MCAT Biochemistry Review

Chapter 6: DNA and Biotechnology

Practice Questions

1.    In a single strand of a nucleic acid, nucleotides are linked by:

1.    hydrogen bonds.

2.    phosphodiester bonds.

3.    ionic bonds.

4.    van der Waals forces.

2.    Which of the following statements regarding differences between DNA and RNA is FALSE?

1.    DNA is double-stranded, whereas RNA is single-stranded.

2.    DNA uses the nitrogenous base thymine; RNA uses uracil.

3.    The sugar in DNA is deoxyribose; the sugar in RNA is ribose.

4.    DNA strands replicate in a 5′ to 3′ direction, whereas RNA is synthesized in a 3′ to 5′ direction.

3.    Which of the following DNA sequences would have the highest melting temperature?

1.    CGCAACCATCCG

2.    CGCAATAATACA

3.    CGTAATAATACA

4.    CATAACAAATCA

4.    Which of the following biomolecules is LEAST likely to contain an aromatic ring?

1.    Proteins

2.    Purines

3.    Carbohydrates

4.    Pyrimidines

5.    For a compound to be aromatic, all of the following must be true EXCEPT:

1.    the molecule is cyclic.

2.    the molecule contains 4n + 2 π electrons.

3.    the molecule contains alternating single and double bonds.

4.    the molecule is planar.

6.    Which of the following enzymes is NOT involved in DNA replication?

1.    Primase

2.    DNA ligase

3.    RNA polymerase

4.    Telomerase

7.    How is cDNA best characterized?

1.    cDNA results from a DNA transcript with noncoding regions removed.

2.    cDNA results from the reverse transcription of processed mRNA.

3.    cDNA is the abbreviation for deoxycytosine.

4.    cDNA is the circular DNA molecule that forms the bacterial genome.

8.    Which of the following statements regarding the polymerase chain reaction is FALSE?

1.    Human DNA polymerase is used because it is the most accurate.

2.    A primer must be prepared with a complementary sequence to part of the DNA of interest.

3.    Repeated heating and cooling cycles allow the enzymes to act specifically and replaces helicase.

4.    Each cycle of the polymerase chain reaction doubles the amount of DNA of interest.

9.    Restriction endonucleases are used for which of the following?

1.    Gene therapy

2.    Southern blotting

3.    DNA repair

1.    I only

2.    II only

3.    II and III only

4.    I, II, and III

10.How does prokaryotic DNA differ from eukaryotic DNA?

1.    Prokaryotic DNA lacks nucleosomes.

2.    Eukaryotic DNA has telomeres.

3.    Prokaryotic DNA is replicated by a different DNA polymerase.

4.    Eukaryotic DNA is circular when unbound by centromeres.

1.    I only

2.    IV only

3.    II and III only

4.    I, II, and III only

11.Why might uracil be excluded from DNA but NOT RNA?

1.    Uracil is much more difficult to synthesize than thymine.

2.    Uracil binds adenine too strongly for replication.

3.    Cytosine degradation results in uracil.

4.    Uracil is used as a DNA synthesis activator.

12.Tumor suppressor genes are most likely to result in cancer through:

1.    loss of function mutations.

2.    gain of function mutations.

3.    overexpression.

4.    proto-oncogene formation.

13.Which of the following is an ethical concern of gene sequencing?

1.    Gene sequencing is invasive, thus the potential health risks must be thoroughly explained.

2.    Gene sequencing impacts relatives, thus privacy concerns may be raised.

3.    Gene sequencing is very inaccurate, which increases anxiety related to findings.

4.    Gene sequencing can provide false-negative results, giving a false sense of security.

14.Which of the following is NOT a difference between heterochromatin and euchromatin?

1.    Euchromatin has areas that can be transcribed, whereas heterochromatin is silent.

2.    Heterochromatin is tightly packed, whereas euchromatin is less dense.

3.    Heterochromatin stains darkly, whereas euchromatin stains lightly.

4.    Heterochromatin is found in the nucleus, whereas euchromatin is in the cytoplasm.

15.During which phase of the cell cycle are DNA repair mechanisms least active?

1.    G1

2.    S

3.    G2

4.    M

PRACTICE QUESTIONS

Answers and Explanations

1.    BNucleotides bond together to form polynucleotides. The 3′ hydroxyl group of one nucleotide's sugar joins the 5′ hydroxyl group of the adjacent nucleotide's sugar by a phosphodiester bond. Hydrogen bonding, choice (A), is important for holding complementary strands together, but does not play a role in the bonds formed between adjacent nucleotides on a single strand.

2.    DBecause we are looking for the false statement, we have to read each choice to eliminate those that are true or find one that is overtly false. Let's quickly review the main differences between DNA and RNA. DNA is double-stranded, with a deoxyribose sugar and the nitrogenous bases A, T, C, and G. RNA, on the other hand, is usually single-stranded, with a ribose sugar and the bases A, U, C, and G. Choice (D) is false because both DNA replication and RNA synthesis proceed in a 5′ to 3′ direction.

3.    AThe melting temperature of DNA is the temperature at which a DNA double helix separates into two single strands (denatures). To do this, the hydrogen bonds linking the base pairs must be broken. Cytosine binds to guanine with three hydrogen bonds, whereas adenine binds to thymine with two hydrogen bonds. The amount of heat needed to disrupt the bonding is proportional to the number of bonds. Thus, the higher the GC-content in a DNA segment, the higher the melting point.

4.    CAromatic rings must contain conjugated π electrons, which require alternating single and multiple bonds, or lone pairs. In carbohydrate ring structures, only single bonds are present, thus preventing aromaticity. Nucleic acids contain aromatic heterocycles, while proteins will generally contain at least one aromatic amino acid.

5.    CFor a compound to be aromatic, it must be cyclic, planar, and conjugated, and contain 4n + 2 π electrons, where n is any integer. Conjugation requires that every atom in the ring have at least one unhybridized p-orbital. While most examples of aromatic compounds have alternating single and double bonds, compounds can be aromatic if they contain triple bonds as well; this would still permit at least one unhybridized p-orbital.

6.    CDuring DNA replication, the strands are prepared by topoisomerases to relieve torsional strain. At the replication fork, primase, choice (A), creates a primer for the initiation of replication, which is followed by DNA polymerase. On the lagging strand, Okazaki fragments form which are joined by DNA ligase, choice (C). After the chromosome has been processed, the ends, called telomeres, are replicated with the assistance of the enzyme telomerase, choice (D). RNA polymerase, distinct from primase, is responsible for transcription.

7.    BcDNA (complementary DNA) is formed from a processed mRNA strand by reverse transcription. cDNA is used in DNA libraries and contains only the exons of genes that are transcriptionally active in the sample tissue.

8.    AThe polymerase chain reaction is used to clone a sequence of DNA using a DNA sample, a primer, free nucleotides, and enzymes. The polymerase from Thermus aquaticus is used because the reaction is regulated by thermal cycling, which would denature human enzymes.

9.    DEndonucleases are enzymes that cut DNA. They are used by the cell for DNA repair. They are also used by scientists during DNA analysis, as restriction enzymes are endonucleases. Restriction enzymes are used to cleave DNA before electrophoresis and Southern blotting, and to introduce a gene of interest into a viral vector for gene therapy.

10.DProkaryotic DNA is circular and lacks histone proteins, and thus does not form nucleosomes. Both prokaryotic and eukaryotic DNA are replicated by DNA polymerases, although these polymerases differ in identity. Eukaryotic DNA is organized into chromatin, which can condense to form linear chromosomes; only prokaryotes have circular chromosomes. Only eukaryotic DNA has telomeres.

11.COne common DNA mutation is the transition from cytosine to uracil in the presence of heat. DNA repair enzymes recognize uracil and correct this error by excising the base and inserting cytosine. RNA exists only transiently in the cell, such that cytosine degradation is insignificant. Were uracil to be used in DNA under normal circumstances, it would be impossible to tell if a base should be uracil or if it is a damaged cytosine nucleotide.

12.AOncogenes are most likely to result in cancer through activation, choice (B), while tumor suppressor genes are most likely to result in cancer through inactivation.

13.BOne of the primary ethical concerns related to gene sequencing is the issue of consent and privacy. Because genetic screening provides information on direct relatives, there are potential violations of privacy in communicating this information to family members who may be at risk. There are not significant physical risks, eliminating choice (A), and gene sequencing is fairly accurate, eliminating choices (C) and (D).

14.DEuchromatin has a classic “beads on a string” appearance that stains lightly, while heterochromatin is tightly packed and stains darkly. Heterochromatin is primarily composed of inactive genes or untranslated regions, while euchromatin is able to be expressed. All chromatin is found in the nucleus, not the cytoplasm.

15.DMismatch repair mechanisms are active during S phase (proofreading) and G2 phase (MSH2 and MLH1), eliminating choices (B) and (C). Nucleotide and base excision repair mechanisms are most active during the G1 and G2 phases, also eliminating choice (A). These mechanisms exist during interphase because they are aimed at preventing propagation of the error into daughter cells during M phase (mitosis).