General Biology
General Biology in NBE Sciences tests cell structure and organelles, the cell cycle, the central dogma of molecular biology, and Mendelian versus non-Mendelian inheritance. Most items are one-step applications, so know the rules precisely rather than long descriptive lists.
Core rules
- The Na+/K+-ATPase moves 3 Na+ out and 2 K+ in per ATP, keeping the cytosol negative.
- Mitochondria carry circular DNA and their own ribosomes; mitochondrial traits are maternally inherited.
- Cell cycle order is G1, S (DNA replication), G2, M. The G1/S and G2/M checkpoints arrest cells with damaged DNA.
- Mitosis preserves chromosome number; meiosis I separates homologues, and meiosis II separates sister chromatids.
- Replication is semiconservative; transcription makes RNA; translation uses codons. The code is degenerate but not ambiguous.
Worked example
In a cross AaBb x AaBb with independent assortment, what fraction is A_bb? Handle each gene separately. Aa x Aa gives 3/4 A_ and 1/4 aa; Bb x Bb gives 3/4 B_ and 1/4 bb. Multiply independent probabilities: 3/4 x 1/4 = 3/16. If the genes were linked, the 9:3:3:1 ratio would fail and recombination frequency would be needed.
Traps
- Dominant does not mean more frequent in the population.
- Meiosis I separates homologues, not sister chromatids.
- Mitochondrial disease does not follow Mendelian ratios.
- Degeneracy allows several codons per amino acid; it does not allow one codon to specify two amino acids.