Q.Define external fertilization with an example.
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🔒 Start your 14-day free trial to unlock the full solution →Concept understanding — Meiosis And Fertilization
Let’s begin with something you already know: every living thing comes from other living things. A mango tree grows from a seed, and that seed came from a parent mango tree. A puppy is born from its mother. But how does a single cell — a fertilized egg — turn into a whole new person or plant, with the right number of chromosomes and a mix of traits from both parents?
That’s where meiosis and fertilization come in. They are the two halves of the same story: making a new individual while keeping the species stable.
The chromosome problem
Every cell in your body (except sperm and eggs) has 46 chromosomes — 23 from your mother and 23 from your father. That’s the diploid number (2n). If a sperm and an egg each had 46 chromosomes, their fusion would give the baby 92. That would double every generation — impossible. So nature has a neat solution: before sperm and egg meet, each must halve its chromosome count.
That halving is meiosis.
What meiosis does
Meiosis is a special kind of cell division that happens only in the reproductive organs (testes in males, ovaries in females). It takes one diploid cell (2n = 46) and produces four haploid cells (n = 23) — each with one complete set of chromosomes.
Think of it like splitting a deck of cards into two halves, then shuffling each half separately. You end up with four half-decks, each unique.
The process has two rounds of division (meiosis I and meiosis II), but the key point for you is this: meiosis reduces the chromosome number by half and shuffles the genetic material so that each sperm or egg is genetically different from every other.
Why does shuffling matter? Because it creates variation. No two siblings (except identical twins) are exactly alike, and that variety is what allows a species to adapt and survive.
What fertilization does
Fertilization is the opposite of meiosis. It’s the fusion of two haploid cells — a sperm (n = 23) and an egg (n = 23) — to form a single diploid cell called a zygote (2n = 46). That zygote is the first cell of the new individual.
Fertilization restores the full chromosome number. Without meiosis, fertilization would double the count. Without fertilization, meiosis would leave cells with only half the needed chromosomes. The two processes are a matched pair.
Why this matters for you
- Continuity of life: Every human being starts as a single fertilized egg. Meiosis and fertilization are the biological machinery that makes that possible.
- Genetic uniqueness: Because meiosis shuffles chromosomes and fertilization brings together two different sets, every person (except identical twins) has a unique combination of genes. That’s why you look like a blend of both parents but are not identical to either. …
Fertilisation is called external when the gametes are shed into the surrounding water and fuse outside the female's body, which is why such animals must release very large numbers of gametes. A familiar example is the frog. …
Fusion of egg and sperm outside the female's body (usually in water) is external fertilization; e.g. frog.
Definition. External fertilization is the type of fertilization in which the male and female gametes are released into the external medium (water) and their fusion (syngamy) takes place outside the body of the female.
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Showing the 12 most recent of 26 on this concept.
- CBSE 2026Set ANNUAL1 markQ.What is the number of chromosomes in spermatogonial cells and spermatids of a human?
›Reveal solutionSolution
Spermatogonia are diploid stem cells (46 chromosomes) inside the seminiferous tubules; after meiosis they give rise to haploid spermatids (23 chromosomes), which then differentiate into sperm.
In the human testis, spermatogonial cells are diploid (2n = 46 chromosomes) stem cells located along the inner wall of the seminiferous tubules. Some spermatogonia divide mitotically to maintain their own numbers, while others (type B spermatogonia) enter meiosis as primary spermatocytes.
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- CBSE 2026Set ANNUAL1 markMCQQ.How many Y-Chromosomes are present in human spermatozoa?(a) 2(b) 23(c) 1(d) 46
›Reveal solutionSolution
Human sperm are haploid (23 chromosomes total): 22 autosomes + 1 sex chromosome, which is either X or Y — never more than one Y.
Spermatogenesis reduces the diploid (2n = 46) germ cell to haploid (n = 23) sperm through meiosis. Each sperm therefore carries only one sex chromosome: about half the sperm population carries an X chromosome (produces a daughter on fertilizing the egg) and the other half carries a Y chromosome (produces a son). So a Y-bearing spermatozo …
- CBSE 2026Set ANNUAL1 markMCQQ.The process of fertilization completed in region of fallopian tube:(a) Uterine Cavity(b) Isthmus(c) Ampulla(d) Infundibulum
›Reveal solutionSolution
Fertilisation is completed in the ampulla of the fallopian tube.
After ovulation the ovum is released into the fallopian tube (oviduct). Sperms deposited in the female tract swim up and meet the ovum. The actual fusion of the sperm and the egg normally occurs at the ampullary-isthmic junction, that is, in the ampulla, the wider part of the oviduct. Fertilisation must occur here for the zygote to be tran …
- CBSE 2026Set ANNUAL1 markQ.State whether True or False: 50 primary spermatocytes formed 200 spermatozoas.
›Reveal solutionSolution
True - 50 primary spermatocytes give 50 x 4 = 200 spermatozoa.
During spermatogenesis, one primary spermatocyte (diploid) undergoes the first meiotic division to form 2 secondary spermatocytes, and each of these undergoes the second meiotic division to form 2 spermatids, giving 4 spermatids that mature into 4 spermatozoa. Therefore each primary sperma …
- CBSE 2025Set F1 markMCQQ.Where does capacitation of human sperms occur?(a) Testis(b) Vagina(c) Vas deferens(d) Female genital tract
›Reveal solutionSolution
Capacitation of human sperm happens in the female genital tract, not in the male ducts.
Sperm leaving the testis are motile but not yet able to fertilize. Capacitation is the set of biochemical changes (removal of glycoprotein coat and cholesterol from the sperm membrane) that occurs while the sperm travel through the female reproductive tract. Only capacitated sperm can undergo the acrosom …
- CBSE 2025Set F1 markMCQQ.Which of the following can lead to the development of twin brothers?(a) One zygote(b) Two zygotes(c) Without zygote(d) Gamete
›Reveal solutionSolution
Fraternal (dizygotic) twins arise from two zygotes; being genetically like ordinary siblings they can be two brothers.
Twins are of two types:
- Monozygotic (identical) twins arise from one zygote that splits; they are genetically identical and always the same sex. …
- CBSE 2025Set ANNUAL1 markMCQQ.Number of chromosomes in secondary spermatocyte are -(a) 23(b) 24(c) 46(d) 47
›Reveal solutionSolution
Meiosis I converts the diploid (2n = 46) primary spermatocyte into two haploid (n = 23) secondary spermatocytes.
Spermatogenesis begins with diploid spermatogonia (2n = 46), which enlarge into primary spermatocytes (also 2n = 46). Each primary spermatocyte undergoes meiosis I (a reductional division) to form two secondary spermatocytes, each with the haploid number, n = 23 chromosomes …
- CBSE 2025Set ANNUAL1 markMCQQ.Acrosomal reaction of the sperm occurs due to :(a) its contact with zona pellucida of the ova(b) reactions within the uterine environment of the female(c) reactions within the epididymal environment of the male(d) androgens produced in the uterus
›Reveal solutionSolution
The acrosomal reaction is triggered when the sperm head contacts the zona pellucida surrounding the ovum.
When a capacitated sperm reaches the secondary oocyte, its plasma membrane comes in contact with the zona pellucida — the thick glycoprotein coat around the ovum. This contact triggers the acrosomal reaction: the acrosome (a cap-like vesicle over the sperm head containing hydrolytic enzymes such as hyaluronidase and acrosin) ruptures and releases these enzymes. The enzymes digest a path through the zona pellucida, allowing the sperm to penetrate and fuse wi …
- CBSE 2025Set ANNUAL1 markQ.What would happen if sperms are devoid of their tail ?
›Reveal solutionSolution
The sperm tail (flagellum) provides motility; without it the sperm cannot travel to the egg, so fertilization cannot occur.
A mature human sperm has four structural regions: head, neck, middle piece and tail. The tail (flagellum) is essential for motility — it propels the sperm forward through the female reproductive tract (cervix, uterus, oviduct) to reach the secondary oocyte in the ampullary-isthmic junction of the fallopian tube, where fertilization normally occurs. The middle piece, packed with mitochondria, supplies the ATP that powers the whip-like beating of this tail.
If sperm were devoid of their tail:
- They would lose the ability to swim/move actively.
- They could not travel up the female reproductive tract to reach the ovum. …
- CBSE 2024Set ANNUAL1 markMCQQ.Secondary spermatocytes are(a) Haploid(b) Diploid(c) Triploid(d) Tetraploid
›Reveal solutionSolution
Spermatogonia are diploid, but after meiosis I converts a primary spermatocyte into two secondary spermatocytes, the chromosome number is reduced to haploid (n) - even though each chromosome still has two chromatids at that stage.
Spermatogenesis proceeds as: spermatogonia (diploid, 2n) -> primary spermatocytes (diploid, 2n, undergo meiosis I) -> secondary spermatocytes (haploid, n, formed after the reductional division of meiosis I) -> spermatids (haploid, n, formed after meiosis II, the equational …
- CBSE 2023Set A1 markQ.Fill in the blank: Fertilization is taken place in ______.
›Reveal solutionSolution
Sperm meets the ovum in the ampullary region of the fallopian tube, not in the uterus or ovary.
After ovulation, the secondary oocyte is picked up by the finger-like fimbriae of the fallopian tube and moved into the tube. Sperm deposited in the vagina swim up through the cervix and uterus into the fallopian tube. Fertilisation (fusion of sperm and secondary oocyte to form a zygote) normally takes place in the wider ampullary region of the fallopian tube, specifically at the junction …
- CBSE 2023Set ZOOLOGY1 markMCQQ.What do you call the embryo at 16-celled stage?(a) Blastocyst(b) Morula(c) Blastula(d) Gastrula
›Reveal solutionSolution
After fertilisation, the zygote undergoes rapid mitotic cleavage; by about the 16-cell stage it forms a solid ball of cells called the morula, which resembles a mulberry.
After fertilisation in the ampullary-isthmic junction of the fallopian tube, the zygote begins to move towards the uterus, dividing mitotically (cleavage) as it travels. Cleavage produces daughter cells called blastomeres:
- 2-celled stage -> 4-celled -> 8-celled -> 16-celled stage, at which point the solid ball of blastomeres is called the morula (Latin for 'mulberry', due to its resemblance to the fruit).
- The morula continues to divide and transforms into the blastocyst, a hollow, fluid-filled stage in which blastomeres arrange into an outer layer (trophoblast) and an inner cell mass. …
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