Gametes are specialized reproductive cells involved in sexual reproduction. In humans, the male gamete is called the sperm or spermatozoon, while the female gamete is called the ovum or egg cell.
Gametes are haploid cells, meaning they contain only one set of chromosomes. In humans, each gamete normally contains 23 chromosomes. During fertilization, a sperm fuses with an ovum to form a diploid zygote containing 46 chromosomes.
These notes explain the structure, formation, functions, characteristics, differences, and important biological facts about sperm and ovum in an easy, NCERT-oriented manner.
What Are Gametes?
Gametes are reproductive cells that participate in sexual reproduction.
In humans:
- Male gamete → Sperm
- Female gamete → Ovum (egg)
- Both are haploid (n).
- Each normally contains 23 chromosomes.
- They are produced through a specialized cell division process called meiosis.
- Their fusion during fertilization produces a diploid (2n) zygote.
Gametes and Chromosome Number
Human body cells generally contain:
46 chromosomes = 23 pairs
Gametes contain:
23 chromosomes = 22 autosomes + 1 sex chromosome
The sperm may carry either:
- 22 autosomes + X chromosome, or
- 22 autosomes + Y chromosome
The ovum normally carries:
- 22 autosomes + X chromosome
Therefore, the sperm determines whether the resulting embryo receives an X or Y chromosome from the father, while the ovum contributes an X chromosome.
Male Gamete: Sperm
The sperm is the male reproductive cell. It is produced in the seminiferous tubules of the testes through a process called spermatogenesis.
Sperm cells are extremely small and are specialized for reaching and fertilizing the ovum.
A mature human sperm consists mainly of:
- Head
- Neck
- Middle piece
- Tail
Structure of Sperm
1. Head
The head contains:
- A highly condensed haploid nucleus
- An acrosome at its anterior end
The nucleus contains the genetic material contributed by the father.
The acrosome is a cap-like structure derived from the Golgi apparatus. It contains enzymes that help the sperm interact with and penetrate the coverings of the ovum during fertilization.
Important acrosomal enzymes include enzymes with proteolytic activity that help sperm pass through the egg’s surrounding layers.
2. Neck
The neck is a short region connecting the head to the middle piece.
It contains structures associated with the sperm’s connecting region and centriolar components.
3. Middle Piece
The middle piece contains numerous mitochondria.
Mitochondria produce ATP, which provides energy required for sperm movement.
The arrangement of mitochondria around the axoneme makes the middle piece an important energy-producing region.
4. Tail
The tail helps the sperm move through the female reproductive tract.
The sperm tail contains a specialized internal structure called the axoneme.
The axoneme is based on the characteristic 9 + 2 arrangement of microtubules found in motile cilia and flagella.
The movement of the tail produces the characteristic swimming motion of sperm.
Important Features of Sperm
A mature sperm is:
- Haploid
- Usually motile
- Very small
- Highly specialized
- Produced in very large numbers
- Capable of carrying either an X or Y chromosome
- Adapted for transporting paternal genetic material to the ovum
Sperm production begins at puberty and can continue throughout much of adult life, although sperm production and quality may change with age and other factors.
Spermatogenesis: Formation of Sperm
The process of formation of sperm is called spermatogenesis.
It occurs in the seminiferous tubules of the testes.
The basic sequence is:
Spermatogonium → Primary spermatocyte → Secondary spermatocytes → Spermatids → Spermatozoa
Step 1: Spermatogonia
Spermatogonia are diploid germ cells.
They contain:
46 chromosomes (2n)
Some spermatogonia continue dividing by mitosis, while others differentiate and enter meiosis.
Step 2: Primary Spermatocyte
A spermatogonium that enters meiosis develops into a primary spermatocyte.
The primary spermatocyte is diploid.
It undergoes meiosis I.
Step 3: Secondary Spermatocytes
Meiosis I produces two secondary spermatocytes.
Each is haploid.
Therefore:
1 primary spermatocyte → 2 secondary spermatocytes
Each secondary spermatocyte undergoes meiosis II.
Step 4: Spermatids
Meiosis II produces a total of four haploid spermatids.
Therefore:
1 primary spermatocyte → 4 spermatids
Step 5: Spermiogenesis
Spermatids are not yet mature sperm.
They undergo a process of differentiation called spermiogenesis, during which they develop the specialized structure of sperm.
During this process:
- The nucleus becomes condensed.
- The acrosome develops.
- The flagellum develops.
- Mitochondria become organized in the middle piece.
- Excess cytoplasm is removed.
The resulting cells are called spermatozoa or mature sperm.
Spermiation
The release of mature spermatozoa from the supporting cells into the lumen of the seminiferous tubules is called spermiation.
Sperm then undergo further maturation and acquire important functional properties as they pass through the epididymis.
Role of Hormones in Spermatogenesis
Spermatogenesis is regulated by hormones of the hypothalamic-pituitary-gonadal axis.
Important hormones include:
- GnRH
- FSH
- LH
- Testosterone
The hypothalamus releases GnRH, which stimulates the pituitary gland to release FSH and LH.
Role of LH
LH acts on Leydig cells of the testes and stimulates them to produce testosterone.
Role of FSH
FSH acts mainly on Sertoli cells, which support and nourish developing germ cells and participate in regulating spermatogenesis.
Role of Testosterone
Testosterone is essential for normal development and maintenance of male reproductive functions and supports spermatogenesis.
Female Gamete: Ovum
The ovum, commonly called the egg, is the female reproductive cell.
It is produced in the ovaries through the process of oogenesis.
Compared with sperm, the human ovum is:
- Much larger
- Generally non-motile
- Rich in cytoplasm
- Surrounded by specialized coverings
- Haploid
- Normally carries an X chromosome
The ovum contains cellular components and stored resources that support the early stages of embryonic development before implantation and establishment of the embryo’s own support systems.
Structure of the Ovum
A mature human egg is surrounded by several important structures.
From inside outward, important components include:
- Nucleus
- Ooplasm/cytoplasm
- Plasma membrane
- Perivitelline space
- Zona pellucida
- Corona radiata
1. Nucleus
The nucleus contains the maternal genetic material.
The ovulated human secondary oocyte is haploid, but at the time of ovulation it is arrested in metaphase II of meiosis.
It completes meiosis II only after successful fertilization.
2. Cytoplasm
The cytoplasm of the egg is called the ooplasm.
It contains:
- Mitochondria
- Ribosomes
- Organelles
- Nutrients and other cellular components
- Maternal molecules important for early development
3. Plasma Membrane
The plasma membrane surrounds the oocyte and separates its contents from the external environment.
It also participates in important events during sperm-egg interaction.
4. Perivitelline Space
The space between the plasma membrane and zona pellucida is called the perivitelline space.
A small structure called the first polar body can be found in this region after completion of meiosis I.
5. Zona Pellucida
The zona pellucida is a glycoprotein-rich layer surrounding the oocyte.
It plays an important role in:
- Sperm recognition
- Sperm binding
- Fertilization
- Prevention of polyspermy after fertilization
6. Corona Radiata
The corona radiata is a layer of cells surrounding the zona pellucida.
These cells are derived from the ovarian follicle and provide support to the developing and ovulated oocyte.
Oogenesis: Formation of the Female Gamete
The formation of the female gamete is called oogenesis.
It begins during fetal development and follows a different pattern from spermatogenesis.
The simplified sequence is:
Oogonium → Primary oocyte → Secondary oocyte → Ovum
However, the process involves important stages and meiotic arrests.
Step 1: Oogonia
During fetal development, germ cells develop into oogonia.
Oogonia are diploid cells.
They multiply by mitosis during fetal life.
Step 2: Primary Oocytes
Oogonia differentiate into primary oocytes.
Primary oocytes begin meiosis I but become arrested in prophase I.
A large number of primary oocytes are present in the ovaries at birth.
Step 3: Completion of Meiosis I
After puberty, during each reproductive cycle, usually one dominant follicle completes maturation and the primary oocyte completes meiosis I.
This produces:
- One large secondary oocyte
- One small first polar body
The division is unequal because most of the cytoplasm remains in the secondary oocyte.
Step 4: Meiosis II
The secondary oocyte begins meiosis II and becomes arrested at metaphase II.
This is the cell that is normally ovulated.
Step 5: Completion After Fertilization
If fertilization occurs, the secondary oocyte completes meiosis II.
It forms:
- The mature female gamete nucleus
- A second polar body
The male and female genetic material then participate in the formation of the zygote.
Sperm vs Ovum: Major Differences
| Feature | Sperm | Ovum / Secondary Oocyte |
|---|---|---|
| Sex | Male gamete | Female gamete |
| Formation | Spermatogenesis | Oogenesis |
| Main organ | Testes | Ovaries |
| Size | Very small | Much larger |
| Shape | Streamlined | Spherical |
| Motility | Motile | Non-motile |
| Cytoplasm | Very little | Abundant |
| Number produced | Very large numbers | Usually one dominant oocyte ovulated per cycle |
| Chromosome number | 23 | 23 |
| Sex chromosome | X or Y | X |
| Main function | Delivers paternal genetic material | Provides maternal genetic material and cellular contents |
| Mitochondria | Concentrated in middle piece | Present in cytoplasm |
| Tail | Present | Absent |
| Acrosome | Present | No sperm-like acrosome |
| Meiosis | Produces four functional spermatids per primary spermatocyte | Produces one large functional female gamete plus polar bodies |
Sperm and Ovum: Similarities
Despite their differences, sperm and ovum have several similarities.
Both:
- Are reproductive cells.
- Are haploid.
- Normally contain 23 chromosomes in humans.
- Are produced through processes involving meiosis.
- Carry genetic information.
- Participate in fertilization.
- Contribute genetic material to the offspring.
- Contain a nucleus with chromosomes.
- Are highly specialized for sexual reproduction.
Sperm and Ovum: Why Are They So Different?
Sperm and ovum have very different structures because they perform different roles.
The sperm is designed mainly for transport and delivery of paternal DNA.
Therefore, it has:
- A compact head
- Very little cytoplasm
- A flagellum for movement
- Many mitochondria in the middle piece
- An acrosome for interaction with the egg coverings
The ovum is designed to:
- Carry maternal DNA
- Provide a large cytoplasmic environment
- Support the earliest stages of development
- Interact with sperm during fertilization
Therefore, it has:
- Abundant cytoplasm
- Numerous organelles
- Specialized surrounding layers
- Maternal molecules important for early development
Fertilization: Fusion of Sperm and Ovum
Fertilization is the process in which male and female gamete nuclei ultimately combine to restore the diploid chromosome number.
In humans, fertilization normally occurs in the ampullary region of the fallopian tube.
The basic sequence is:
Sperm + Secondary oocyte → Fertilization → Zygote
Major Events During Fertilization
1. Sperm Reaches the Oocyte
After ejaculation, sperm travel through the female reproductive tract.
Only a small fraction of sperm reach the region where the oocyte is present.
2. Capacitation
Before a sperm can efficiently fertilize an oocyte, it undergoes functional changes in the female reproductive tract known as capacitation.
Capacitation prepares the sperm for the acrosome reaction and successful fertilization.
3. Interaction With Corona Radiata
Sperm encounter the cells of the corona radiata surrounding the oocyte.
4. Interaction With Zona Pellucida
Sperm bind to molecules in the zona pellucida.
This interaction contributes to sperm activation and the acrosome reaction.
5. Acrosome Reaction
The acrosome releases enzymes and undergoes membrane changes that help the sperm pass through the egg’s surrounding layers.
6. Fusion of Gamete Membranes
A sperm successfully reaching the oocyte can fuse with the oocyte plasma membrane.
The sperm nucleus and other components enter the oocyte.
7. Completion of Meiosis II
Sperm entry triggers the secondary oocyte to complete meiosis II.
The second polar body is formed.
8. Prevention of Polyspermy
Changes in the oocyte and its surrounding zona pellucida help prevent additional sperm from entering.
This is essential because normal human development requires genetic contribution from one sperm.
9. Formation of Pronuclei
The paternal and maternal genetic material forms separate pronuclei before their chromosomes participate in the first mitotic division.
10. Formation of Zygote
The fertilized egg becomes a zygote, which is diploid.
The zygote normally contains:
46 chromosomes
It then begins cleavage divisions and embryonic development.
What Happens to the Sperm After Fertilization?
The sperm’s primary contribution is its genetic material and the cellular structures required to initiate development.
The paternal chromosomes combine with maternal chromosomes during the formation of the zygote.
The sperm’s mitochondria generally do not contribute to the embryo’s mitochondrial inheritance in the usual pattern of human reproduction.
Human mitochondrial DNA is therefore inherited predominantly through the maternal line.
What Happens to the Ovum During Fertilization?
The secondary oocyte:
- Receives the sperm.
- Completes meiosis II.
- Forms the female pronucleus.
- Prevents additional sperm entry.
- Contributes maternal chromosomes and cytoplasmic components.
- Participates in formation of the zygote.
Haploid and Diploid Conditions
Understanding chromosome number is very important.
Haploid
A haploid cell has one set of chromosomes.
In humans:
n = 23
Examples:
- Sperm
- Secondary oocyte/female gamete lineage at the relevant stage
Diploid
A diploid cell has two sets of chromosomes.
In humans:
2n = 46
Examples:
- Somatic cells
- Zygote
- Most cells of the developing embryo
Fertilization Restores Diploid Number
The basic chromosome calculation is:
23 chromosomes from sperm + 23 chromosomes from the female gamete = 46 chromosomes in the zygote
This restoration of the diploid chromosome number is one of the major biological functions of meiosis followed by fertilization.
X and Y Chromosomes in Human Gametes
The ovum normally contains:
22 autosomes + X
Sperm can contain:
22 autosomes + X
or
22 autosomes + Y
Therefore:
X-bearing sperm + X-bearing ovum
XX
This generally corresponds to a female chromosomal sex.
Y-bearing sperm + X-bearing ovum
XY
This generally corresponds to a male chromosomal sex.
Thus, in terms of chromosomal contribution, the sperm determines whether the zygote receives an X or Y sex chromosome from the father.
Polar Bodies
A polar body is a small haploid cell produced during oogenesis.
Polar bodies receive chromosomes but very little cytoplasm.
The unequal division of cytoplasm during oogenesis allows the main female gamete to retain most of the cellular resources.
This differs from spermatogenesis, in which one primary spermatocyte ultimately gives rise to four functional sperm cells.
Gametogenesis: Spermatogenesis vs Oogenesis
| Feature | Spermatogenesis | Oogenesis |
|---|---|---|
| Occurs in | Testes | Ovaries |
| Begins | At puberty | Begins before birth |
| Pattern | Continuous after puberty, with age-related changes | Cyclic maturation after puberty |
| Primary cell | Primary spermatocyte | Primary oocyte |
| Meiotic products | Four spermatids | One large female gamete plus polar bodies |
| Cytoplasmic division | Approximately equal | Highly unequal |
| Mature gamete | Spermatozoon | Female gamete formed after completion of meiosis II |
| Motility | Sperm is motile | Female gamete is non-motile |
| Gamete reserve | New sperm production continues | Oocytes are formed before birth; no new large-scale production of oogonia after birth |
Important Terms Related to Gametes
Gamete
A reproductive cell that participates in sexual reproduction.
Spermatozoon
A mature male gamete.
Ovum
A mature female gamete; in human reproductive biology, the cell released at ovulation is actually a secondary oocyte arrested in metaphase II, which becomes a mature female gamete after completion of meiosis II.
Spermatogenesis
Formation and development of sperm in the testes.
Oogenesis
Formation and development of female gametes in the ovaries.
Spermiogenesis
Transformation of spermatids into mature spermatozoa.
Spermiation
Release of mature spermatozoa from Sertoli cells into the seminiferous tubule lumen.
Fertilization
The process leading to fusion of male and female genetic material and formation of a zygote.
Zygote
The diploid cell formed after fertilization.
Haploid
Having one set of chromosomes.
Diploid
Having two sets of chromosomes.
Acrosome
A cap-like structure at the anterior part of sperm containing molecules important for interaction with the oocyte coverings.
Zona Pellucida
A specialized glycoprotein layer surrounding the oocyte.
Corona Radiata
Cells surrounding the zona pellucida.
Polar Body
A small haploid cell formed during oogenesis as a result of unequal meiotic division.
Capacitation
Functional maturation of sperm in the female reproductive tract that enables them to undergo the changes required for fertilization.
Pronucleus
A haploid nucleus derived from the male or female gamete after fertilization, before the first embryonic division.
Important NCERT-Based Points
- Sperm is the male gamete.
- Ovum is the female gamete.
- Human gametes are haploid.
- Each human gamete normally contains 23 chromosomes.
- Sperm are produced in the seminiferous tubules of testes.
- Sperm formation is called spermatogenesis.
- Female gamete formation is called oogenesis.
- A mature sperm has a head, neck, middle piece and tail.
- The sperm head contains the haploid nucleus and acrosome.
- The sperm middle piece contains many mitochondria.
- The sperm tail helps in motility.
- The acrosome contains enzymes important for sperm-oocyte interaction.
- The ovulated human cell is normally a secondary oocyte arrested at metaphase II.
- The zona pellucida surrounds the oocyte.
- The corona radiata lies outside the zona pellucida.
- The secondary oocyte completes meiosis II following successful fertilization.
- Fertilization results in formation of a diploid zygote.
- A zygote normally has 46 chromosomes.
- Sperm can carry either X or Y.
- The ovum normally carries X.
- One primary spermatocyte produces four spermatids.
- Oogenesis involves unequal cytokinesis.
- Polar bodies are formed during oogenesis.
- Sperm undergo capacitation in the female reproductive tract.
- The zona pellucida plays an important role in preventing polyspermy after fertilization.
Frequently Asked Questions
What are gametes?
Gametes are specialized reproductive cells that participate in sexual reproduction. In humans, sperm and female gametes are haploid and normally contain 23 chromosomes.
What is the male gamete called?
The male gamete is called the sperm or spermatozoon.
What is the female gamete called?
The female gamete is commonly called the ovum or egg. Strictly speaking, the cell normally ovulated in humans is a secondary oocyte, which completes meiosis II following fertilization.
How many chromosomes are present in a human sperm?
A normal human sperm contains 23 chromosomes.
How many chromosomes are present in a human ovum?
The female gamete is haploid and contains 23 chromosomes.
Which chromosome can a sperm carry?
A sperm can carry either an X chromosome or a Y chromosome.
Which sex chromosome does an ovum carry?
A normal human ovum contributes an X chromosome.
Where are sperm produced?
Sperm are produced in the seminiferous tubules of the testes.
What is spermatogenesis?
Spermatogenesis is the process of formation and development of sperm from male germ cells.
What is oogenesis?
Oogenesis is the process of formation and development of the female gamete from female germ cells.
What is the function of the sperm tail?
The sperm tail provides the motility required for movement of sperm through the female reproductive tract.
What is the function of mitochondria in sperm?
Mitochondria in the sperm middle piece generate ATP needed to support sperm movement.
What is the acrosome?
The acrosome is a cap-like structure on the sperm head containing molecules important for interaction with and penetration of the oocyte’s surrounding layers.
What is the zona pellucida?
The zona pellucida is a glycoprotein-rich layer surrounding the oocyte. It participates in sperm recognition and helps prevent polyspermy after fertilization.
What is the corona radiata?
The corona radiata is a layer of cells surrounding the zona pellucida of the oocyte.
Why is the ovum larger than sperm?
The female gamete contains abundant cytoplasm, organelles, and maternal molecules required to support early development, whereas sperm is highly specialized for delivering paternal genetic material.
How many sperm are produced from one primary spermatocyte?
One primary spermatocyte normally produces four spermatids, which differentiate into four spermatozoa.
How many functional female gametes are produced from one primary oocyte?
Oogenesis produces one large female gamete and small polar bodies because the cytoplasm is distributed unequally during meiosis.
When does the secondary oocyte complete meiosis II?
The secondary oocyte normally completes meiosis II after fertilization.
Where does fertilization normally occur in humans?
Fertilization normally occurs in the ampullary region of the fallopian tube.
Quick Revision Chart
Male Gamete
Sperm → Haploid → 23 chromosomes → Testes → Motile → Head + Neck + Middle piece + Tail
Female Gamete
Ovum/Female gamete → Haploid → 23 chromosomes → Ovary → Non-motile → Large cytoplasm + Zona pellucida + Corona radiata
Formation
Spermatogenesis → Sperm
Oogenesis → Female gamete
Fertilization
Sperm + Female gamete → Zygote → Diploid → 46 chromosomes
One-Line Revision Points
- Gametes = reproductive cells.
- Sperm = male gamete.
- Ovum = female gamete.
- Gametes = haploid.
- Human gamete chromosome number = 23.
- Human somatic chromosome number = 46.
- Sperm formation = spermatogenesis.
- Female gamete formation = oogenesis.
- Spermatid maturation = spermiogenesis.
- Sperm release = spermiation.
- Sperm head = nucleus + acrosome.
- Sperm middle piece = mitochondria.
- Sperm tail = movement.
- Oocyte covering = zona pellucida.
- Outer cell layer = corona radiata.
- Ovulated cell = secondary oocyte.
- Secondary oocyte = arrested at metaphase II.
- Fertilization = completion of meiosis II + formation of zygote.
- Zygote = diploid.
- Zygote chromosome number = 46.
- Sperm = X or Y.
- Ovum = X.
- 1 primary spermatocyte → 4 spermatids.
- Oogenesis → one large female gamete + polar bodies.
- Fertilization restores diploid chromosome number.
