this reduced translation.

In contrast to translation, transcription __________.

Possible Answers:

produces an rRNA transcript

takes place in the nucleus

uses tRNA, not rRNA

takes place in the cytosol

relies on soluble factors 

Correct answer:

takes place in the nucleus

Explanation:

Transcription takes place in the nucleus, where DNA access is ensured. DNA is then turned into an mRNA transcript, which can exit the nucleus and move to the cytosol for translation.

Example Question #7 : Transcription

Which class of upstream DNA element is responsible for increasing transcription of target genes?

Possible Answers:

Repressor

Enhancer

Insulator

Activator

Correct answer:

Enhancer

Explanation:

An enhancer is a cis-acting element that is responsible for activating or increasing expression of a target gene. An insulator is a boundary element between inactive and active domains of DNA. Both activators and repressors are trans-acting (protein) factors that modulate gene expression.

Example Question #4 : Transcription

Prokaryotic mRNA usually includes several genes on the same transcript. An operon is a genetic unit which typically consists of a promoter, an operator, and all of the functional genes that will be coded for by a single mRNA. In biology, the lac operon is the most commonly used example. The lac operon is transcribed when a prokaryote has a glucose deficiency, and requires lactose in order to create glucose.

The lac operon is modulated by specific proteins which can regulate the amount of transcription. A lac repressor will attach to the operator and reduce transcription if it is not necessary at a given time. The lac repressor will be removed in the presence of lactose by becoming attached to a lactose molecule. Meanwhile, catabolite activator protein (CAP) will attach upstream to the promoter and signal RNA polymerase to begin transcription if transcription is needed.

How would you describe the levels of lac operon transcription when a prokaryote has high glucose and lactose levels available?

Possible Answers:

Transcription would be very high

Transcription would be very low

More information is needed in order to answer the question

Transcription would be at a normal rate

Correct answer:

Transcription would be very low

Explanation:

While it is true that the lac operon is used when lactose is present, the prokaryote in this scenario also has high levels of glucose present. As a result, the lac operon is not necessary in the given environment, so it will be transcribed at a very low rate. It is energetically favorable to use the glucose already present, rather than to convert lactose to glucose; this causes the lac operon to be activated only when glucose levels are low and lactose is available.

Example Question #5 : Transcription

Prokaryotic mRNA usually includes several genes on the same transcript. An operon is a genetic unit which typically consists of a promoter, an operator, and all of the functional genes that will be coded for by a single mRNA. In biology, the lac operon is the most commonly used example. The lac operon is transcribed when a prokaryote has a glucose deficiency, and requires lactose in order to create glucose.

The lac operon is modulated by specific proteins which can regulate the amount of transcription. A lac repressor will attach to the operator and reduce transcription if it is not necessary at a given time. The lac repressor will be removed in the presence of lactose by becoming attached to a lactose molecule. Meanwhile, catabolite activator protein (CAP) will attach upstream to the promoter and signal RNA polymerase to begin transcription if transcription is needed.

In a low glucose, high lactose environment, which of the following statements is false?

Possible Answers:

The lac operon will be transcribed at a high rate

Lactose will attach to the lac repressor, removing it from the operator

RNA polymerase will attach to the promoter and begin transcription

Glucose will attach to the promoter and signal transcription to begin

Correct answer:

Glucose will attach to the promoter and signal transcription to begin

Explanation:

In a low glucose, high lactose environment, there is only lactose to metabolize. Lactose will attach to the lac repressor and remove it from the operator. Glucose does NOT attach to any of the lac operon regulators. It is glucose's absence, rather than its presence, which promotes the transcription of the lac operon.

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