Biology · Plant Kingdom · NEET
It is when a plant's life cycle has TWO different multicellular bodies that take turns (alternate). One is the haploid (n) gametophyte, the other is the diploid (2n) sporophyte. The gametophyte forms gametes, these fuse to make the sporophyte; the sporophyte forms spores, which grow into the next gametophyte. Because these two phases keep following each other, we say the generations 'alternate'. This pattern is seen in all plants, but which body is bigger or dominant changes from group to group.
The GAMETOPHYTE is haploid (n). The SPOROPHYTE is diploid (2n). Easy trick: 'gameto-' has the same idea as gametes, which are haploid, so gametophyte = haploid. The sporophyte comes from a zygote (2n), so it is diploid. NEET loves to test this ploidy, so lock it in.
By MITOSIS. This is the point most students get wrong. The gametophyte is already haploid (n), so it just copies its cells by mitosis to make haploid gametes. Meiosis is NOT needed here because the cells are already n. Meiosis happens in the SPOROPHYTE, when it makes spores. Rule: gametophyte (n) → mitosis → gametes (n); sporophyte (2n) → meiosis → spores (n).
Meiosis occurs in the SPOROPHYTE, inside the spore mother cells, to produce haploid spores. This is called sporic (spore-forming) meiosis in plants with a haplo-diplontic cycle. It reduces the chromosome number from 2n back to n, so the new gametophyte is haploid. So in one full cycle: fertilisation raises n to 2n, and meiosis lowers 2n back to n — the two events balance each other.
Gametophyte = haploid (n) plant body that produces GAMETES by mitosis; the gamete-making phase. Sporophyte = diploid (2n) plant body that produces SPORES by meiosis; the spore-making phase. Remember: gametophyte → gametes, sporophyte → spores. Both are multicellular bodies, just at different ploidy levels of the same life cycle.
Two haploid gametes fuse in fertilisation to form a diploid (2n) ZYGOTE. The zygote grows by mitosis into the diploid SPOROPHYTE. Inside the sporophyte, meiosis makes haploid SPORES. Each spore grows by mitosis into a haploid GAMETOPHYTE. The gametophyte makes gametes again — and the loop repeats. So the zygote links gametophyte→sporophyte, and the spore links sporophyte→gametophyte.
Because Plant Kingdom questions almost always test which phase is dominant in each group (algae, bryophytes, pteridophytes, gymnosperms, angiosperms) and the ploidy at each stage. Knowing that gametophyte = n = makes gametes by mitosis, and sporophyte = 2n = makes spores by meiosis, lets you solve match-the-column and assertion-reason questions quickly. It is the base idea behind haplontic, diplontic and haplo-diplontic life cycles.
Match the plant with the kind of life cycle it exhibits: (a) Spirogyra (b) Fern (c) Funaria (d) Cycas | (i) Dominant diploid sporophyte vascular plant, with highly reduced male or female gametophyte (ii) Dominant haploid free-living gametophyte (iii) Dominant diploid sporophyte alternating with reduced gametophyte called prothallus (iv) Dominant haploid leafy gametophyte alternating with partially dependent multicellular sporophyte
Zygotic meiosis is characteristic of
Try the real previous-year questions from this chapter — each with the answer and a full solution.
No. In bryophytes (mosses, liverworts) and many algae the gametophyte is the bigger, dominant phase. In pteridophytes, gymnosperms and angiosperms the sporophyte is dominant and the gametophyte gets smaller and smaller. So which phase dominates depends on the plant group.
The zygote is diploid (2n) because it forms when two haploid gametes fuse. The spore is haploid (n) because it forms by meiosis inside the diploid sporophyte. So zygote = 2n (starts the sporophyte), spore = n (starts the gametophyte).
Typical animals do not. In most animals the body is diploid and meiosis directly makes gametes (gametic meiosis), with no separate multicellular haploid body. Plants have a true multicellular gametophyte and sporophyte that alternate, which animals usually lack.
The zygote. When gametes made by the gametophyte fuse, the diploid zygote forms and grows into the sporophyte. Going the other way, the haploid spore made by the sporophyte grows into the next gametophyte.
