Reproduction | Asexual reproduction
Asexual reproduction may be defined as the production of offsprings by a single parent without meiosis, formation of gametes, fertilization and transfer of genetic materials between individuals. Mostly it occurs in unicellular organisms, e.g. bacteria, protozoa, some plants (e.g. algae, fungi, bryophytes, etc) and certain multicellular organism (e.g. sponges and Hydra).
Types of Asexual Reproduction
Fission
It is defined as the splitting of a unicellular organism into two or more separate daughter cells. Example: bacteria, yeast, diatoms, mycoplasmas and protozoans.
Fission is of two types:
(a) Binary fission:
In binary fission, the parent organism splits to form two new organisms.
During binary fission, the DNA molecules replicate. It is followed by nuclear division in eukaryotes. The nuclear division is followed by the appearance of constriction in cell membrane. The membrane then grows centripetally from near the middle of dividing cell which separates the cytoplasm into two equal parts, each with one nucleus, e.g. Amoeba, Paramecium, Leishmania etc.
Binary Fission
(b) Multiple fission:
In multiple fission, the parent organism splits to form many new organisms at the same time.
A cyst is formed around the cell of a single celled organism especially during unfavorable conditions. During multiple fission, the nucleus of parent cell divides several times into many daughter nuclei. The daughter nuclei then get arranged along the periphery of the parent cell followed by division of cytoplasm into as many pieces as the number of nuclei e.g. Plasmodium.
Reproduction by multiple fission
Budding
It is the process of production of new organism from an outgrowth of the parent individual e.g. Hydra, Yeast, Scypha, etc.
(a) Budding in Hydra:
A bulge appears on the body as a result of repeated mitotic division in the cells resulting in the formation of lateral outgrowth called bud. This bud enlarges in size by further division of cells and attains the shape of parent. It then separates from the parent body and starts behaving as new Hydra.
Budding in Hydra
(b) Budding in yeast:
A small bud like out growth appears at one end of the parent cell which gradually enlarges in size. The nucleus also enlarges and divides into two daughter nuclei. One nuclei remains in the parent cell and other goes to the daughter. When the bud attains almost similar size like parent a constriction appears at the base of the bud separating it from the parent.
Budding in Yeast
Spore formation
A spore is a single or several celled reproductive structure that detaches from the parent and gives rise, directly or indirectly to a new individual.
Spore formation takes place mostly in bacteria and fungi. In fungi, spores are formed in a sac-like structure called sporangium at the tips of fungal hyphae.
The nucleus divides inside the sporangium and gets surrounded by a small mass of cytoplasm forming a spore. After attaining maturity, the sporangial wall ruptures releasing the spores.
Formation of sporangia and spores in a fungus (Rhizopus)
Regeneration
Regeneration is defined as a natural ability of some simple multicellular organisms to replace worn out parts, to repair damaged parts or to regrow cast off organs. It takes place mostly in Hydra, Planaria, sponges, etc.
Regeneration is performed by specialised cells. These specialised cells proliferate and make large number of cells. Different cells from the mass of cells undergo change to become various cell types and tissues.
These changes takes place in an organized sequence referred to as development
Regeneration in Planaria
Fragmentation
In some multicellular organisms with simple body organization, yet another method of asexual reproduction works. Spirogyra (algae) breaks up into smaller pieces upon maturation.
These pieces (fragments) grow into new individuals.
Vegetative propagation in plants
Plant undergoing vegetative reproduction propagates by a part of their body other than a seed. This part is called propagule. Vegetative reproduction is of different types.
(a) Natural method:
There are many plants which propagate naturally. Some plants propagate by roots e.g. sweet potato, guava, etc.; some by stems like ginger, banana, potato, strawberry; etc.; some by leaves like Bryophyllum, Begonia, etc.
(b) Artificial method:
There are some plants which propagates artificially by following methods:-
(i) Cutting: In this method, a small portion of the parent plant (stem or root) is cut and buried partly in the moist soil. After some days, the cutting develops into a new plant exactly similar to the parent plant. Many plants like rose, grapes, etc are propagated by means of cutting.
(ii) Layering: In this process, roots are artificially induced on the stem branches before they are detached from the parent plant for propagation. Layering is of two types.
Mound layering: In this process the lower flexible portion is pulled and a portion of it is covered by soil to develop roots, e.g. strawberry.
Air layering or gootee: A ring of bark is removed from the plant and this portion is covered with clay, cow dung, hay, etc. wrapped with a polythene sheet. This portion is called gootee, e.g. litchi, pomegranate, lemon, jasmine, orange, etc.
A-B Vegetative propagation by layering
(iii) Grafting:
In this process, two parts of the plant are joined in such a way that they grow as one plant. It is done between two closely related plants with vascular cambium.
The rooted plant in which grafting is done is called stock and the portion that is grafted is called scion,
e.g. Mango, roses, citrus, apple, grapes, etc.
Different stages in grafting
(iv) Micropropagation:
It is defined as, ‘The production of plants from a small piece of plant tissue removed from the growing tip of a plant in a suitable growth medium (culture solution). It is also called tissue culture. This technique is used for the production of ornamental plants like orchids, dahlia, etc.
Mechanism of micro propagation
- A small piece of plant tissue placed in a culture medium divides rapidly to form a shapeless lump called ‘callus’.
- The callus is then placed in different culture media to stimulate the development of root and shoot.
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Tiny plantlets are formed from just few cells which are transplanted into pots or soil where they can grow to form mature plants.
Advantages of micro propagation
- It is a fast technique producing many plantlets from a small plant tissue in few weeks and using very little space. In other words, it is quite economical.
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The plants produced by tissue culture are disease free.
(c) Significance of vegetative propagation:
- It is the only method of reproduction in those plants which have lost their capacity to produce seeds e.g. banana, orange, rose, etc.
- By this process plants retain their original types without variation.
- It is used to produce disease free plants.
- Plants produced by this process can give flowers and fruits earlier than those produced from seeds.
Do organisms create exact copies of themselves in reproduction ?
Reproduction produces new individuals that look much similar to the parent.
We have already studied that chromosomes contain units (genes) for inheritance from one generation to another.
Chemically genes are made up of DNA which encodes for different proteins.
If somehow, the code for a particular protein is changed, a different protein would be produced.
This would result in alternation of body designs.
Therefore, a basic event in reproduction is production of a DNA copy.
The replication (copying) of DNA prior to cell division involves many biochemical reactions.
These reactions do not produce same results all the time and hence slight variations are always likely in the two copies formed.
The two copies of DNA so formed are separated and daughter cells are formed from a single parent cell.
The offsprings formed from the daughter cells will also show slight variations.
Importance of Variation
The importance of variation lies in the fact that it helps the species of various organisms to survive and flourish even in adverse environment.
The various niches (well-defined places) in the ecosystem are filled by populations of organisms using their reproductive ability.
In case of drastic changes like excessive heat, cold or shortage of water, etc there is danger that all of them may die and no one would survive under these conditions.
This will eliminate the species from the habitat completely.
However, if some variations are present in some individual organisms to tolerate the extremities there is chance for them to survive in such conditions.
Thus variations during reproduction provide stability to the populations of various species by preventing them for getting wiped.
For example, some variant mosquito has developed resistance to DDT (Dichloro diphenyl trichloro ethane) which prevented their population from being wiped out.