Question
Which of the following methods of horizontal transmission of genetic material would be most likely to lead to new variants of the COVID- 19 virus?
(A) transformation
(B) transduction
(C) transposition
(D) conjugation
▶️Answer/Explanation
Ans:
(B) Transduction involves the transfer of genetic material by viral
transmission. Transformation, transposition, and conjugation do not
involve viruses, so choices (A), (C), and (D) are incorrect.
Question
Which of the following is a reasonable hypothesis to explain why mutant 1 did not produce β-galactosidase in the presence or absence
of lactose?
(A) The β-galactosidase gene had a mutation that produced an early
stop codon in the mRNA.
(B) Mutant 1 is unable to metabolize glucose and galactose.
(C) Mutant 1 is unable to break down lactose.
(D) Mutant 1 prefers glucose, but will use lactose when no other
substrate is available.
▶️Answer/Explanation
Ans: A
The data for the wild-type suggests that β-gal expression in
inducible. The presence of lactose induces the synthesis of β-gal, the enzyme
that breaks down lactose. The fact that mutant 1 cannot make β-gal in the
presence of lactose suggests that either the protein is not made (or it is made
in a non-functional form). If a mutant never expresses a gene, it is likely that
there is a mutation in the gene sequence or in the regulatory region (the
promoter/operator) of the gene. (Please see preceding table.)
Since the latter is not an answer option, the former must be true. Choice A
is a type of mutation that would prevent a protein from being produced in a
cell.
Choice B is incorrect because the bacteria were grown in a glucose
medium before lactose was added.
Choice C is true, but it is does not explain why mutant 1 did not produce β- gal; it states the consequence of the inability of mutant 1 to express β-gal in the presence of lactose (mutant 1 can’t break down lactose because it doesn’t
express β-gal).
There is no indication in the data that choice D is true.
Question
Refer to the following information.
About 98% of the volume of red blood cells is occupied by the protein hemoglobin. Hemoglobin is a protein that carries oxygen from the lungs to
the tissues. It is composed of four subunits—two identical α subunits and two identical β subunits.
Sickle-cell anemia is a heritable disease that results from a point mutation in the hemoglobin-β (HBB) gene. The mutant gene contains a thymine base instead of an adenine base and causes a missense mutation (an amino acid substitution). The mutant polypeptide contains a valine, a nonpolar amino acid, on the exterior surface of the polypeptide in the position occupied by glutamic acid in the wild-type protein.
Which of the following is the most likely result of the mutation?
(A) abnormal DNA structure as a result of noncomplementary base
pairing
(B) altered protein properties resulting from abnormal interactions
between hemoglobin molecules in red blood cells
(C) altered protein structure resulting from abnormal hydrophobic
interactions between amino acid backbones
(D) altered red blood cell function resulting from an acid-base
reaction between the mutant hemoglobin and the red blood cell
membrane
▶️Answer/Explanation
Ans: B
Red blood cells contain a large amount of the hemoglobin protein,
so any mutation on the surface of the protein could interfere with interactions
between hemoglobin molecules within the cell.
Choice A is incorrect because the base substitution of adenine to thymine
would result in the placement of adenine to the complementary strand of
DNA during replication. Choice C is incorrect because there are no
hydrophobic interactions that occur between the amino acid backbones.
Choice D is incorrect because glutamic acid was replaced with valine, not the other way around, so no “acid-base” reaction would occur between the red blood cell membrane and the hemoglobin surface.
Question
Refer to the following information.
About 98% of the volume of red blood cells is occupied by the protein hemoglobin. Hemoglobin is a protein that carries oxygen from the lungs to
the tissues. It is composed of four subunits—two identical α subunits and two identical β subunits.
Sickle-cell anemia is a heritable disease that results from a point mutation in the hemoglobin-β (HBB) gene. The mutant gene contains a thymine base instead of an adenine base and causes a missense mutation (an amino acid substitution). The mutant polypeptide contains a valine, a nonpolar amino acid, on the exterior surface of the polypeptide in the position occupied by glutamic acid in the wild-type protein.
Which of the following true statements most accurately and comprehensively describes how a single base-pair substitution in a polypeptide can have massive effects on a phenotype?
(A) Hemoglobin is an abundant protein in the body.
(B) Hemoglobin is the most abundant protein in red blood cells.
(C) The alteration in the protein causes oxygen transport to be
compromised.
(D) People that are heterozygous for sickle-cell anemia are resistant
to malaria.
▶️Answer/Explanation
Ans: C
Red blood cells are the only cells in the human body that cannot
use oxygen. Fast-twitch muscle fibers can work anaerobically for short
periods of time, but otherwise all cells in the body need a constant supply of
oxygen to function. Just a few minutes of oxygen deprivation can kill you!
Question
Refer to the following information.
Antibodies are the one of the most abundant proteins in the blood (they make up about 20% of the plasma proteins). One of the five classes of
antibodies is shown as follows.
Each antibody contains four polypeptide chains: two identical heavy chains and two identical light chains. Each heavy chain has a 110 amino
acid variable region \((V_H)\) and a 330–440 amino acid constant region \((C_H1\), 2, and 3). Each light chain has a 110 amino acid variable \((V_L)\) region and a 110 amino acid constant region \((C_L )\). There are several disulfide bonds holding the structure together (indicated by S–S).
These antibodies are made by B-lymphocyte cells. During development of a B cell, the randomly chosen V gene is moved to lie next to a J gene
segment (3 in the diagram). The extra J segment (J is joining segment) and the intron are transcribed and then removed by splicing, and the V3 and J3 are joined together.
Which of the following is the fundamental process by which evolution produces such a great variety of genes for heavy and light chains?
(A) gene duplication
(B) homologous recombination
(C) gene rearrangement
(D) clonal selection
▶️Answer/Explanation
Ans: A
Gene duplication and mutation of the duplicates are the
primary source of new alleles in eukaryotes.