Biology: Molecular and Systems Reasoning
Biology: molecular and systems reasoning
The OAT Biology section tests molecular and cellular biology, genetics, microbiology, and human anatomy and physiology, with smaller amounts of evolution and ecology. Most items ask you to connect a structure to a function or predict the result of a perturbation, not just recall a definition.
Core rules
- Central dogma: DNA to RNA to protein. Transcription uses RNA polymerase and a DNA template; translation uses ribosomes, mRNA, and tRNAs. Mutations in coding DNA may alter protein sequence; silent mutations do not.
- Membrane transport: simple diffusion follows concentration gradients without energy; facilitated diffusion uses channels or carriers; active transport uses ATP, as in the Na+/K+ ATPase, and can move substances against a gradient.
- Genetics: Mendelian ratios assume independent assortment and random fertilization. A testcross reveals an unknown genotype. In pedigrees, autosomal recessive traits can skip generations; X-linked recessive traits affect males more.
- Physiology: use negative feedback. Low blood glucose triggers glucagon release, glycogen breakdown, and a rise in glucose; insulin does the reverse.
- Microbiology: bacteria are prokaryotes with no nucleus and 70S ribosomes; viruses require a host; antibiotics target bacterial structures such as cell walls or 70S ribosomes.
Worked example
A couple, both carriers of an autosomal recessive allele (Aa x Aa), has one affected child (aa). What is the probability their next child is a carrier? Each parent passes A or a with equal probability. Offspring genotypes are AA 1/4, Aa 1/2, and aa 1/4. The next child is independent of the first. Carrier probability is 1/2, not 2/3; 2/3 applies only when you know the child is unaffected.
Traps
- Confusing probability before versus after knowing a phenotype.
- Saying active transport for facilitated diffusion.
- Mixing up afferent and efferent or arteries and veins by oxygen content rather than direction.
- Assuming all mutations change protein function.