Biology (Scientific Branch) - علم الأحياء
Biology - علم الأحياء - tests cell structure and division, Mendelian and molecular genetics, DNA and protein synthesis, photosynthesis and respiration, and the organ systems emphasised in the curriculum. Genetics problems and diagram-labelling questions carry a large share of the marks.
Core rules
- Mitosis produces two genetically identical diploid cells; meiosis produces four haploid cells, with crossing over in prophase I and independent assortment in metaphase I.
- Monohybrid cross Aa × Aa gives genotype ratio 1 AA : 2 Aa : 1 aa and phenotype ratio 3 : 1.
- A test cross (crossing with the recessive homozygote) reveals whether a dominant phenotype is homozygous or heterozygous.
- In DNA, A pairs with T by two hydrogen bonds and G pairs with C by three; therefore %A = %T and %G = %C.
- Transcription occurs in the nucleus and produces mRNA; translation occurs on ribosomes and uses tRNA to add amino acids.
- Aerobic respiration: glycolysis in the cytoplasm, Krebs cycle in the matrix, electron transport on the inner mitochondrial membrane.
Worked example
A DNA sample contains 30% adenine. Find the percentages of the other bases. Since A pairs with T, %T = 30%. A + T = 60%, so G + C = 40%. Because G = C, each is 20%. The answer is T = 30%, G = 20%, C = 20%, and the sum is checked as 100%.
Traps
- Counting chromatids as chromosomes after replication - the centromere, not the arm, defines the chromosome.
- Writing 'dominant' to mean 'more common in the population'.
- Swapping the products of mitosis and meiosis in gamete formation questions.
- Reversing transcription and translation when describing where each happens.
- Misapplying sex-linked ratios: a carrier mother × unaffected father gives 50% of sons affected, not 50% of all children.