Survey of the Natural Sciences - Biology
What the biology section tests
The OAT biology section samples cell and molecular biology, genetics, microbiology, anatomy and physiology, evolution, and ecology. Questions reward recall of specific structures and processes, but many items ask you to predict an outcome-for example, what happens when a transport protein is inhibited or how a mutation changes a protein.
Core rules to keep straight
- Membranes: small nonpolar molecules diffuse; water moves by osmosis; ions and polar molecules need channels or carriers. Active transport uses ATP and can move against a gradient.
- Energy flow: glycolysis occurs in the cytosol and yields 2 ATP and 2 NADH per glucose; the Krebs cycle and oxidative phosphorylation occur in mitochondria.
- Central dogma: DNA → RNA → protein. Replication is semiconservative; transcription uses RNA polymerase; translation uses mRNA codons and tRNA anticodons.
- Genetics: alleles segregate during meiosis; independent assortment applies to genes on different chromosomes. Use the product rule for independent events.
Worked example
In a dihybrid cross AaBb × AaBb, what fraction of offspring is A_bb? Treat each gene separately. Aa × Aa gives 3/4 A_ and 1/4 aa; Bb × Bb gives 3/4 B_ and 1/4 bb. Because the genes assort independently, multiply: (3/4)(1/4)=3/16. The underscore means the dominant phenotype, so A_bb includes AAbb and Aabb.
Traps that cost marks
- Confusing mitosis (2 identical diploid cells) with meiosis (4 genetically varied haploid cells).
- Saying transcription produces protein; translation does.
- Reversing the direction of osmosis: water moves toward higher solute concentration.
- Treating a dominant allele as more common; dominance is about phenotype, not frequency.
- Forgetting that enzymes lower activation energy but do not change ΔG.