Fertilisation and Implantation
Fertilisation is the fusion of a haploid sperm with a haploid ovum to form a diploid zygote, while implantation is the attachment of the blastocyst to the uterine endometrium.
In one line: Fertilisation makes zygote; cleavage makes blastocyst; uterus receives blastocyst.
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- In humans, fertilisation usually occurs in the ampullary-isthmic junction of the oviduct.
- Sperm must reach the secondary oocyte, and one sperm fuses with it, triggering completion of meiosis II and formation of the ovum.
- Fusion of male and female pronuclei forms the diploid zygote.
- The zygote undergoes cleavage divisions while moving towards the uterus, forming a morula and then a blastocyst.
- The blastocyst embeds in the endometrium, beginning implantation.
- Exams commonly test the site of fertilisation, difference between zygote and blastocyst, and the order from fertilisation to implantation.
Example
Rule to remember
Sequence rule: sperm entry → completion of meiosis II → zygote formation → cleavage → morula → blastocyst → implantation.
Sequence to remember
See it worked out
Follow the example line by line, then use the same method on new questions.
Worked example
- 1For a sequence question, arrange these correctly: blastocyst, zygote, implantation, cleavage.
- 2Correct order: zygote → cleavage → blastocyst → implantation.
Method you can reuse
Visual tip
Important labels are sperm, ovum, ampullary-isthmic junction, zygote, cleavage stages, morula, blastocyst, trophoblast, inner cell mass and endometrium. Notice location changes from oviduct to uterus.
Check yourself
Answer the question in your head before opening the answer.
Try it yourself
Where does fertilisation usually occur in humans, and where does implantation occur?
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Common wrong answer
A common wrong answer is saying the zygote implants immediately. The zygote first undergoes cleavage and reaches the blastocyst stage before implantation.
Don't mix this up
Exam tip
Write it in the exam
Each point is worth about one mark. Write point 1 for a 1-mark question and keep adding points as the marks go up.
How CBSE asks it
Answer plan
- 1
Enough for 1 mark
Fertilisation is the fusion of a haploid sperm with a haploid ovum to form a diploid zygote, while implantation is the attachment of the blastocyst to the uterine endometrium.
- 2
Add for 2 marks
In humans, fertilisation usually occurs in the ampullary-isthmic junction of the oviduct.
- 3
Add for 3 marks
Sperm must reach the secondary oocyte, and one sperm fuses with it, triggering completion of meiosis II and formation of the ovum.
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