Test: DNA - 1 - MCAT MCQ

Nucleotides 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.

The 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.

For 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.

cDNA (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.

Endonucleases 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.

One 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.

One 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).

Because 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.

Aromatic 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.

During 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.