The nucleus of a cell contain information for inheritance of features from parents to next generation in the form of DNA (Deoxyribonucleic Acid) molecules.
The DNA in the cell nucleus is the information source for making proteins.
If the information is changed, different proteins will be made. Different proteins will eventually lead to altered body designs.
1. DO ORGANISMS CREATE EXACT COPIES OF THEMSELVES?
Organisms look similar because their body designs are similar. If body designs are to be similar, the blueprints for these designs should be similar. Thus, reproduction at its most basic level will involve making copies of the blueprints of body design.
A basic event in reproduction is the creation of a DNA copy. Cells use chemical reactions to build copies of their DNA.
1.1 The Importance of Variation
- If a population of reproducing organisms were suited to a particular niche and if the niche were drastically altered, the population could be wiped out. However, if some variations were to be present in a few individuals in these populations, there would be some chance for them to survive.
- Example:- Thus, if there were a population of bacteria living in temperate waters, and if the water temperature were to be increased by global warming, most of these bacteria would die, but the few variants resistant to heat would survive and grow further. Variation is thus useful for the survival of species over time.
2. MODES OF REPRODUCTION USED BY SINGLE ORGANISMS
The modes by which various organisms reproduce depend on the body design of the organisms.
Types of Asexual mode of reproduction
This methode of reproduction is used by unicellular organisms.
In this mode of reproduction cell division, or fission, leads to the creation of new individuals.
Fission mode of reproduction is divided into two parts
1. Binary fission: In binary fission, a single organism produces two parts. Example: Amoeba , paramecium
Binary fission in Amoeba
2. Multiple fission: in which a single organism produces multiple parts. Example: Leishmania , Plasmodium
Multiple fission in Plasmodium
It is a form of asexual reproduction wherein a parent organism breaks into fragments, each capable of growing independently into a new organism.
Many fully differentiated organisms have the ability to give rise to new individual organisms from their body parts. That is, if the individual is somehow cut or broken up into many pieces, many of these pieces grow into separate individuals.
For example, simple animals like Hydra and Planaria can be cut into any number of pieces and each piece grows into a complete organism. This is known as regeneration
Regeneration in Planaria
Budding is a type of asexual reproduction in which a new organism develops from an outgrowth or bud due to cell division at one particular site.
These buds develop into tiny individuals and when fully mature, detach from the parent body and become new independent individuals.
Example : Budding in Hydra
Budding in Hydra
5. Vegetative Propagation
In this mode of reproduction the parts of many plants like the root, stem and leaves develop into new plants under appropriate conditions.
This property of vegetative propagation is used in methods such as layering or grafting to grow many plants like sugarcane, roses, or grapes for agricultural purposes.
Plants raised by vegetative propagation can bear flowers and fruits earlier than those produced from seeds. Such methods also make possible the propagation of plants such as banana, orange, rose and jasmine that have lost the capacity to produce seeds.
Another advantage of vegetative propagation is that all plants produced are genetically similar enough to the parent plant to have all its characteristics.
Example: sugarcane, rose, grapes, bryophyllum
Leaf of Bryophyllum with buds
6. Spore Formation
A mode of reproduction resembling multiple fission, common among Protozoa, in which the organism breaks up into a number of pieces, or spores, each of which eventually develops into an organism like the parent form.
Spore formation in Rhizopus
3. SEXUAL REPRODUCTION
Sexual reproduction means production of new living organisms by combining genetic information from two individuals of different types (sexes).
Why the Sexual Mode of Reproduction?
Variation : Every individual organism cannot be protected by variations, but in a population, variations are useful for ensuring the survival of the species.
Sexual Reproduction in Flowering Plants
- The reproductive parts of flowering plants (angiosperms) are located in the flower.
- Different parts of a flower – sepals, petals, stamens and pistil. Stamens and pistil are the reproductive parts of a flower which contain the germ-cells.
- The flower may be unisexual (papaya, watermelon) when it contains either stamens or pistil or bisexual (Hibiscus, mustard) when it contains both stamens and pistil.
- Stamen is the male reproductive part and it produces pollen grains that are yellowish in colour.
- Pistil is present in the centre of a flower and is the female reproductive part. It is made of three parts.
- The swollen bottom part is the ovary, middle elongated part is the style and the terminal part which may be sticky is the stigma. The ovary contains ovules and each ovule has an egg cell.
- The male germ-cell produced by pollen grain fuses with the female gamete present in the ovule. This fusion of the germ-cells or fertilisation gives us the zygote which is capable of growing into a new plant.
- Thus the pollen needs to be transferred from the stamen to the stigma. If this transfer of pollen occurs in the same flower, it is referred to as self-pollination. On the other hand, if the pollen is transferred from one flower to another, it is known as cross-pollination. This transfer of pollen from one flower to another is achieved by agents like wind, water or animals.