Biology - Molecular Mechanisms, Genetics and Energy Metabolism
Biology in the ENP Alger access contest covers the molecular and cellular basis of life, genetics with its probability calculations, energy metabolism and immunity. Questions often come as a document-based exercise - a pedigree, an experimental result, a diagram of a metabolic pathway - followed by short justifications, so the skill tested is linking a specific observation to a named mechanism rather than recalling a definition.
Core content
- Cell: the fluid-mosaic plasma membrane, key organelles, mitosis versus meiosis.
- Molecular biology: semi-conservative DNA replication, transcription, translation, the genetic code, mutation types (substitution, insertion, deletion).
- Genetics: Mendel's laws, test cross, dihybrid 9:3:3:1, sex-linked transmission, pedigree analysis.
- Metabolism: photosynthesis (light reactions, Calvin cycle) and respiration (glycolysis, Krebs cycle, oxidative phosphorylation).
- Immunity: innate versus acquired responses, antibody structure, vaccination, HIV as a model of immune evasion.
Worked example
Two parents are heterozygous for albinism, a recessive condition: Aa × Aa. Each parent produces A and a gametes with probability ½. The offspring are ¼ AA, ½ Aa and ¼ aa, so ¾ show the dominant phenotype and ¼ are albino. The probability that two successive children are both albino is ¼ × ¼ = 1/16, because the two fertilisations are independent events. The probability that at least one of two children is albino is 1 − (3/4)² = 7/16. The independence applies to the two conceptions, not to two phenotypes of the same child.
Traps that cost marks
- Giving the genotype ratio when the question asks for the phenotype ratio.
- Assuming independent assortment for two genes located on the same chromosome (linkage).
- Confusing transcription (DNA to RNA) with translation (RNA to protein), or reading the template strand in the wrong direction.
- Mixing up mitosis (two identical diploid cells) and meiosis (four haploid, genetically different cells).
- Overstating ATP yields: current estimates for complete aerobic oxidation of one glucose are about 30–32 ATP, not the older figure of 38.