Fertilisation: How Sperm and Ovum Fuse in the Ampulla
Biology · Human Reproduction · NEET
Fertilisation is the fusion of a sperm with an ovum to form a single diploid cell called the zygote. In humans it happens in the ampulla (the widest part) of the fallopian tube, NOT in the uterus. Memory hook: "AMPulla is the AMPle wide space where sperm meets egg" — the egg waits here and sperm swim up to it.
Fertilisation happens in the ampulla of the fallopian tube, where the ovum and sperm meet at the ampullary-isthmic junction. The resulting diploid zygote then moves down to the uterus for implantation — fusion in the tube, growth in the uterus.
Your doubts, answered
Where exactly does fertilisation take place in humans?
Fertilisation takes place in the ampulla of the fallopian tube (oviduct). The ampulla is the widest, longest part of the tube. This is a fixed NCERT fact and NEET asks it directly. It does NOT happen in the ovary, uterus, cervix or vagina.
Uterus or fallopian tube — which is the site of fertilisation?
Fallopian tube. Many students wrongly say uterus because the baby grows there. But fusion of sperm and ovum happens earlier, in the ampulla of the fallopian tube. The zygote later travels down and only IMPLANTS in the uterus. So: fuse in tube, grow in uterus.
What is the ampullary-isthmic junction and why is it important?
The oviduct has an ampulla (wide part) and an isthmus (narrow part). Their meeting point is the ampullary-isthmic junction. For fertilisation to happen, BOTH the ovum and the sperms must reach this region at the same time. If they do not arrive together, no fertilisation occurs. This is why not every mating leads to pregnancy.
What is formed immediately when sperm and ovum fuse?
A zygote is formed. It is a single cell and it is DIPLOID (2n), because it gets one haploid set (n) of chromosomes from the sperm and one haploid set (n) from the ovum. The fusion of the two haploid nuclei is called syngamy.
Why does the secondary oocyte complete meiosis only at fertilisation?
The egg released at ovulation is a secondary oocyte, still arrested in the middle of meiosis-II. When the sperm enters and touches the ovum, it triggers the completion of the second meiotic division. This releases the second polar body and forms the mature haploid ovum, whose nucleus then fuses with the sperm nucleus.
Is fertilisation in humans internal or external?
Internal. The sperm is deposited in the female tract and swims up to meet the ovum inside the body, in the fallopian tube. External fertilisation (in water) is seen in frogs and fish, not humans.
⚠️ The NEET trap ✗ Fertilisation in humans occurs in the uterus, so the ovum and sperm must reach the uterus together. ✓ Fertilisation occurs at the ampullary-isthmic junction of the FALLOPIAN TUBE. The ovum and sperms must be transported there simultaneously. 🧠 NEET 2016 gave 'cervix' and 'uterus' as tempting options. Always pick the fallopian tube / ampullary-isthmic junction. Uterus is only for implantation, never for fusion.
Real NEET questions
2016
Fertilization in humans is practically feasible only if:
A · The sperms are transported into vagina just after the release of ovum in fallopian tube
B · The ovum and sperms are transported simultaneously to ampullary-isthmic junction of the fallopian tube ✓
C · The ovum and sperms are transported simultaneously to ampullary-isthmic junction of the cervix
D · The sperms are transported into cervix within 48 hrs of release of ovum in uterus
Solution: Both the ovum and the motile sperms must reach the ampullary-isthmic junction of the fallopian tube at the SAME time for fusion to occur. This is why not every copulation results in fertilisation. Options naming the cervix or uterus as the site are wrong, since fertilisation occurs in the fallopian tube, not the cervix or uterus.
2019
Extrusion of second polar body from egg nucleus occurs:
A · after entry of sperm but before fertilization ✓
B · after fertilization
C · before entry of sperm into ovum
D · simultaneously with first cleavage
Solution: Entry of the sperm through the zona pellucida and plasma membrane induces the secondary oocyte to COMPLETE meiosis-II, forming the second polar body and the haploid ovum. Only then do the sperm and ovum nuclei fuse (fertilisation). So the second polar body is released AFTER sperm entry but BEFORE fusion is complete.
2020
Meiotic division of the secondary oocyte is completed:
A · After zygote formation
B · At the time of fusion of a sperm with an ovum ✓
C · Prior to ovulation
D · At the time of copulation
Solution: The secondary oocyte stays arrested in meiosis-II until the sperm contacts the ovum. Sperm entry triggers completion of meiosis, releasing the second polar body and forming the haploid ovum, which then fuses with the sperm. So meiosis of the secondary oocyte is completed at the moment of fusion of sperm with ovum.
Solved Human Reproduction NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Fertilisation is the fusion of a male gamete (sperm) with a female gamete (ovum) to form a single diploid cell called the zygote. In humans it is internal and happens in the fallopian tube.
What is the exact site of fertilisation in humans?
The ampulla of the fallopian tube, specifically at the ampullary-isthmic junction. This is a common NEET one-word answer.
What is syngamy?
Syngamy is the fusion of the haploid nucleus of the sperm with the haploid nucleus of the ovum, forming the diploid zygote nucleus. It is the final step of fertilisation.
Is the zygote haploid or diploid?
Diploid (2n). It receives n chromosomes from the sperm and n from the ovum, restoring the full 2n number (46 chromosomes in humans).
Does fertilisation happen in the ovary?
No. The ovary only releases the ovum at ovulation. The ovum is then picked up by the fimbriae and fertilised in the ampulla of the fallopian tube.
What happens right after fertilisation?
The diploid zygote begins cleavage (mitotic divisions) as it moves toward the uterus, forming a morula and then a blastocyst, which later implants in the uterus.
