(a) Explain conjugation and transformation in bacteria. Write a brief note on their significance. (20 marks) (b) Explain…

(a) Conjugation and Transformation in Bacteria Bacteria lack meiosis, but exchange DNA by horizontal gene transfer.

Conjugation (discovered by Lederberg and Tatum, 1946) is one-way DNA transfer from a donor to a recipient by direct contact. A donor with the F (fertility) plasmid, which carries the tra genes, makes a sex pilus. The pilus attaches to an F⁻ cell and pulls it close, forming a conjugation bridge. In F⁺ × F⁻ mating, one strand of the plasmid is nicked at oriT and transferred 5' to 3' by rolling-circle replication; each cell makes the complementary strand, so the recipient becomes F⁺, but chromosomal genes are not transferred. In Hfr (high frequency of recombination) cells, F is integrated into the chromosome, so chromosomal genes are transferred linearly from the integrated origin, in a fixed order. The pair usually breaks before complete transfer, so the recipient stays F⁻, but recombines the donor segment (RecA). Jacob and Wollman's interrupted-mating experiment (1957) used the time of gene entry to map the E. coli chromosome. F′ factors carry bacterial genes.

Transformation is uptake of free DNA by competent cells, naturally in Streptococcus pneumoniae, Bacillus subtilis, Haemophilus and Neisseria, and artificially in E. coli using CaCl₂ or electroporation. DNA binds, one strand is degraded and the other is taken in and integrated by recombination. Griffith (1928) showed transformation in pneumococcus; Avery, MacLeod and McCarty (1944) proved DNA is the transforming principle.

Significance: spread of antibiotic resistance and virulence genes, bacterial evolution, genetic mapping, and recombinant DNA technology (plasmid cloning).

(b) Evolution of Sex in Algae Sexual reproduction in algae shows a trend from isogamy through anisogamy to oogamy, with increasing specialisation of gametes.

Isogamy: the gametes are alike in size and shape and motile, as in Chlamydomonas (most species), Ulothrix and Ulva. In Spirogyra the gametes are non-flagellate but differ physiologically (conjugation), a sign of physiological anisogamy, as in Ectocarpus.

Anisogamy: both gametes are motile, but the female (macrogamete) is larger than the male (microgamete), as in Chlamydomonas braunii, Pandorina, Eudorina and Cutleria.

Oogamy: a large, non-motile egg is fertilised by a small motile sperm (non-motile spermatia in red algae), as in Volvox, Oedogonium, Chara, Fucus and Vaucheria.

The volvocine line (Chlamydomonas → Gonium → Pandorina → Eudorina → Pleodorina → Volvox) shows this progression with growing colony size and division of labour. Selection explains it: small, numerous gametes improve encounter, while larger zygotes with food reserves increase survival, so gamete size diverges (disruptive selection). Oogamy protects the egg and prepares for the embryo and the seed habit of land plants. Sex is often triggered by stress, such as nutrient depletion, and gamete attraction involves pheromones.

(c) Progymnosperms Progymnosperms are extinct Devonian-Carboniferous plants that had the secondary wood of gymnosperms (a cambium producing xylem with pitted tracheids and phloem) and fern-like, spore-bearing reproduction, so they link pteridophytes and gymnosperms.

Examples: Aneurophyton and Protopteridium (Aneurophytales) were small, with a protostele-based, homosporous; Archaeopteris (Archaeopteridales, Late Devonian), a large tree with a pith-containing eustele and secondary wood (Callixylon is its wood), which bore fern-like leaves and was heterosporous in some species; Protopitys (Protopityales) had a Carboniferous record.

Significance: they show how secondary growth and heterospory arose before seeds, leading to seed ferns and conifers, and Archaeopteris formed the first forests. Beck (1960) proved the *Archaeopteris*-*Callixylon* connection.