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Key Facts: Senegal Baccalauréat Série S2 Exam
10/20
Minimum weighted average required for immediate admission (Premier Groupe)
Office du Baccalauréat du Sénégal
Coeff 6
Coefficient for SVT and Physical Sciences in Série S2
Décret portant organisation du Baccalauréat
5,000 FCFA
Registration fee; optional papers cost an additional 1,000 FCFA each
Office du Baccalauréat — 2026 registration conditions
Sample Senegal Baccalauréat Série S2 Practice Questions
Try these sample questions to review concepts for the Senegal Baccalauréat Série S2 exam. Each question includes a detailed explanation. Start the interactive quiz above for the full 100+ question experience with AI tutoring.
1During DNA replication in a dividing cell, which enzyme is primarily responsible for synthesizing the nascent leading strand by continuously adding deoxyribonucleotides in the 5' to 3' direction?
A.DNA polymerase III
B.DNA ligase
C.RNA primase
D.DNA helicase
Explanation: DNA polymerase III is the primary prokaryotic replicative enzyme (functionally analogous to DNA polymerase delta and epsilon in eukaryotes) that synthesizes the leading strand continuously in the 5' to 3' direction with high processivity and 3' to 5' proofreading activity.
2In the classic Meselson-Stahl experiment, bacteria cultured in 15N medium were transferred to 14N medium. What distribution of DNA buoyant density was observed after exactly one generation of replication?
A.Two distinct bands: one corresponding to pure 15N-15N DNA and one to pure 14N-14N DNA
B.A single intermediate-density hybrid band corresponding to 15N-14N duplex DNA
C.A single heavy band of pure 15N-15N DNA because replication takes two cycles to incorporate light nitrogen
D.Three distinct bands of heavy, intermediate, and light density in equal proportions
Explanation: Meselson and Stahl demonstrated semi-conservative replication: after one generation in 14N medium, every replicated DNA duplex contains one original heavy parent strand (15N) and one newly synthesized light strand (14N), producing a single band of intermediate density.
3In eukaryotic gene expression, which post-transcriptional processing event protects the 3' terminus of pre-mRNA from exonucleolytic degradation and promotes its nuclear export to the cytoplasm?
A.Addition of a 7-methylguanosine cap to the 5' triphosphate end
B.Excision of coding exons and retention of non-coding intervening sequences
C.Synthesis of a polyadenine (poly-A) tail by poly(A) polymerase following cleavage at the AAUAAA signal
D.Binding of DNA topoisomerase to the downstream promoter element
Explanation: Cleavage of the nascent pre-mRNA downstream of the AAUAAA polyadenylation signal followed by the addition of approximately 150 to 250 adenine residues by poly(A) polymerase forms the 3' poly-A tail, which enhances transcript stability, nuclear export, and translation initiation.
4The amino acid Leucine is specified by six distinct mRNA codons (UUA, UUG, CUU, CUC, CUA, CUG). Which fundamental property of the genetic code does this observation demonstrate?
A.Ambiguity
B.Degeneracy (redundancy)
C.Universality
D.Overlapping reading frame
Explanation: The genetic code is degenerate (redundant) because 61 sense codons encode only 20 standard amino acids, meaning multiple synonymous codons can direct the incorporation of the same amino acid.
5In sickle cell anemia (drépanocytose), an adenine-to-thymine transversion in the beta-globin gene converts codon GAG to GTG in the non-template strand. What molecular classification best describes this mutation?
A.Frameshift mutation resulting in premature termination of the peptide chain
B.Silent mutation with no phenotypic effect due to codon degeneracy
C.Missense mutation resulting in the substitution of glutamic acid by valine at position 6
D.Nonsense mutation that generates an in-frame stop codon (UGA)
Explanation: The point mutation alters codon 6 from GAG (hydrophilic glutamic acid) to GUG (hydrophobic valine). Because it changes one amino acid to another, it is a missense mutation. The exposed hydrophobic patch on deoxy-HbS promotes pathological polymer formation.
6In Escherichia coli, how does the presence of lactose induce transcription of the lac operon structural genes (lacZ, lacY, lacA)?
A.Allolactose binds directly to the beta-galactosidase enzyme to increase its catalytic turnover rate
B.Allolactose binds the LacI repressor, causing an allosteric conformational shift that reduces its affinity for the operator sequence
C.Allolactose phosphorylates the CAP-cAMP complex to force RNA polymerase off the promoter
D.Allolactose hydrolyzes the lac operator DNA sequence to create an open promoter complex
Explanation: Allolactose, a metabolic byproduct of lactose isomerization by beta-galactosidase, acts as an inducer. It binds to the LacI repressor protein, triggering an allosteric structural transition that causes LacI to dissociate from the operator (lacO), freeing RNA polymerase to transcribe the operon.
7In pea plants, seed shape is determined by alleles R (round, dominant) and r (wrinkled, recessive), and cotyledon color by Y (yellow, dominant) and y (green, recessive). In a dihybrid cross between two RrYy plants on separate chromosomes, what is the expected phenotypic ratio among the offspring?
A.9 round yellow : 3 round green : 3 wrinkled yellow : 1 wrinkled green
B.1 round yellow : 1 round green : 1 wrinkled yellow : 1 wrinkled green
C.3 round yellow : 1 wrinkled green
D.9 wrinkled green : 3 round green : 3 wrinkled yellow : 1 round yellow
Explanation: According to Mendel's law of independent assortment, the selfing or crossing of two independent dihybrids (RrYy x RrYy) generates a 9:3:3:1 phenotypic distribution among the 16 combinations: 9 R_Y_, 3 R_yy, 3 rrY_, and 1 rryy.
8A dihybrid female Drosophila heterozygous in coupling (b+ vg+ / b vg) is test-crossed with a homozygous recessive male (b vg / b vg). The test-cross yields 965 wild-type, 945 black vestigial, 205 black normal, and 195 gray vestigial flies. What is the recombination frequency between the black and vestigial loci?
A.8.7%
B.17.3%
C.34.6%
D.50.0%
Explanation: Total offspring = 965 + 945 + 205 + 195 = 2310. The recombinant classes are the non-parental combinations (black normal + gray vestigial) = 205 + 195 = 400. Recombination frequency = (400 / 2310) * 100% = 17.3%, indicating a genetic map distance of 17.3 cM (centimorgans).
9Red-green color blindness is an X-linked recessive condition. A woman with normal color vision whose father was color-blind marries a man with normal color vision. What is the probability that their first biological son will be color-blind?
A.0%
B.25%
C.50%
D.100%
Explanation: The woman's father was hemizygous affected (X^d Y), so she must be an obligate carrier (X^D X^d). Her husband is unaffected (X^D Y). Any son inherits the Y chromosome from the father and has a 50% chance of inheriting the maternal X^d allele, resulting in a 50% probability of being affected.
10Which of the following describes the correct chronological sequence of stages during phagocytosis carried out by macrophages and neutrophils in innate immunity?
A.Adherence -> Chemotaxis -> Exocytosis -> Phagolysosome digestion -> Ingestion
B.Chemotaxis -> Adherence -> Ingestion (engulfment) -> Phagolysosome fusion & digestion -> Exocytosis of indigestible debris
C.Ingestion -> Chemotaxis -> Phagolysosome fusion -> Adherence -> Antigen presentation
D.Degranulation -> Opsonization -> Chemotaxis -> Endocytosis -> Clonal selection
Explanation: Phagocytes first migrate along chemoattractant gradients toward pathogens (chemotaxis), bind to surface markers or opsonins (adherence), extend pseudopodia to engulf the microbe into a phagosome (ingestion), fuse the phagosome with lysosomes containing hydrolytic enzymes (digestion), and eliminate residual material via exocytosis.
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