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Biology · 2024 · 5 marks
CBSE 2024 · Region 2 · Set 1 · Q31
(a) (i) Explain the process of double fertilization in an angiosperm starting from the germination of pollen grains on the stigma, mentioning the…
(i)
Explain the process of double fertilization in an angiosperm starting from the germination of pollen grains on the stigma, mentioning the ploidy of the end products formed at the end. State the role of synergids during the course of the process.
(ii)
Why does the development of endosperm precede that of the embryo ?
(i)
Mention the site where fertilisation of the ovum occurs in a human female. Explain the process of fertilization and mention how polyspermy is prevented.
(ii)
Name the embryonic stage that gets implanted in the uterus. Explain the process of implantation in a human female.
Marking-scheme solution
(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.
(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.
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CBSE Class 12 Biology past-paper question from the 2024 board exam, with the answer as CBSE’s own marking scheme gives it. Where our answers come from.