MCAT Biology : DNA, RNA, and Proteins
Study concepts, example questions & explanations for MCAT Biology
All MCAT Biology Resources
Example Questions
Example Question #1 : Dna, Rna, And Proteins
A culture of human tissue is being grown in a lab to study mitosis. A solution containing radioactively labelled cytosines was added to the culture in the middle of prophase, and then growth was halted at the end of telophase. Where would the scientists see radioactively labelled DNA?
In the nuclei of every cell
In the cells produced at the end of telophase—only the daughter cells
No where
In the mother cells only—not in the cells produced at the end of telophase
Only in the nuclei of half of the cells
No where
DNA is replicated in S phase. Prophase is a part of mitosis, or M phase. Since all of the DNA that would be present at the end of telophase had already been synthesized in S phase, none of the radioactively labelled cytosines would be incorporated into the DNA of any cells in the culture.
Example Question #2 : Dna Replication And Repair
Which answer choice correctly matches the molecule with its function in DNA replication?
DNase—adds nucleotides to new strands
Single-stranded binding proteins—untangles supercoils
Topoisomerase—prevents reannealing of DNA during replication
Single-stranded binding proteins—untangles supercoils
Topoisomerase—untangles supercoils
Single-stranded binding proteins—prevents reannealing of DNA during replication
Polymerase—adds RNA primers prior to replication
Primase—adds nucleotides to new strands
Polymerase—adds nucleotides to new strands
Primase—unzips DNA
Topoisomerase—untangles supercoils
Single-stranded binding proteins—prevents reannealing of DNA during replication
Topoisomerase functions to untangle the supercoiling of DNA, which is when DNA overwinds into itself. This mechanism facilitates the unwinding action of helicase during replication. Single-stranded binding proteins bind to the two unzipped DNA strands to prevent them from prematurely coming back together into a whole molecule; otherwise replication would be interrupted.
The other proteins discussed serve the following functions.
Polymerase—adds nucleotides to new strands
Primase—adds RNA primers prior to replication
DNase—cleaves and degrades DNA molecules
Example Question #2 : Dna, Rna, And Proteins
Which of the following is the first to act during DNA replication?
Primase
DNA polymerase
Helicase
DNA ligase
Helicase
Helicase is the first component of the DNA replication machinery to act during replication. It works by "unzipping" the double-stranded DNA so that replication can subsequently occur. Following the work of helicase, primase creates a primer to which the DNA polymerase will subsequently add deoxynucleotides and elongate the strand. DNA ligase acts at the end of replication by joining together the Okazaki fragments of the lagging strand.
Example Question #4 : Dna Replication And Repair
The Meselson-Stahl experiment provided the necessary evidence to discover the mechanism by which DNA replicates. They accomplished this discovery by first culturing DNA with the heavy 15N nitrogen isotope. They then allowed the "heavy" DNA to replicate with DNA grown in normal 14N nitrogen. The density of each generation of replicated DNA was recorded by marking its position in a test tube after centrifugation. The position of each generation was then compared to the positions of pure 15N DNA and pure 14N DNA.
Suppose that the first generation after replication revealed two bands after being centrifuged: one at the pure 14N mark, and one at the pure 15N mark. Which method of replication would this observation support?
Semiconservative replication
Dispersive replication
Conservative replication
Another generation would be needed in order to find a viable mechanism
Conservative replication
Conservative replication proposes that both strands of DNA act as the template, but do not separate during replication. If the heavy strands were to stay together, we would expect to see a "heavy" set of DNA at the 15N mark as well as a "normal" set of DNA at the 14N mark.
Both semiconservative and dispersive replication would predict a singular band of DNA in between the two marks.
Example Question #5 : Dna Replication And Repair
Compared to RNA polymerase, DNA polymerase has a much lower error rate for nucleotide incorporation. What structural difference between the two polymerases accounts for this?
RNA molecules are proofread after they are synthesized, whereas DNA molecules are not.
DNA polymerase contains a proof-reading domain that allows it to recognize incorrect base-pair insertion before moving on; RNA polymerase does not.
RNA polymerase incorporates the nucleic acids into sequences in such a way that they are more tightly bound to their partner nucleic acid, thus making it much more difficult to replace incorrect insertions.
RNA is much less stable that DNA, and this instability makes it much harder for RNA polymerase to proofread as it incorporates bases into the sequence.
DNA polymerase contains a proof-reading domain that allows it to recognize incorrect base-pair insertion before moving on; RNA polymerase does not.
RNA polymerase does not contain a proof reading domain, making it much more error prone than DNA polymerase. This domain in DNA polymerase prevents incorrect nucleotide insertion, reducing the errors made in DNA replication.
Example Question #3 : Dna, Rna, And Proteins
Several enzymes are required for DNA replication. What is the class of enzymes that is required for unwinding the DNA at the replication fork?
Telomerase
Topoisomerase
DNA helicase
DNA polymerase
DNA helicase
DNA helicases use ATP to break the hydrogen bonds that separate complementary strands of DNA. During DNA replication, DNA helicases move along the DNA backbone with the replication fork and are responsble for unwinding the DNA at the fork.
Example Question #6 : Dna Replication And Repair
Prions are the suspected cause of a wide variety of neurodegenerative diseases in mammals. According to prevailing theory, prions are infectious particles made only of protein and found in high concentrations in the brains of infected animals. All mammals produce normal prion protein, PrPC, a transmembrane protein whose function remains unclear.
Infectious prions, PrPRes, induce conformational changes in the existing PrPC proteins according to the following reaction:
PrPC + PrPRes → PrPRes + PrPRes
The PrPRes is then suspected to accumulate in the nervous tissue of infected patients and cause disease. This model of transmission generates replicated proteins, but does so bypassing the standard model of the central dogma of molecular biology. Transcription and translation apparently do not play a role in this replication process.
This theory is a major departure from previously established biological dogma. A scientist decides to test the protein-only theory of prion propagation. He establishes his experiment as follows:
Homogenized brain matter of infected rabbits is injected into the brains of healthy rabbits, as per the following table:
Rabbit 1 and 2: injected with normal saline on days 1 and 2
The above trials serve as controls.
Rabbit 3 and 4: injected with homogenized brain matter on days 1 and 2
The above trials use unmodified brain matter.
Rabbit 5 and 6: injected with irradiated homogenized brain matter on days 1 and 2
The above trials use brain matter that has been irradiated to destroy nucleic acids in the homogenate.
Rabbit 7 and 8: injected with protein-free centrifuged homogenized brain matter on days 1 and 2
The above trials use brain matter that has been centrifuged to generate a protein-free homogenate and a protein-rich homogenate based on molecular weight.
Rabbit 9 and 10: injected with boiled homogenized brain matter on days 1 and 2
The above trials use brain matter that have been boiled to destroy any bacterial contaminants in the homogenate.
In the material used with Rabbits 5 and 6, irradiation was used to destroy DNA. In functioning, normal cells, what types of genes typically code for DNA repair proteins?
I. Tumor suppresor genes
II. Proto-onco genes
III. Pro-apoptotic genes
I, only
I, II, and III
I and III, only
II and III, only
I and II, only
I, only
Tumor suppresor genes, like p53 and Rb, usually code for DNA repair enzymes. Proto-oncogenes typically code for cell growth factors or receptors, and pro-apoptotic proteins would not lead to DNA repair, but would prevent tumor development via cell death pathways.
Example Question #7 : Dna Replication And Repair
DNA replication is much more accurate than RNA transcription. In replication, only one base in every ten billion, on average, is inaccurately placed.
What is the primary reason that transcription results in more errors than DNA replication?
DNA polymerase is able to repair mismatched nucleotides.
Transcription proceeds much more quickly than replication. This results in more mistakes by RNA polymerase.
DNA polymerase synthesizes a new strand. Immediately after, a proofreading enzyme attaches and "checks" the new strand for errors.
Replication is done very slowly, only a couple base pairs per second, in order to prevent mistakes by DNA polymerase.
DNA polymerase is able to repair mismatched nucleotides.
In addition to creating a new DNA strand, DNA polymerase can function as an exonuclease. DNA polymerase I has the ability to remove mismatched nucleotides from the new strand and correct them. As a result, DNA replication is very accurate, because DNA polymerase has a proofreading mechanism.
Example Question #8 : Dna Replication And Repair
Which statement best describes the function of the enzyme DNA helicase?
It proofreads the newly synthesized DNA for errors.
It links nucleotide subunits together.
It begins replication of the leading strand during DNA synthesis.
It opens and unwinds the DNA double helix by disrupting the hydrogen bonds.
It prevents excess twisting of the DNA during replication.
It opens and unwinds the DNA double helix by disrupting the hydrogen bonds.
DNA helicases are enzymes that separate the two DNA strands, and unwinds them as it progresses along the helix. It functions much like a zipper unwinding the DNA.
Example Question #2 : Dna Replication And Repair
Which base pair requires the least amount of energy to break?
T-C
G-C
A-T
T-G
G-T
A-T
The adenine and thymine base pairing forms 2 hydrogen bonds. Both cytosine and guanine form three hydrogen bonds. Thus the A-T base pair has the weakest interaction, and requires the least amount of energy to break.
All MCAT Biology Resources
