(a) What is sex-determination? Give an account of genic balance theory of sex-determination in Drosophila with examples from…

(a) Sex Determination and Genic Balance Theory in Drosophila

Sex-determination is the genetic mechanism establishing the sexual phenotype of an organism, distinct from sex differentiation, which is the downstream morphological and physiological development of sex characteristics. In Drosophila melanogaster, C.B. Bridges formulated the Genic Balance Theory, postulating that sex is determined by the numerical ratio between X chromosomes (carrying female-determining factors) and sets of autosomes (A, carrying male-determining factors), expressed as the X:A ratio.

An X:A ratio of 1.0 produces a female, while a ratio of 0.5 produces a male. Ratios between 0.5 and 1.0 result in intersex individuals, while ratios exceeding 1.0 or falling below 0.5 result in inviable or sterile metafemales and metamales, respectively. The Y chromosome plays no role in sex determination, although it is required for male fertility.

Abnormal karyotypes substantiate this theory. Individuals with 2X:2A (ratio 1.0) or XXY:2A (ratio 1.0) develop as fertile females despite the presence of a Y chromosome. Individuals with XY:2A (ratio 0.5) develop as normal males, whereas XO:2A (ratio 0.5) develops into a morphologically normal but sterile male due to the absence of the Y-borne fertility factors. Flies with 3X:2A (ratio 1.5) develop as metafemales (superfemales), which are sterile and weakly viable. A 2X:3A karyotype (ratio 0.67) produces an intersex fly displaying a mosaic of intermediate male and female traits. A 1X:3A karyotype (ratio 0.33) yields a sterile, inviable metamale (supermale), while a triploid 3X:3A fly (ratio 1.0) is a normal female, confirming that the ratio, not the absolute number of X chromosomes, matters.

At the molecular level, the X:A ratio is read through X-linked signal elements (such as sisterless-a and sisterless-b/scute) whose double dose in XX embryos switches on the master gene Sex-lethal (Sxl); Sxl protein controls splicing of transformer (tra), which in turn produces the female form of doublesex (dsx). In XY embryos Sxl stays off, and male-specific dsx and dosage compensation by hypertranscription of the single X follow.

(b) DNA Replication in Prokaryotes

DNA replication is the biological process of duplicating a double-stranded DNA molecule into two identical copies through a semiconservative mechanism, wherein each daughter duplex retains one parental and one newly synthesized strand, as shown by Meselson and Stahl (1958) with ¹⁵N-labelled E. coli. The circular chromosome replicates bidirectionally from a single origin in the theta (θ) mode.

In E. coli, replication initiates at a unique locus, oriC. DnaA initiator proteins bind to specific 9-mer repeats, causing localized denaturation at 13-mer AT-rich regions. DnaB helicase unwinds the double helix bidirectionally, while single-stranded DNA-binding (SSB) proteins stabilize the exposed templates and DNA gyrase (topoisomerase II) relieves positive supercoiling ahead of the fork.

DnaG primase synthesizes short RNA primers complementary to the DNA template. The replisome core, DNA polymerase III, extends these primers in the 5' to 3' direction. On the leading strand, synthesis is continuous toward the replication fork. On the lagging strand, synthesis occurs discontinuously away from the fork, generating short Okazaki fragments. DNA polymerase I removes the RNA primers utilizing its 5' to 3' exonuclease activity and fills the resulting gaps via its 5' to 3' polymerase activity. Finally, DNA ligase catalyzes the formation of phosphodiester bonds to seal the remaining nicks. The Pol III holoenzyme, held on the template by the sliding β-clamp, adds about 1,000 nucleotides per second and proofreads with its 3′→5′ exonuclease, giving an error rate near 10⁻⁹–10⁻¹⁰. The two forks meet in the terminus region, where Ter sites bound by Tus protein halt them, and topoisomerase IV unlinks (decatenates) the daughter circles.

(c) Gene Regulation Mechanisms in E. coli

Gene regulation in E. coli coordinates metabolic adaptation primarily at the level of transcription through multi-cistronic operons containing regulatory genes, promoters, operators, and structural genes.

The lac operon exemplifies negative inducible control. In the absence of lactose, the *lacI*-encoded repressor binds the operator, physically blocking RNA polymerase. When lactose is present, its isomer allolactose acts as an inducer, binding the repressor and causing an allosteric shift that releases it from the operator to allow transcription of lacZ, lacY, and lacA. The operon is further regulated by positive control via catabolite repression; low glucose levels elevate cyclic AMP (cAMP), which complexes with the Catabolite Activator Protein (CRP/CAP) to bind the promoter and enhance RNA polymerase recruitment.

The trp operon operates under negative repressible control for tryptophan biosynthesis. The active corepressor, tryptophan, binds the inactive TrpR aporepressor, enabling it to attach to the operator and repress transcription when end-product levels are sufficient. Fine-tuning occurs via transcriptional attenuation, where translation of a leader peptide responds to charged tRNA^Trp levels; excess tryptophan causes the ribosome to advance rapidly, allowing the formation of an intrinsic 3-4 transcription terminator hairpin that prematurely aborts transcription; when tryptophan is scarce the ribosome stalls at the Trp codons, the 2–3 antiterminator forms instead, and transcription continues.

Other mechanisms include the ara operon, where the AraC protein acts as both repressor and activator; alternative sigma factors such as σ³² for heat-shock genes; and post-transcriptional control by small regulatory RNAs and riboswitches.