2.1: Gametes and fertilization - Biology

2.1: Gametes and fertilization - Biology

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The gametes, or oocytes and sperm, are the cells types responsible for reproduction. They are essentially an unbroken chain of life.

Oocytes and sperm are similar in that they both undergo meiosis to produce haploid cells, but the timing and specific details of oogenesis and spermatagenesis differ between males and females. Additionally, each species often has unique features of gametogenesis.

A summary can be found in the section,The structure of the gametes, from Gilbert's Developmental biology here:


Fertilization might seem like it "just happens" but it is actually a complex process process, some aspects of which are still not entirely charactertized.

Review the chapter,The structure of the gametes, from Gilbert's Developmental biology here:

2.1: Gametes and fertilization - Biology

In simple terms, reproduction is the process by which organisms create descendants. This miracle is a characteristic that all living things have in common and sets them apart from nonliving things. But even though the reproductive system is essential to keeping a species alive, it is not essential to keeping an individual alive.

In human reproduction, two kinds of sex cells or gametes are involved. Sperm, the male gamete, and a secondary oocyte (along with first polar body and corona radiata), the female gamete must meet in the female reproductive system to create a new individual. For reproduction to occur, both the female and male reproductive systems are essential. It is a common misnomer to refer to a woman’s gametic cell as an egg or ovum, but this is impossible. A secondary oocyte must be fertilized by the male gamete before it becomes an “ovum” or “egg.”

While both the female and male reproductive systems are involved with producing, nourishing and transporting either the oocyte or sperm, they are different in shape and structure. The male has reproductive organs, or genitals, that are both inside and outside the pelvis, while the female has reproductive organs entirely within the pelvis.

Fertilization Process

There are three stages to fertilization which ensure that the appropriate egg and sperm are able to find each other and to warrant that only one sperm enters the egg: chemotaxis, sperm activation/acrosomal reaction and sperm/egg adhesion.

Ovulation must occur before fertilization can happen in humans, ovulation occurs once a month during the menstrual cycle. This cycle releases an egg cell from the ovaries and the first stage of fertilization begins. In other animals, ovulation can occur in cycles of different length, or is triggered by the occurrence of sexual intercourse.

In mammals, after ejaculation, the sperm locates the oocyte (the immature egg), through changes in temperature and chemical gradients. Sperm chemotaxis, a type of interaction in which sperm cells are guided to the oocyte to the hormone progesterone, which is secreted by the oocyte, and sperm thermotaxis, which involves the response to changes in temperature, ensure that the sperm are able to locate the oocyte (usually within the ampulla of the fallopian tube. While the sperm is in the reproductive tract, it undergoes capacitation, which increases its movement ability and destabilizes its membrane, preparing it for the acrosome reaction.

Once the sperm locates the oocyte, it binds to the zona pellucida, which is a thick layer of jelly-like, extra-cellular matrix consisting of glycoproteins, surrounding the egg. A specialized molecule on the surface of the sperm binds to a ZP3 glycoprotein in the zona pellucida, triggering the acrosome reaction. The acrosome reaction releases hyaluronidase, which digests the hyaluronic acid around the oocyte, allowing the sperm to pass through.

Upon successful implantation of a sperm, the cortical granules within the oocyte fuse with the plasma membrane of the cell, and are expelled into the zona pellucida, causing the surface to become hard and impenetrable. This process is called the cortical reaction and is responsible for ensuring that only one sperm cell can enter and fertilize the egg.

Once the sperm has successfully penetrated the oocyte, the outer coating and the tail of the sperm disintegrates. The oocyte undergoes meiosis to produce the haploid ovum. The two haploid cells, each containing 23 chromosomes, undergo fusion of their genetic material, ultimately creating a diploid cell containing 46 chromosomes, called a zygote. The zygote then begins mitosis, the repeated cellular division necessary for the growth of an organism, forming a blastocyst, which is implanted into the wall of the uterus, beginning the pregnancy.

Types of Sexual Reproduction

The type of sexual reproduction of an organism is largely dependent on the size and shape of its gametes. Some male and female gametes are of similar size and shape, while others are vastly different. In some species of algae and fungi, for example, male and female sex cells are almost identical and both are usually motile. The union of similar gametes is known as isogamy.

The process of gametes of dissimilar size and shape joining is called anisogamy or heterogamy. Higher plants, animals, and some species of algae and fungi exhibit a special type of anisogamy called oogamy. In oogamy, the female gamete is non-motile and much larger than the fast-moving male gamete. This is the type of reproduction that occurs in humans.

14.7 Human fertilization: from gametes to a zygote

As we discussed in chapter 13, fertilization in humans happens in the oviducts. For this to happen, the sperm need to arrive in the oviducts when there is an egg there. Sperm can stay alive in the female reproductive tract for 3-5 days. An egg needs to be fertilized within about 12 hours of ovulation and while some fast-swimming sperm can reach the egg within an hour, many will take a day or more to swim that far. Based on these sperm swimming and egg survival times, the most likely timing for vaginal sex to occur to achieve fertilization is from 1-3 days prior to ovulation.

Note: while there are birth control methods that take advantage of this timing, there are a LOT of babies conceived by people thinking that it is a “safe” time to have sex. Keep in mind that the drive to have sex is increased for both males and females during times of high fertility. So if you are trying to convince yourself you are “safe” from pregnancy, remember there are evolutionary drivers for reproduction that may be greater than your ability to calculate pregnancy risk.

In a typical ejaculate there are about 100 million sperm. When these sperm are ejaculated in semen into a vagina, they begin swimming toward the cervix, through the cervix, through the uterus and into the oviducts. This is a perilous journey for the sperm. Many never make it through the cervix, some are attacked by immune cells in the uterus, and roughly half of those that remain enter the empty oviduct (remember, in general only one follicle in one ovary matures per menstrual cycle). Out of the 100 million-plus contenders only several dozen sperm actually reach the egg. When sperm and egg meet in the oviduct, the head of the sperm (or acrosome) secretes an enzyme that helps the sperm swim through the jelly-like coating of the egg. Once through this layer, the sperm fuses with the cell membrane of the egg the membrane then undergoes chemical changes, blocking other sperm. Only the genetic material from the sperm enters the egg (mitochondria and all other parts of the sperm remain outside the egg). At this point the egg completes meiosis II. The genetic material from the sperm fuses with the genetic material from the egg and a fertilized egg, or zygote, is formed.

Figure 14.6 A zygote

Male Reproductive System

The main structures of the male reproductive system are external to the body and illustrated in Figure 18.2.3. The two testes (singular, testis) hang between the thighs in a sac of skin called the scrotum . The testes produce both sperm and testosterone . Resting atop each testis is a coiled structure called the epididymis (plural, epididymes). The function of the epididymes is to mature and store sperm. The penis is a tubular organ that contains the urethra and has the ability to stiffen during sexual arousal. Sperm passes out of the body through the urethra during a sexual climax (orgasm). This release of sperm is called ejaculation.

In addition to these organs, the male reproductive system consists of several ducts and glands that are internal to the body. The ducts, which include the vas deferens (also called the ductus deferens), transport sperm from the epididymis to the urethra . The glands, which include the prostate gland and seminal vesicles , produce fluids that become part of semen. Semen is the fluid that carries sperm through the urethra and out of the body. It contains substances that control pH and provide sperm with nutrients for energy.

Figure 18.2.3 Most of the major male reproductive organs are located outside of the body.

Related Biology Terms

  • Axoneme – Central strand of cytoplasmic filaments seen in organelles such as cilia or flagella, usually formed by microtubules.
  • Follicle Cells – Also known as granulosa cells, these cells surround the growing oocyte within the ovary and are though to help the oocyte respond to the hormonal cues of the body.
  • Spermatids – Haploid cells that are formed from spermatocytes through meiosis. Spermatids undergo further differentiation before becoming mature sperm.
  • Zygote – A eukaryotic diploid cell formed by the fusion of two haploid gametes.

1. Which of these is a protective membrane around an ovum?
A. Acrosome
B. Vitelline membrane
C. Corona radiata
D. All of the above

2. Why do sperm need mitochondria but not ribosomes?
A. They can take up proteins from the external environment but not ATP
B. Mitochondria are larger organelles that can provide mechanical support for the sperm.
C. Sperm do not synthesize proteins, but they generate ATP to power the movement of flagella
D. All of the above

3. Why are people having aneuploid disorders frequently sterile?
A. They cannot produce sex hormones
B. It is difficult, and highly unlikely that an aneuploid cell can undergo meiosis successfully and produce viable gametes that can undergo fertilization
C. The presence of a vitelline membrane prevents aneuploid sperm from accessing the cytoplasm of the egg
D. All of the above


Mammalian fertilization begins when the head of a sperm binds in a species-specific manner to the zona pellucida surrounding the egg. This induces the acrosome reaction in the sperm, which releases the contents of its acrosomal vesicle, exposing enzymes that help the sperm to digest its way through the zona to the egg plasma membrane in order to fuse with it. The fusion of the sperm with the egg induces a Ca 2+ signal in the egg. The Ca 2+ signal activates the egg to undergo the cortical reaction, in which cortical granules release their contents, including enzymes that alter the zona pellucida and thereby prevent the fusion of additional sperm. The Ca 2+ signal also triggers the development of the zygote, which begins after sperm and egg haploid pronuclei have come together, and their chromosomes have aligned on a single mitotic spindle, which mediates the first division of the zygote.

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Free Response

List and briefly describe the three processes that lead to variation in offspring with the same parents.

a. Crossover occurs in prophase I between non-sister homologous chromosomes. Segments of DNA are exchanged between maternally derived and paternally derived chromosomes, and new gene combinations are formed. b. Random alignment during metaphase I leads to gametes that have a mixture of maternal and paternal chromosomes. c. Fertilization is random, in that any two gametes can fuse.

Compare the three main types of life cycles in multicellular organisms and give an example of an organism that employs each.

a. In the haploid-dominant life cycle, the multicellular stage is haploid. The diploid stage is a spore that undergoes meiosis to produce cells that will divide mitotically to produce new multicellular organisms. Fungi have a haploid-dominant life cycle. b. In the diploid-dominant life cycle, the most visible or largest multicellular stage is diploid. The haploid stage is usually reduced to a single cell type, such as a gamete or spore. Animals, such as humans, have a diploid-dominant life cycle. c. In the alternation of generations life cycle, there are both haploid and diploid multicellular stages, although the haploid stage may be completely retained by the diploid stage. Plants have a life cycle with alternation of generations.

2.1: Gametes and fertilization - Biology

The process of fusion of male and female gametes to form the zygote is called fertilization. In angiosperms, the male gametes are carried to the egg cell by pollen tube, this phenomenon is called siphonogamy.

The pollen grains reach the surface of stigma by pollination. Pollen grains absorbs moisture from the surface of stigma. As a result, pollen grains swells and the intine comes out in the form of a tube called pollen tube. Pollen tube is driven forward by dissolving the tissue of style with the help of enzymes present at its tip. The direction of the pollen tube is controlled by the chemicals secreted by synergids. Such movement by chemicals is known as achemotactic movement.

The pollen tube enters the ovary by three routes -

The entry of pollen tube through the micropyle is the most common and is called as aporogamy.

The entry of pollen tube through the chalaza is called as chalazogamy.

The entry of pollen tube through the integuments is called as mesogamy.

Chalazogamy and mesogamy are collectively called aporogamy.

The pollen tube enters through synergids and breaks up to release its two male gametes.Out of these two male gametes, one fuses with the egg cell to form a zygote. This process is called syngamy. Another male gamete travels little farther and fuses with the secondary nucleus to form endosperm mother cell. This process is called triple fusion. Since two types of fusion i:e syngamy and triple fusion, takes place in the embryo sac the phenomenon is called double fertilization.

Syngamy and triple fusion together are known as double fertilization which is the characteristics of angiosperms.

Syngamy results in the formation of the zygote which later divides to give rise to an embryo. Triple fusion results in the formation of endosperm mother cell which later divides to form endosperm tissue. Endosperm tissue is the nutritive tissue for the growing embryo.

Double fertilization is the fusion of two male gametes brought by a pollen tube to two different cells of the same female gametophyte in order to produce two different structures.

source: Fig: Fertilization


The process of formation of an embryo from a zygote is known as embryogenesis. It can be studied under two headings:

Dicot Embryogenesis

The zygote elongates and then divides by a transverse wall into two unequal cells. The larger nasal cell is called suspensor cell and the other towards the chalaza is called terminal or embryo cell. The suspensor cell divides transversely a few times to produce a filamentous suspensor of 6-10 cells. The suspensor helps in pushing the embryo in the endosperm. The first cell of suspensor towards the micropylar end becomes swollen and functions as haustorium. The haustorium helps to draw nutrition. The last cell of the suspensor towards chalaza is called hypophysis which later gives rise to the radicle and root cap.

The embryo cell undergoes two vertical divisions and one transverse division to form eight-celled octant structure. Four cells towards the chalaza are known as epibasal cells and four towards micropyle are called hypobasal cells. The epibasal cells later give rise to two cotyledons and plumule. The hypobasal cells produce the hypocotyl.

The octant divides periclinally to produce an outer layer of protoderm. The inner cells differentiate into procambium and ground meristem. Protoderm forms epidermis, procambium gives rise to vascular strand and ground meristem produces cortex and pith,

Initially, the embryo is globular and undifferentiated. Early embryo with radial symmetry is called proembryo. It later becomes heart-shaped. The growth rate of cotyledons is very high and plumule remains at the place from where cotyledons are separated.

Dicot embryo

  • A typical dicotyledonous embryo consists of an embryonal axis and cotyledons.
  • Embryonal axis above the cotyledon is the epicotyls.
  • The terminal part of the epicotyls is the plumule (gives rise to the shoot).
  • The embryonal axis below the cotyledon is the hypocotyl.
  • The terminal part of the hypocotyl is called the radicle (root tip).
  • The root tip is covered by the root cap called calyptras.

Monocot Embryogenesis

The zygote elongates and then divides transversely to form basal and terminal cells. The basal cell towards the micropylar end becomes large and called as suspensor cell. It may function as haustorium. The terminal cell divides by transverse wall to form two cells. The top cell after a series of divisions forms plumule and a single cotyledon. Cotyledon called scutellum, grows rapidly and pushes the terminal plumule to one side. The plumule comes to lie in a depression. The middle cell, after many divisions forms hypocotyl and radical. It also adds a few cells to the suspensor. In some cereals, both plumule and radicle get covered by sheaths developed from scutellum called coleoptiles and coleorhiza respectively.

Monocot embryo

  • A monocot embryo possesses only one cotyledon.
  • In grass family, the cotyledon is called scutellum.
  • The scutellum is situated towards one side of the embryonal axis.
  • Radicle and the root cap is enclosed by a sheath called coleorhiza.
  • The portion of the embryonal axis above the level of attachment of scutellum is called epicotyls.
  • The epicotyl has the shoot apex and plumule enclosed by a hollow foliar structure called coleoptile.
  • The seed is the final product of the sexual reproduction.
  • The seed consists of a seed coat, cotyledon, and an embryo axis.
  • Cotyledon stores the reserve food material for the development and germination.
  • Matured seed without endosperm is called non-aluminous.
  • A part of the endosperm retained in matured seed is calleda luminous.
  • Remainants of nucellus in the matured seed is called perisperm.
  • The wall of the ovary develops into the wall of fruit called pericarp.
  • The fruit developed from the ovary is called true fruit.
  • In apple, strawberry, cashew, the thalamus contributes to the fruit formation, such type of fruit is called false fruit.
  • The fruit developed without fertilization is called parthenocarpic fruits.

Apomixis and polyembryony

  • Apomixis is very common in Asteraceae and grasses.
  • Seeds are produced without fertilization.
  • Apomixis is a type of asexual reproduction which mimics the sexual reproduction.
  • The diploid egg cell is formed without meiosis and develops into seed without fertilization.
  • In Citrus and Mango, the nucellar cells start dividing, protrude into the embryo sac and develop into an embryo.
  • Ovule having more than one embryo is termed as polyembryony.
  • Hybrid plants are developed by apomixis to maintain the genetic identity.

Keshari, Arvind K. and Kamal K. Adhikari. A Text Book of Higher Secondary Biology(Class XII). 1st. Kathmandu: Vidyarthi Pustak Bhandar, 2015.

Mehta, Krishna Ram.Principleof biology.2nd edition.Kathmandu: Asmita, 2068,2069.

Jorden, S.L.principle of biology.2nd edition . Kathmandu: Asmita book Publication, 2068.2069.

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Things to remember
    The process of fusion of male and female gametes to form the zygote is called fertilization. In angiosperms, the male gametes are carried to the egg cell by pollen tube, this phenomenon is called siphonogamy.

The entry of pollen tube through the micropyle is the most common and is called as aporogamy.

The entry of pollen tube through the chalaza is called as chalazogamy.

The entry of pollen tube through the integuments is called as mesogamy.

The process of formation of an embryo from a zygote is known as embryogenesis. It can be studied under two headings: Dicot embryogenesis and Monocot embryogenesis.

  • It includes every relationship which established among the people.
  • There can be more than one community in a society. Community smaller than society.
  • It is a network of social relationships which cannot see or touched.
  • common interests and common objectives are not necessary for society.

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Watch the video: Meiosis, Gametes, and the Human Life Cycle (August 2022).