Reproduction and Asexual Modes

Introduction — Why Do Organisms Reproduce?

A single bacterium divides and within a day becomes a colony of millions. A mustard seed, no bigger than a full stop, becomes an entire plant bearing hundreds of new seeds. A human baby, born helpless, one day becomes a parent. The common thread is reproduction — the one life process which, unlike digestion or breathing, is not needed for the survival of the individual, yet without which no species would continue.

Why then do organisms spend so much energy on it? Three big reasons:

• Continuity of species — every individual eventually dies; only by producing offspring is the species kept alive.
• Transfer of genetic information — the "body design" stored in DNA is passed on to the next generation.
• Introducing variation — small differences between offspring give a species the flexibility to survive a changing environment.

7.1 Do Organisms Create Exact Copies of Themselves?

At first glance reproduction looks like simple "copying". A bacterium divides into two bacteria, both looking identical to the parent. But biology is never as precise as a photocopier. Every time DNA is copied, tiny errors creep in. These small differences are called variations.

Variations are not mistakes — they are an opportunity. Suppose a population of bacteria living in warm water suddenly faces a rise in temperature. Most bacteria will die, but a few with a variation that lets them tolerate heat will survive and multiply. The species, as a whole, survives the crisis. Without variation, one bad day could wipe out an entire species.

7.2 Modes of Reproduction Used by Single Organisms

Organisms that live as single cells, or which have a simple body plan, reproduce using a single parent — this is asexual reproduction. No gametes, no partner, no fertilisation. The main methods seen in Class 10 are fission, fragmentation, regeneration, budding, vegetative propagation and spore formation.

7.2.1 Fission

In unicellular organisms the simplest thing to do is split the whole cell into two (or many) new cells. This is called fission.

Binary fission — the parent cell divides into two equal daughter cells. Seen in Amoeba, Leishmania (which causes kala-azar) and bacteria. In Amoeba the split can happen in any plane; in Leishmania (with its whip-like flagellum) the split occurs along a definite axis.

Multiple fission — under unfavourable conditions the parent cell produces a tough cyst wall and inside it the nucleus divides many times. When conditions become favourable the cyst ruptures and releases many daughter cells at once. Example: Plasmodium, the malarial parasite.

7.2.2 Fragmentation

Some multicellular organisms with a simple body, such as the filamentous green alga Spirogyra, simply break into two or more pieces when they grow big. Each piece then grows into a complete organism. This is called fragmentation. It works only when the body is made of a uniform tissue — in a complex animal, a fragment would be missing vital organs.

7.2.3 Regeneration

Some simple animals like Planaria (a flatworm) and Hydra can be cut into small pieces and each piece grows back the missing parts to form a complete new organism. This is called regeneration. It is possible because these animals contain specialised cells that can divide and turn into any body part. Regeneration is NOT the same as true reproduction — it happens only when an accident cuts the body. But the principle shows how powerful simple body plans can be.

Complex animals (humans, dogs) cannot regenerate because their bodies are organised into highly specialised organs — a piece of stomach cannot grow a brain or a heart.

7.2.4 Budding

In Hydra and in yeast, a small outgrowth called a bud appears on the side of the parent. The bud grows, develops all the parent's features, and finally detaches as a new individual. In yeast buds can appear while the previous bud is still attached, giving a short chain of cells.

7.2.5 Vegetative Propagation

Many flowering plants can produce new plants from a piece of root, stem or leaf, without the need for seeds. This is called vegetative propagation.

Natural methods:

• Roots — sweet potato, dahlia form buds on the roots.
• Stems — potato tubers have "eyes" (buds); ginger and turmeric have underground rhizomes; onion has a bulb.
• Leaves — Bryophyllum leaves carry tiny buds at their notches. When the leaf touches moist soil, each bud grows into a new plant.

Artificial methods used by farmers and gardeners:

• Cutting — a piece of stem with a node (sugarcane, rose) is planted in soil where it develops roots.
• Layering — a branch (jasmine) is bent into the soil; once roots form it is cut off from the parent.
• Grafting — the stem of one plant (the scion, e.g. a desired rose variety) is joined to the rooted stem of another plant (the stock). Useful for mango, apple, rose.

7.2.6 Spore Formation

Many fungi and some simple plants produce tiny, light-weight spores inside a knob-like structure called the sporangium. When the sporangium bursts, the spores are scattered by wind or water. If a spore lands on a moist substrate (like bread) it germinates and grows a new fungus. The classic Class 10 example is Rhizopus, the black mould that appears on stale bread.