🌳 Concept tree

├── Importance of reproduction

├── Reproductive capacity

├── Methods of reproduction

├── Asexual reproduction

├── Sexual reproduction

• ├── Comparison between asexual and sexual reproduction

└── Alternation of generations

1. ├── 2.4.1 Life cycle of Plasmodium (malaria parasite)
2. └── 2.4.2 Life cycle of a fern (Polypodium, Adiantum)

• It means separation of part of the body — a spore, one cell or many cells or a tissue — that grows into a new organism fully identical to the parent.
• The characteristics of the next generations remain the same even if the environment changes; so, when the environment changes unfavourably, most of the offspring are exposed to destruction unless the parents were already adapted.
• It is common in the plant kingdom and limited to primitive animals.

• It depends on mitosis, so the cells formed receive a complete copy (2N) of the chromosomes. There are no gametes and only one parent is needed.
• Types of asexual reproduction:

1. Binary fission. The nucleus divides by mitosis, then the cytoplasm divides and the cell separates into two cells, each a new individual.
2. Occurs in protozoa (Amoeba, Paramecium), simple algae and bacteria, under suitable conditions.
3. In unfavourable conditions Amoeba secretes a chitinous cyst around itself and divides repeatedly inside it; many small amoebae are liberated when conditions improve.
4. 2. Budding.

• In unicellular organisms, e.g. yeast: the nucleus divides mitotically, a small lateral projection (bud) appears; one of the two nuclei moves into the bud. The bud grows and may stay connected (forming a colony) or separates.
• In multicellular organisms, e.g. sponges and Hydra: a bud grows out as a cellular projection by division of the interstitial cells, which differentiate; it becomes a small copy of the parent and usually separates to live independently. Hydra and sponges also reproduce sexually and can regenerate.

1. Regeneration. The ability to replace lost parts or even to grow a whole new individual from a fragment.
2. sponges, Hydra, some worms, starfish (sea star).
3. If the body of some of these animals is cut into pieces, each piece may become a new individual.
4. in some crustaceans and amphibians it is restricted to replacing a lost part (e.g.
5. a lizard’s tail, a crab’s claw); in higher vertebrates it is no more than healing of wounds in the skin, blood vessels and muscles.

• Planaria (a freshwater flatworm) regenerates when cut into several transverse pieces or two longitudinal halves — each grows into a complete individual.
• Hydra regenerates when cut transversely or longitudinally.
• A starfish arm with a part of the central disc can regrow a complete star (taking up to a year).
• Regeneration is not necessarily reproduction (in lower animals it is accidental), though in some forms (Planaria, starfish) it is a means of multiplication.

1. Sporogony (spore formation). Primitive organisms form single cells called spores, each with a small amount of cytoplasm and water, a nucleus and a thick coat.
2. When ripe the spore is liberated from the parent and carried by air.
3. On reaching a suitable medium it absorbs water, the coat ruptures, and it divides by mitosis to form a new individual.
4. bread mould (Mucor/Rhizopus), mushroom, some algae and ferns.
5. rapid multiplication, resistance to harsh conditions, and wide distribution to distant places.

1. Parthenogenesis. Development of an egg into a new individual without fertilisation by a male gamete; the progeny comes only from the mother.
2. It is regarded as a special type of asexual reproduction.

1. Tissue culture. Growing living cells or tissues in a semi-natural nutrient medium and following their differentiation until a complete organism is obtained.
2. In a well-known experiment, small pieces of carrot were placed in flasks containing coconut milk (which supplies nutrients and plant hormones); they grew into whole carrot plants.
3. Isolated cells of a carrot or tobacco leaf also gave whole plants.
4. any somatic cell of a plant contains the whole genetic information (it is totipotent) and, if given a suitable medium with the right ratio of plant hormones, can develop into a whole plant.
5. rapid propagation of rare plants, desirable strains, or disease-resistant plants, with identical offspring (clones).

✍️ Worked examples

• Worked example 2.
• A farmer wants thousands of identical, disease-free date palm plantlets.
• Best method tissue culture of cells of one selected plant on a nutrient medium with the proper ratio of hormones (auxin: cytokinin).
• Why identical?
• Mitosis copies the DNA exactly.
• Worked example 3.
• A drone bee has 16 chromosomes in its body cells (haploid).
• How many chromosomes does a worker have? 32.
• How many does the drone sperm have? 16, formed by mitosis (no reduction possible).

• Plasmodium is a sporozoan protozoan parasite that infects man (intermediate host, asexual cycle) and the female Anopheles mosquito (definitive host, sexual cycle).
• In man (asexual):

1. The infected female mosquito bites a person and injects sporozoites (tiny spindle-shaped cells) with saliva into the blood.
2. Sporozoites go to the liver where they multiply asexually by schizogony (multiple fission) and form many merozoites.
3. Merozoites enter the red blood cells and multiply again by schizogony, so a large number of merozoites are produced and released together every 2 days (or 3, depending on the species) when the red cells burst, releasing toxins. This causes the typical malaria fever with chills, high temperature and sweating.
4. Some merozoites develop into gametocytes (male and female) in the blood.

• In the mosquito (sexual, then asexual):

1. When another mosquito feeds on the patient, the gametocytes pass with the blood into its stomach and develop into gametes.
   6.
2. The male and female gametes fuse (fertilisation) to form a zygote, which changes into a motile ookinete.
   7.
3. The ookinete penetrates the stomach wall and forms an oocyst under the outer layer.
4. The zygote nucleus divides by meiosis (reduction division), so the cells are haploid.
   8.
5. The oocyst nucleus divides many times by mitosis (sporogony) forming numerous sporozoites, which break out and migrate to the mosquito’s salivary glands, ready to infect another person.
6. The life cycle thus contains one sexual generation (gametes, in the mosquito) and asexual generations: schizogony in man (liver and red cells) and sporogony in the mosquito.

✍️ Worked examples

• Worked example 4.
• Why does malaria fever appear at regular intervals (every 48 hours in some types)?
• Because a batch of merozoites matures and bursts out of the red cells synchronously, releasing toxins; the cycle needs about 48 hours to repeat.