Cell Biology, Molecular Biology, and Genetics
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MCAT Biology › Cell Biology, Molecular Biology, and Genetics
A man with type A– blood and a woman with type AB+ blood have a child. Which blood type is impossible for that child to have?
O-
B+
A+
AB+
A-
Explanation
Blood type is inherited as a codominant trait and relies on alleles for blood antibodies as well as Rh (Rhesus) factor. The father's blood type is A–, so he has no Rh factor and must be either AA or AO. The mother must be AB with an Rh factor.
Father possibilities: A-A- or A-O-
Mother possibilities: A-B+ or A+B- or A+B+
Based on these possibilities, we cannot conclude if the child will be positive or negative for Rh factor; however, since the mother has no allele for O blood type, we can conclude that the child cannot have O type blood. The child could receive AA, AO, BO, or AB.
Pea plants have two independently assorted genes that code for seed shape (round or wrinkled) and seed color (yellow or green), respectively. A researcher crosses two pea plants and observes that all F1 offspring have the same phenotype: round shape and yellow seeds. He then performs a test cross with an F1 offspring and observes four different phenotypes in a 1:1:1:1 ratio. Based on this information the researcher concludes the genotypes of the parents.
Which of the following is the correct pairing of recessive alleles?
Wrinkled and green
Round and green
Round and yellow
Wrinkled and yellow
Explanation
The passage states that the F1 generation only had round, yellow seeds. Test crossing an F1 offspring lead to an equal ratio of four different phenotypes. When you test cross, you are crossing the F1 offspring with a homozygous recessive individual; therefore, the test cross individual had recessive seed shape and seed color.
???? x aabb
If round and yellow were recessive, then the F2 offspring would all be round and yellow (because you would cross round/yellow with round/yellow).
aabb x aabb (all offspring round and yellow)
If only round was recessive then you wouldn’t get any wrinkled seeds in F2 generation; similarly, if only yellow was recessive then you wouldn’t get any green seeds.
Aabb x aabb (all offspring have one recessive trait, bb)
If round and yellow are both dominant, then means that wrinkled and green are recessive. If the F1 offspring is heterozygous for both traits, then we can see the observed ratios from the test cross.
AaBb x aabb
1 AaBb (round/yellow), 1 aaBb (wrinkled/yellow), 1 Aabb (round/green), 1 aabb (wrinkled/green)
We can conclude that green and wrinkled must be recessive to yellow and round.
The cell cycle is the series of events a cell undergoes during its lifetime. It involves four main phases: G1, G2, S phases, and mitosis. Each phase is characterized by a specific set of events. These events include cell growth, genetic material replication, and cell division. Several cellular machineries such as organelles and cytoskeletal elements are involved in each phase. In addition to these phases, the cell cycle has specific checkpoints to ensure that the cell is ready to proceed to the subsequent steps in the cycle. This decreases errors during replication and division. G0 phase is a special phase of the cell cycle that is characterized by a quiescent cell.
Cyclin-dependent kinases are special molecules that facilitate the progression of a cell through the cell cycle. Many molecules such as p53 and kinase inhibitors regulate the cell cycle. Unregulated cell cycle can lead to rapid growth of cells that may, eventually, lead to cancer.
Which of the following is/are true regarding the cell cycle?
More than one of these
G0 phase and G1 phase both involve increasing the amount of genetic material
G1 and G2 phase both involve cell growth
G0 phase involves no cell growth
Explanation
There are four main phases in the cell cycle: G1 phase, S phase, G2 phase, and mitosis (M phase). G1 involves cell growth and preparation for DNA replication, S phase involves replication of the genetic material (DNA), G2 phase involves more cell growth and preparation for mitosis, and mitosis involves the division of the cell into two identical daughter cells. G0 phase is another phase that cells can undergo where they remain quiescent. This phase usually occurs after mitosis, when the cell is preparing to enter the G1 phase. Genetic material is increased in S phase only and cell growth occurs in G1 and G2 phases only, thus two of the answer choices are correct.
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.
Which intermediates of the central dogma of molecular biology below are present in normal cellular replication, but apparently absent in the above model of prion replication?
I. mRNA
II. tRNA
III. Protein
I and II
I and III
I, only
II, only
I, II, and III
Explanation
The above reaction shows an actual transfer of information. The information contaiend in the specific structure of PrPC is converted to that contained in PrPRes; in other words, PrPRes is effectively replicating. In all other forms of cellular replication, based on the central dogma of molecular biology, replication occurs based on the following pattern:
Parental DNA → mRNA → Protein, carried by tRNA
Which of the following experimental methods cannot be used to measure the relative abundance of a particular mRNA?
Western blot
Northern blot
Microarray
Reverse transcription polymerase chain reaction (rt-PCR)
Explanation
Western blots are used to measure the relative abundance of proteins. While there is a correlation between the amount of mRNA and protein, using a western blot to measure mRNA would be inconclusive due to the variability of protein half life.
Northern blots are used to run mRNA samples on gels, DNA microarrys give the expression levels of certain genes, and rtPCR is used to detect RNA expression levels. Any of these methods could provide the relative abundance of a particular mRNA.
Cryptosporidium is a genus of gastrointestinal parasite that infects the intestinal epithelium of mammals. Cryptosporidium is water-borne, and is an apicomplexan parasite. This phylum also includes Plasmodium, Babesia, and Toxoplasma.
Apicomplexans are unique due to their apicoplast, an apical organelle that helps penetrate mammalian epithelium. In the case of cryptosporidium, there is an interaction between the surface proteins of mammalian epithelial tissue and those of the apical portion of the cryptosporidium infective stage, or oocyst. A scientist is conducting an experiment to test the hypothesis that the oocyst secretes a peptide compound that neutralizes intestinal defense cells. These defense cells are resident in the intestinal epithelium, and defend the tissue by phagocytizing the oocysts.
She sets up the following experiment:
As the neutralizing compound was believed to be secreted by the oocyst, the scientist collected oocysts onto growth media. The oocysts were grown among intestinal epithelial cells, and then the media was collected. The media was then added to another plate where Toxoplasma gondii was growing with intestinal epithelial cells. A second plate of Toxoplasma gondii was grown with the same type of intestinal epithelium, but no oocyst-sourced media was added.
The apicoplast that defines the phylum Apicomplexa is a membrane bound organelle. Which of the following is true of membrane-bound organelles?
I. They are only present in eukaryotes
II. They are bound by a single phospholipid layer
III. They do not have membrane-associated proteins attached
I only
I and III
II and III
I, II, and III
II only
Explanation
Membrane-bound organelles are a key distinction between eukaryotic cells and prokaryotic cells. Membrane-bound organelles serve diverse purposes, and often have associated protein structures to help carry out enzymatic reactions or other functions.
Cell membranes are almost invariably at least bilayers, however, making choice 2 incorrect. A bilayer functions to sequester the lipid tails common to membranes away from the aqueous cytosol. Incidentally, the apicoplast is surrounded by four membranes, but the effect is the same.
When a solute moves down its concentration gradient across a non-permeable barrier, the process is known as __________.
facilitated diffusion
simple diffusion
active transport
osmosis
Explanation
A solute moving down its concentration gradient across a non-permeable barrier is an example of facilitated diffusion. It requires a carrier protein, but no energy. Any particle crossing a non-permeable barrier will require a protein, and cannot pass via diffusion or osmosis. ATP will not be required to transport a particle down its gradient.
If the particle were travelling against its gradient, it would require ATP AND a protein, and active transport would be the correct answer. Simple diffusion and osmosis require no energy or protein.
Which of the following experimental methods cannot be used to measure the relative abundance of a particular mRNA?
Western blot
Northern blot
Microarray
Reverse transcription polymerase chain reaction (rt-PCR)
Explanation
Western blots are used to measure the relative abundance of proteins. While there is a correlation between the amount of mRNA and protein, using a western blot to measure mRNA would be inconclusive due to the variability of protein half life.
Northern blots are used to run mRNA samples on gels, DNA microarrys give the expression levels of certain genes, and rtPCR is used to detect RNA expression levels. Any of these methods could provide the relative abundance of a particular mRNA.
What happens at the minus-end of actin filaments when the concentration of G-actin is above its critical concentration?
Monomers add on to it.
Monomers are lost from it.
Monomers are not lost from it or added on to it.
Monomers bind GTP.
Monomers undergo dynamic instability.
Explanation
Monomers are lost when concentration of G-actin is below its critical concentration. Monomers are gained when concentration of G-actin is above its critical concentration. If it is in between the critical concentrations, the actin filaments will undergo treadmilling, which is the addition of monomers on the (+) end and loss of monomers on the (–) end.
Cryptosporidium is a genus of gastrointestinal parasite that infects the intestinal epithelium of mammals. Cryptosporidium is water-borne, and is an apicomplexan parasite. This phylum also includes Plasmodium, Babesia, and Toxoplasma.
Apicomplexans are unique due to their apicoplast, an apical organelle that helps penetrate mammalian epithelium. In the case of cryptosporidium, there is an interaction between the surface proteins of mammalian epithelial tissue and those of the apical portion of the cryptosporidium infective stage, or oocyst. A scientist is conducting an experiment to test the hypothesis that the oocyst secretes a peptide compound that neutralizes intestinal defense cells. These defense cells are resident in the intestinal epithelium, and defend the tissue by phagocytizing the oocysts.
She sets up the following experiment:
As the neutralizing compound was believed to be secreted by the oocyst, the scientist collected oocysts onto growth media. The oocysts were grown among intestinal epithelial cells, and then the media was collected. The media was then added to another plate where Toxoplasma gondii was growing with intestinal epithelial cells. A second plate of Toxoplasma gondii was grown with the same type of intestinal epithelium, but no oocyst-sourced media was added.
The apicoplast that defines the phylum Apicomplexa is a membrane bound organelle. Which of the following is true of membrane-bound organelles?
I. They are only present in eukaryotes
II. They are bound by a single phospholipid layer
III. They do not have membrane-associated proteins attached
I only
I and III
II and III
I, II, and III
II only
Explanation
Membrane-bound organelles are a key distinction between eukaryotic cells and prokaryotic cells. Membrane-bound organelles serve diverse purposes, and often have associated protein structures to help carry out enzymatic reactions or other functions.
Cell membranes are almost invariably at least bilayers, however, making choice 2 incorrect. A bilayer functions to sequester the lipid tails common to membranes away from the aqueous cytosol. Incidentally, the apicoplast is surrounded by four membranes, but the effect is the same.