Question

(i) Explain the process of double fertilization in an angiosperm starting from the germination of pollen grains on the stigma, mentioning the ploidy…

Accepted Answer

(a) (i) • Pollen tube passes down the style, generative cell divides mitotically into two male gametes and male gametes are discharged into the embryo sac through the micropyle, -One of the male gamete + Egg/ovum → Zygote (2n)/zygote is diploid, Process is known as syngamy. -Other male nucleus (n) + secondary nucleus (2n) → PEN/Primary Endosperm Nucleus (3n)/PEN is triploid, this is known as triple fusion. Since two types of fusion (syngamy and triple fusion) in an embryo sac called double fertilization. • Filiform apparatus of synergids guide entry of the pollen tube (at the micropylar end). (ii) To provide assured nutrition to the developing embryo. OR (b) (i) • Ampullary-isthmic junction/ampulla of fallopian tube, • A sperm comes in contact with zona pellucida (layer of ovum), the secretion of the acrosome of the sperm helps the sperm to enter into the cytoplasm of the ovum, this induces completion of meiosis II to form haploid ovum (ootid), haploid nucleus of the sperm and of the ovum fuse together to form the diploid zygote. -On contact of sperm with zona pellucida induces changes in the membrane of the ovum that blocks the entry of the additional sperms (ii) • Blastocyst • Trophoblast layer of the blastocyst gets attached to the endometrium, inner cell mass gets differentiated into an embryo, the uterine cells divide rapidly and blastocyst gets embedded in the endometrium of the uterus.

Marking-scheme solution

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