Sexual Reproduction at the Cellular Level
Science & Technology › Cell Biology and Genetics · Topic 1.7
Sexual reproduction at the cellular level is the formation of male and female gametes by meiosis and their fusion in fertilisation to form a diploid zygote. In animals, gametes form by gametogenesis: spermatogenesis in the testes produces sperm, and oogenesis in the ovaries produces eggs. Fertilisation, or syngamy, restores the diploid number. In flowering plants, pollen grains carry the male gametes and the ovule contains the egg, and a special process called double fertilisation produces both the embryo and the endosperm. Sexual reproduction mixes the genes of two parents through crossing over, independent assortment and random fertilisation, which gives variation that helps species to adapt.
5 min read · UPSC Prelims · Science & Technology · Cell Biology and Genetics
At a Glance
- Gametes formed by Meiosis (gametogenesis)
- Male gamete Sperm (spermatogenesis)
- Female gamete Egg (oogenesis)
- Fertilisation Fusion gives diploid zygote
- Plants Double fertilisation
- Exam link UPSC Prelims, Science & Technology: Cell Biology and Genetics
Where Does It Fit?
This is the seventh topic of the macro on cell biology and genetics. It applies meiosis from the previous topic to the formation of gametes, and prepares the ground for inheritance. Previous topic: Cell Division: Mitosis and Meiosis. Next topic: Inheritance Mechanisms (Mendelian Genetics).
How Are Gametes Formed in Animals?
| Process | Detail |
|---|---|
| Spermatogenesis | In the seminiferous tubules of the testes. A diploid germ cell divides by mitosis and then meiosis to give four sperm, and the process is continuous from puberty |
| Oogenesis | In the ovary. A primary oocyte starts meiosis before birth and pauses, and one egg and polar bodies form after fertilisation. Each primary oocyte gives only one functional egg |
| Sperm structure | Head with the nucleus and acrosome, a middle piece with mitochondria and a tail for movement |
| Egg structure | A large cell with stored food, surrounded by a protective layer |
| Fertilisation | A sperm enters the egg, the nuclei fuse and a zygote is formed. This usually occurs in the fallopian tube in humans |
| Cleavage | The zygote divides by mitosis to form a ball of cells, which becomes an embryo |
How Does Fertilisation Work in Plants?
- Pollen and ovule: the anther makes pollen grains, and the ovule in the ovary contains the embryo sac with the egg cell.
- Pollination: transfer of pollen to the stigma by wind, water, insects or other agents.
- Pollen tube: a pollen grain on the stigma grows a tube through the style to the ovule and carries two male gametes.
- Double fertilisation: one male gamete fuses with the egg to form the zygote, and the other fuses with two polar nuclei to form the triploid endosperm, which nourishes the embryo. This was discovered by Sergei Nawaschin in 1898 and occurs only in flowering plants.
- Result: the ovule becomes a seed, and the ovary becomes the fruit.
What Else Should You Remember?
- Sources of genetic variation: crossing over, independent assortment of chromosomes in meiosis and random fusion of gametes, together with mutations.
- Alternation of generations: plants alternate between a haploid gamete-producing phase and a diploid spore-producing phase, as in mosses and ferns.
- Parthenogenesis: development of an egg without fertilisation, seen in some insects and lizards, and the male honeybee develops this way.
- Sex determination in humans: the egg always carries an X chromosome, while sperm carry X or Y, so the father’s sperm decides the sex of the child, as in Inheritance Mechanisms (Mendelian Genetics).
- External and internal fertilisation: frogs and fishes fertilise outside the body, while reptiles, birds and mammals fertilise inside.
- Assisted reproduction: in vitro fertilisation, IVF, combines egg and sperm in a laboratory dish, and the first IVF baby, Louise Brown, was born in 1978.
- Chromosome numbers in reproduction: a human sperm and egg each carry 23 chromosomes, a zygote has 46, and and in a flowering plant the endosperm has three sets of chromosomes, which shows the difference between zygote and endosperm.
- Menstrual cycle link: in humans one egg is usually released about the middle of the cycle in ovulation, and if it is not fertilised the uterine lining is shed.
- Twins: identical twins come from one zygote that splits, while fraternal twins come from two eggs fertilised by two sperm.
- Seedless fruits: bananas and seedless grapes are produced without normal fertilisation, using methods such as triploid plants or hormone treatment.
- Self-incompatibility: many flowering plants prevent their own pollen from fertilising their ovules, which promotes cross-pollination and variation.
- India’s law: the Assisted Reproductive Technology (Regulation) Act, 2021 regulates such clinics.
Why Does It Matter for the Exam?
- Concept questions: what is double fertilisation, which cells are haploid and what is the ploidy of the endosperm.
- Evolution link: variation from sexual reproduction is the raw material of natural selection, as in Evolution of Life Forms: Theories of Evolution (Darwinism, Lamarckism).
- Health and law: questions on IVF, surrogacy rules and sex determination.
- Agriculture link: hybrid seeds and breeding programmes depend on controlled fertilisation.
- Common errors: mixing up the number of chromosomes in zygote, gamete and endosperm.
As an analytical point, sexual reproduction is slower and costlier than asexual reproduction, but variation helps populations to survive changes in the environment and diseases.
Why Is This Topic Important?
- Facts Gametogenesis and double fertilisation
- Concept Variation
- Link IVF and ART law
What Came Next?
The next topic covers inheritance mechanisms and Mendelian genetics. Next topic: Inheritance Mechanisms (Mendelian Genetics).
Key People and Terms
- Gamete Haploid reproductive cell
- Zygote Fertilised egg
- Syngamy Fusion of gametes
- Endosperm Food tissue in seed, triploid
- Polar body Small cell from oogenesis
- Parthenogenesis Development without fertilisation
Exam Corner
Points to Remember
- Gametes form by meiosis.
- Double fertilisation found by Nawaschin, 1898.
- Endosperm is triploid.
- Father’s sperm decides the sex.
- First IVF baby: 1978.
Do Not Confuse With
- Spermatogenesis and oogenesis Four sperm versus one egg
- Pollination and fertilisation Pollen transfer versus gamete fusion
- Zygote and endosperm Diploid embryo cell versus triploid food tissue
Memory Hook
Four sperm, one egg per cell; zygote diploid, endosperm triploid, double fertilisation Nawaschin did tell.
Mains Angle
Questions ask about variation and reproductive technology. These points add depth.
- Cells Gametogenesis
- Plants Double fertilisation
- Benefit Variation
A Question You May Face
An original practice question, not a past paper question.
“Explain gametogenesis and fertilisation and discuss how sexual reproduction contributes to genetic variation.”
How to Answer
- Gametes Meiosis.
- Fusion Zygote.
- Plants Double fertilisation.
- Variation Adaptation.
GS Relevance
Frequently Asked Questions
What is gametogenesis?
Gametogenesis is the formation of gametes by meiosis in the reproductive organs. In males it is called spermatogenesis and produces four sperm from each germ cell, and in females it is called oogenesis and produces one functional egg and polar bodies from each primary oocyte.
What is double fertilisation?
Double fertilisation is a process in flowering plants in which one male gamete fuses with the egg to form the diploid zygote, and the second male gamete fuses with two polar nuclei to form the triploid endosperm. It was discovered by Sergei Nawaschin in 1898 and gives the seed its embryo and food store.
How does sexual reproduction create variation?
Sexual reproduction creates variation through crossing over between homologous chromosomes, independent assortment of chromosomes during meiosis, and the random fusion of male and female gametes. Mutations add to this variation, which gives natural selection the material to act on and helps populations to adapt.
Who determines the sex of a child in humans?
The father determines the sex of a child, because the egg always carries an X chromosome and the sperm carries either an X or a Y chromosome. A sperm with X gives a girl, XX, and a sperm with Y gives a boy, XY, so it is a matter of chance which sperm fertilises the egg.
What is in vitro fertilisation?
In vitro fertilisation, or IVF, is a method of assisted reproduction in which eggs and sperm are combined in a laboratory dish and the embryo is placed in the uterus. The first IVF baby, Louise Brown, was born in 1978, and India regulates such clinics through the Assisted Reproductive Technology (Regulation) Act, 2021.
PYQ Practice — Statement Analysis
1 The endosperm in flowering plants is triploid.
It forms by fusion with two polar nuclei.
2 Double fertilisation occurs in gymnosperms.
It occurs in angiosperms.
3 Gametes are formed by meiosis.
They are haploid.
4 The mother's egg decides the sex of the child.
The father's sperm decides it.
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