Reproduction is the ability of organisms to produce offspring like themselves; it ensures that a species persists over time. Exam questions often compare asexual and sexual reproduction and test gametogenesis, double fertilisation, the stages of embryonic development and what each germ layer gives rise to. This lesson covers all of these with calculation examples.
§1. Asexual reproduction
In asexual reproduction a single parent is involved, no gametes are formed, and the new organism develops by mitosis. The offspring are genetically identical to the parent (a clone).
Fission — in unicellular organisms: amoeba, paramecium, bacteria (binary fission).
Spores — fungi, mosses, ferns, many algae.
Budding — hydra, yeast.
Fragmentation — regrowth from body pieces (flatworms, filamentous algae).
Asexual reproduction is fast and efficient but gives no genetic diversity, so offspring may fail to cope when conditions change.
§2. Sexual reproduction and gametogenesis
In sexual reproduction two gametes fuse to form a zygote. The offspring receives its parents' genes in a new combination — the source of combinative variation, which matters for evolution.
Gametogenesis, the formation of sex cells, has four phases:
Multiplication — primordial germ cells divide many times by mitosis (2n).
Growth — the cell enlarges and its DNA doubles, forming a primary spermatocyte or primary oocyte.
Maturation — meiosis: two successive divisions halve the chromosome number (2n → n).
Formation — only in spermatogenesis: the sperm develops a tail and an acrosome.
The egg is large, immobile and rich in stored food (yolk); the sperm is small and motile.
Fertilisation is the fusion of gamete nuclei, n + n = 2n. External fertilisation takes place in water (most fish, amphibians); internal fertilisation occurs in reptiles, birds and mammals.
Double fertilisation in flowering plants (S. G. Navashin, 1898):
first sperm + egg → zygote (2n) → embryo;
second sperm + central cell (2n) → endosperm (3n), a food-storage tissue.
Parthenogenesis is development from an unfertilised egg (honeybee drones, aphids, water fleas). It is a form of sexual reproduction, because development starts from a gamete. Hermaphrodites (earthworm) have both kinds of sex glands but usually cross-fertilise.
Worked example
In a male, 50 primary spermatocytes entered meiosis; in a female, 50 primary oocytes. How many sperm, eggs and polar bodies are formed?
Each spermatocyte gives 4 sperm: 50 × 4 = 200.
Each oocyte gives 1 egg: 50 × 1 = 50.
Polar bodies: 50 × 3 = 150.
Answer: 200 sperm, 50 eggs, 150 polar bodies.
Worked example
A plant leaf cell has 24 chromosomes. How many chromosomes are in its egg cell, zygote and endosperm?
2n = 24, so n = 12.
Egg — n = 12; zygote — 2n = 24.
Endosperm — 3n = 36.
Answer: 12, 24, 36.
§3. Embryonic development
Ontogeny is the individual development of an organism from zygote to death; it is divided into the embryonic and post-embryonic periods.
Stages of the embryonic period:
Cleavage — rapid mitotic divisions of the zygote; the resulting cells (blastomeres) do not grow, so the total size of the embryo hardly changes.
Blastula — a single-layered hollow ball with a cavity (blastocoel).
Gastrula — a two-layered embryo: outer ectoderm and inner endoderm; the primary mouth (blastopore) appears. In most animals a third layer, the mesoderm, then forms.
Neurula — the ectoderm forms the neural plate and then the neural tube; the notochord forms.
Organogenesis — tissues and organs take shape.
Ectoderm → epidermis of the skin, hair, nails, sweat glands, nervous system, sense organs.
Endoderm → lining of the gut, digestive glands (liver, pancreas), lungs.
Mesoderm → muscles, skeleton, blood and blood vessels, heart, kidneys, gonads, the deep layer of the skin (dermis).
The influence of one part of the embryo on the development of another is called embryonic induction (H. Spemann).
§4. Post-embryonic development and human ontogeny
Direct development — the newborn resembles the adult (reptiles, birds, mammals).
Indirect development (metamorphosis) — the larva differs sharply from the adult.
Incomplete metamorphosis: egg → larva (nymph) → adult, with no pupa (grasshoppers, dragonflies, true bugs).
In the frog: spawn → tadpole (breathes with gills, has a tail) → frog.
In humans fertilisation usually takes place in the oviduct (fallopian tube); the embryo embeds in the wall of the uterus (implantation). The fetus receives nutrients and oxygen from the mother’s blood through the placenta and umbilical cord, but the blood of mother and fetus does not mix directly. Pregnancy lasts about 9 months. Alcohol, nicotine, drugs and many medicines cross the placenta and can seriously harm the fetus.
§5. Alternation of generations in plants
In the life cycle of land plants two generations alternate: the sporophyte (2n, produces spores) and the gametophyte (n, produces gametes). Spores are formed on the sporophyte by meiosis, so they are haploid; gametes are formed on the gametophyte by mitosis, because the gametophyte is already haploid.
In mosses (for example, haircap moss) the green plant is the gametophyte; the capsule growing on it (the sporogonium) is the sporophyte, which is nourished by the gametophyte.
In ferns the large leafy plant is the sporophyte; a spore grows into a small heart-shaped prothallus, which is the gametophyte. Sperm cells reach the egg through a film of water.
In flowering plants the gametophyte is greatly reduced and develops inside the sporophyte. A microspore in the anther gives a pollen grain containing a vegetative cell (forms the pollen tube) and a generative cell (divides by mitosis into two sperm cells). One megaspore in the ovule undergoes three successive mitoses to form the embryo sac — the female gametophyte with 7 cells and 8 nuclei.
In plants gametes are formed by mitosis on the gametophyte, not by meiosis; in animals gametes are the direct product of meiosis.
Comparing the two modes: asexual reproduction allows rapid multiplication in stable conditions, while sexual reproduction helps adaptation to changing ones. That is why many organisms (hydra, aphids, fungi, many plants) alternate between both.
Worked example
A fern leaf cell has 2n = 60 chromosomes. How many chromosomes are in its spore, prothallus, sperm cell and zygote?
The leafy plant is the sporophyte, 2n = 60, so n = 30.
A spore is formed by meiosis → n = 30.
The prothallus grows from the spore by mitosis → n = 30; the sperm cell forms on the prothallus by mitosis → n = 30.
The zygote is the fusion of sperm and egg → 2n = 60.