Relationships Among Ideas and Processes Practice Test

Biological Mechanism: synaptic transmission and receptor types

Neurons communicate at synapses, where an electrical signal in the presynaptic cell is converted into a chemical signal and then back into an electrical response in the postsynaptic cell. When an action potential arrives at the presynaptic terminal, voltage-gated Ca$^{2+}$ channels open, allowing Ca$^{2+}$ influx. Elevated Ca$^{2+}$ triggers synaptic vesicle fusion with the membrane via SNARE proteins, releasing neurotransmitter into the synaptic cleft. The neurotransmitter diffuses and binds receptors on the postsynaptic membrane.

Two major receptor classes shape postsynaptic responses. Ionotropic receptors are ligand-gated ion channels that open directly upon neurotransmitter binding, producing rapid changes in membrane potential. For example, AMPA-type glutamate receptors allow Na$^+$ influx, generating fast excitatory postsynaptic potentials. Metabotropic receptors are G protein–coupled receptors (GPCRs) that activate intracellular signaling cascades; they modulate ion channels indirectly and act more slowly but can produce longer-lasting effects, including changes in gene expression.

Signal termination is essential for temporal precision. Neurotransmitters can be cleared by reuptake transporters (e.g., serotonin transporter), enzymatic degradation (e.g., acetylcholinesterase), or diffusion away from the synapse. Pharmacologic agents can target these steps: selective serotonin reuptake inhibitors increase synaptic serotonin by blocking reuptake, while acetylcholinesterase inhibitors prolong acetylcholine action, benefiting some neuromuscular and cognitive disorders.

In the passage, how are ionotropic and metabotropic receptors connected to response timing at synapses?

Medical Innovation: CRISPR-based gene editing for sickle cell disease

Sickle cell disease arises from a single nucleotide substitution in the β-globin gene (HBB) that produces hemoglobin S, which polymerizes under low oxygen and deforms red blood cells. A therapeutic strategy edits hematopoietic stem and progenitor cells (HSPCs) ex vivo so that, after reinfusion, they generate red cells resistant to sickling. Rather than directly correcting HBB in every allele, many approaches disrupt an enhancer of BCL11A, a transcription factor that represses fetal hemoglobin (HbF). HbF interferes with hemoglobin S polymerization; thus, increasing HbF can reduce disease severity.

CRISPR-Cas9 editing uses a guide RNA (gRNA) to direct the Cas9 nuclease to a specific DNA sequence adjacent to a PAM motif. Cas9 creates a double-strand break, which cells repair by non-homologous end joining (NHEJ) or homology-directed repair (HDR). NHEJ is error-prone and often introduces small insertions or deletions (indels) that disrupt regulatory elements. In the BCL11A enhancer strategy, NHEJ-mediated disruption reduces BCL11A expression in erythroid cells, lifting repression of HbF.

Before reinfusion, patients receive myeloablative conditioning chemotherapy to clear bone marrow niches, enabling edited HSPCs to engraft. Efficacy depends on the fraction of long-term repopulating stem cells successfully edited and on sustained HbF production. Safety assessment focuses on off-target edits (unintended cuts at similar sequences), chromosomal rearrangements, and clonal expansion that could predispose to malignancy. Sensitive assays (GUIDE-seq, targeted deep sequencing) map potential off-target sites, while long-term follow-up monitors hematologic parameters.

Clinical trials proceed stepwise: early cohorts test feasibility and dose, later cohorts evaluate vaso-occlusive crisis frequency and transfusion independence. Manufacturing controls ensure consistent editing efficiency and cell viability, and release criteria specify acceptable off-target profiles. If successful, implementation requires specialized centers, because cell collection, editing, conditioning, and reinfusion are complex and resource-intensive.

In the passage, how are NHEJ repair and increased fetal hemoglobin connected?