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Biology · 2023 · 5 marks
Sexual Reproduction in Flowering PlantsPre-fertilisation: Structures and Events5 marksUnderstandlong answer
CBSE 2023 · Region 5 · Set 1 · Q31
(a) (i) Explain the monosporic development of embryo sac in the ovule of an angiosperm. (ii) Draw a diagram of the mature embryo sac of an…
(a)(i)
Explain the monosporic development of embryo sac in the ovule of an angiosperm.
(ii)
Draw a diagram of the mature embryo sac of an angiospermic ovule and label any four parts in it.
OR
(b)(i)
Explain the formation of placenta after the implantation in a human female.
(ii)
Draw a diagram showing human foetus within the uterus and label any four parts in it.
Marking-scheme solution
(i)
Megaspore mother cell (2n) —— ( Meiosis) ——→ Tetrad (n)
↓
Degeneration of $\displaystyle 3$ cells (one functional megaspore left)
↓
$\displaystyle 3$ Mitosis without cell wall formation / Free Nuclear division
↓
$\displaystyle 8$ nuclear stage —— (Cell wall formation) ——→ Embryo sac ($\displaystyle 7$ celled $\displaystyle 8$ nucleated stage) (n)
(ii)
(i)
After implantation finger like projections appear on the trophoblast called chorionic villi which are surrounded by the uterine tissue and maternal blood, these tissues become interdigitated with each other, and jointly form a structural and functional unit between developing embryo and maternal body called placenta.
(ii)
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CBSE Class 12 Biology past-paper question from the 2023 board exam, with the answer as CBSE’s own marking scheme gives it. Where our answers come from.