HESI A2 Elsevier (HESI): HESI Admission Assessment - Set 3 - Part 3

Correct Answer: Double helix

DNA has a double helix shape, with two complementary strands of nucleotides that twist around each other. The sugar phosphate backbones run on the outside, while the nitrogen bases pair on the inside. Option a is incorrect because DNA is not a single straight chain. Although each strand by itself is a chain of nucleotides, two strands wind around each other, so the overall shape involves two strands rather than one. Option b is incorrect because A branched tree shape does not describe DNA at all. DNA strands run linearly with no branches, and the two strands twist together rather than splitting into multiple branches at any point along their length. Option c is incorrect because A triple helix is not the standard structure of DNA in cells. While certain triple helix DNA forms can occur in special situations, the well known biological form has only two intertwined strands.

Correct Answer: Thymine

In DNA, adenine pairs with thymine through two hydrogen bonds across the helix. This pairing rule for A and T, along with C and G pairing, allows the two strands to fit together. Option a is incorrect because Cytosine pairs with guanine in DNA, not with adenine. Each cytosine and guanine pair is held together by three hydrogen bonds, while adenine pairs with a different base across the helix entirely. Option b is incorrect because Guanine pairs with cytosine in DNA, not with adenine. Adenine and guanine are both purines, and base pairing always occurs between a purine and a pyrimidine, not between two purines or two pyrimidines together. Option d is incorrect because Uracil pairs with adenine in RNA, not in DNA. DNA uses thymine in place of uracil, so the correct partner for adenine within a DNA molecule is thymine rather than uracil during pairing.

Correct Answer: DNA polymerase

DNA polymerase reads each parental strand and adds matching nucleotides to build a new complementary strand. It is the central enzyme of DNA replication and ensures that new strands match the template accurately. Option b is incorrect because RNA polymerase synthesizes RNA, not DNA, during the process of transcription in the nucleus. While it adds nucleotides to a growing strand, it does not act on DNA replication, where DNA polymerase works. Option c is incorrect because Helicase unwinds the DNA double helix at the replication fork, separating the two strands so they can be copied. It does not add nucleotides itself, and a different enzyme builds the new strands. Option d is incorrect because Ligase seals the small gaps between Okazaki fragments on the lagging strand, joining the sugar phosphate backbone together. It does not add new nucleotides one by one along the template strand itself.

Correct Answer: Each new molecule has one old strand

Semiconservative replication means each new DNA molecule contains one parental strand and one newly synthesized strand. This pattern was confirmed by classic experiments and ensures that genetic information is preserved accurately across generations. Option a is incorrect because Two completely new strands per molecule would describe conservative replication, which is not how DNA actually replicates in cells. The original strands are not discarded, so this option does not match the semiconservative model. Option c is incorrect because Two completely old strands would describe no replication at all, since no new strands would have been built. Replication produces new strands, so this option does not describe a real replication mechanism. Option d is incorrect because Random mixing of strands describes the dispersive model of replication, which experiments have ruled out long ago. DNA replicates in an organized way, with each daughter molecule keeping one whole parental strand intact.

Correct Answer: Deoxyribose, phosphate, nitrogenous base

A DNA nucleotide is made of a deoxyribose sugar, a phosphate group, and one of four nitrogenous bases. The bases are adenine, thymine, cytosine, or guanine, and the three parts repeat along DNA. Option a is incorrect because Glucose is not part of a DNA nucleotide, and amino acids are the building blocks of proteins, not nucleic acids. The DNA nucleotide uses a different sugar plus a phosphate and a nitrogenous base. Option b is incorrect because Ribose is the sugar found in RNA, not DNA. While the phosphate and nitrogenous base parts are correct, the DNA nucleotide uses a slightly different sugar that distinguishes it from RNA in cells. Option c is incorrect because Glycerol is a component of lipids such as triglycerides and phospholipids, not nucleic acids in any form. DNA nucleotides contain a sugar, not glycerol, along with a phosphate group and a nitrogenous base.

Correct Answer: Copying DNA into a messenger RNA

Transcription is the process in which a DNA sequence is used as a template to make a complementary messenger RNA molecule. RNA polymerase carries out this step inside the nucleus of eukaryotic cells. Option a is incorrect because Building a protein from amino acids using a messenger RNA template describes translation, not transcription. Translation occurs at the ribosome and turns the message carried by RNA into a polypeptide chain of amino acids. Option b is incorrect because Copying DNA into a new DNA molecule describes DNA replication, which is performed by DNA polymerase before cell division. Transcription produces RNA, not DNA, so this option describes a different cellular process inside the nucleus. Option d is incorrect because Breaking down RNA into individual nucleotides is RNA degradation, not transcription. Transcription is a building process that creates new RNA molecules rather than dismantling existing ones, so this option does not describe transcription correctly.

Correct Answer: At the ribosome

Translation occurs at the ribosome, where messenger RNA is read in three nucleotide codons. Transfer RNA molecules deliver the corresponding amino acids, which are linked into a polypeptide chain by the ribosome during protein synthesis. Option b is incorrect because The nucleus is where transcription occurs in eukaryotic cells, not translation of proteins. After messenger RNA is made, it leaves the nucleus and travels to a ribosome, where translation actually takes place. Option c is incorrect because The mitochondrial matrix is the site of the Krebs cycle, not protein synthesis from messenger RNA in general for the cell. Although mitochondria contain their own ribosomes, most translation occurs at cytoplasmic ribosomes. Option d is incorrect because The smooth endoplasmic reticulum lacks ribosomes and is involved in lipid synthesis and detoxification of drugs. Translation occurs at ribosomes, including those bound to the rough endoplasmic reticulum, not the smooth endoplasmic reticul

Correct Answer: Three RNA nucleotides coding one amino acid

A codon is a sequence of three consecutive nucleotides on messenger RNA that the ribosome reads. Each codon either specifies one amino acid to be added during translation or signals start and stop. Option a is incorrect because A pair of nucleotides is not a codon in any genetic code. The genetic code reads in groups of three, so a unit of two nucleotides cannot specify an amino acid at all. Option c is incorrect because A single nucleotide cannot serve as a codon because the genetic code is read in groups of three nucleotides. Stop signals during translation are still made up of three nucleotides, not one nucleotide. Option d is incorrect because A region of DNA that does not code for any product describes a noncoding region, such as an intron of a gene. This is different from a codon, which is a messenger RNA triplet.

Correct Answer: Point mutation

A point mutation, also called a substitution, replaces one nucleotide with another at a single position in DNA. The effect on the protein depends on the codon affected and may be entirely silent. Option a is incorrect because An insertion mutation adds one or more nucleotides into a DNA sequence rather than replacing an existing nucleotide already present. This change can shift the reading frame for codons that come after it. Option b is incorrect because A deletion mutation removes one or more nucleotides from a DNA sequence in a gene. It does not replace one base with another, and like insertion it often shifts the reading frame outward. Option c is incorrect because A frameshift mutation results from insertions or deletions that change how codons are read by the ribosome. It is not itself a single base change, and its effects come from altered codon grouping.

Correct Answer: Several codons specify the same amino acid

The genetic code is redundant because most amino acids are specified by more than one codon during translation. For example, leucine can be coded by six different codons, providing protection against mutations. Option a is incorrect because If each amino acid were coded by only one codon, the code would not be redundant at all in any sense. Redundancy specifically refers to multiple codons mapping to the same amino acid. Option b is incorrect because A single codon does not specify several different amino acids in the standard genetic code at all. That would make protein synthesis unreliable, and each codon specifies one amino acid or a stop. Option d is incorrect because The genetic code is essentially universal across living organisms, with very few exceptions to this rule overall. The same codon does not have different meanings in different cells of the same organism.