Reproduction

How do Organisms Reproduce?

No organism survives forever and therefore, if a species is to continue to exist, its members must reproduce their own kind. Hence, all the living organisms reproduce their own kind to maintain the continuity of their race. Reproduction therefore, helps in the increase in population of the species. It transfers the characteristics of parents into offspring.

REPRODUCTION

The process by which living organisms produce new individuals similar to themselves is called REPRODUCTION. It is one of the most important characteristics of living organisms as it ensures perpetuation and survival of a race. The process of reproduction has ensured continuity of life on earth.

Through reproduction, variations produced in the living body are transferred to the next generation. The accumulation of various forms of variations, generation after generation, ultimately leads to the origin of new forms or species. Reproduction is a bridge to hereditary transmission.

The importance of Variation

The different members of a species look similar in most of their characters, although individual differences do occur. They transmit their characters to the next generation through the process of reproduction. This indicates that the blueprints of these designated characters are similar and, during reproduction, copies of these blueprints are produced to be transmitted to the next generation.

The cellular components of hereditary transmission are in the form of macromolecules called DNA (Deoxyribo Nucleic Acid).

DNA is a very large single molecule. Which is composed two stands, wound around each other in a double helix manner. DNA is present in the chromosomes, which are present in the nucleus. The DNA has complex chemical structures composed of pentose sugars, nitrogenous bases and phosphate. The DNA was first discovered by Frederick Miesher. The DNA contains all the information necessary to make the specific protein which in turn form enzymes required to control the chemical reaction of the body. It is these chemical reactions which govern the metabolism of the body in order to produce specific characters. Thus, in order to transmit the character to the next generation, DNA forms copies of itself prior to reproduction and these copies are transported to the newly formed reproductive cells.

The process of formation of new DNA strands from the older ones is called REPLICATION of DNA. During replication, DNA forms an exact copy of itself but mistakes in the process of replication may occur. In this way, the two cells receiving their DNA content from the parent cells may have DNA possessing some variations. Variations are then passed over to the next generation through the process of reproduction. So, the blueprint of the next generation is actually stored in the DNA macromolecules and it may be considered as the bridge of hereditary transmission.

Types of reproduction

There are two basic types of reproduction: Asexual and Sexual.

Asexual Reproduction

When reproduction occurs without the fusion of gametes, it is called asexual reproduction. In Asexual Reproduction a single organism is capable of producing its own kind.

Asexual reproduction is considered as primitive mode of reproduction, but for several organisms, it is extremely useful as a means of rapid multiplication.

Characteristics of Asexual Reproduction

(a) A single individual is involved in this type of reproduction. Sex differentiation is not required.

(b) Gametes are not formed.

(c) Fertilisation of gametes is not required.

(d) There occurs only mitotic or amitotic type of cell division. Meiosis does not occur.

(e) Daughter organisms are generally identical to the parents.

(f) It is a mode of reproduction.

Different forms asexual reproduction

There are five types of Asexual Reproduction. Fission, Budding, Spore formation, fragmentation and vegetative propagation.

Fission

In the process of Fission, the organism simply divides or splits into two or equal - sized parts. Each part later becomes a complete organism. It is the simplest form of reproduction and is characteristic of protozoa and bacteria.

Fission can further be divided into following two types i.e. Binary Fission and Multiple Fission.

Binary Fission

The term binary means two and the term fission means splitting. The process, in which a number of organisms reproduce by simply splitting into two identical parts, is called BINARY FISSION. Binary fission is a common mode of reproduction in unicellular organism like Amoeba, Paramecium, Euglena, Yeast, Bacteria, etc. This type of reproduction occurs when the environmental condition are favourable for the growth and development of the organism.

In this process the nucleus of the organism first divides into two and then the cytoplasm also divides. The cell finally divides to form two cells, i.e, the daughter organism.

Multiple Fission

This type of fission occurs when an organism faces unfavourable environmental condition. During this process, many new organisms are formed from a single individual. Multiple fission is found in Amoeba, Monocystis, Plasmodium, etc. Multiple Fission has also been observed in many forms of algae.

During multiple fission, the organism forms a protective covering (cyst) over the cell. Inside the cyst, the nucleolus divides many times to form a large number of nuclei. A small amount of cytoplasm surrounds each daughter nucleus to form a large number of daughter cells. On the arrival of favorable conditions, the cyst breaks up and the daughter cells formed inside the cyst get released. Each daughter cell becomes a complete organism.

Budding

Budding is a process in which a new organism is produced as an outgrowth of the parent organism. During this process, a small part of the body of the parent organism grows out as a bud. The bud then detaches and moves away on its own to lead an independent life.

The bud may split away from the parent body as in the case of Hydra. In hydra, a bud develops as an outgrowth due to repeated cell division. These buds developed into new hydra. There are many plants, sycon and yeast etc. are the example of budding.

Spore Formation

In lower plants like algae, bacteria, fungi, etc. spore formation is a common form of asexual reproduction Spores are unicellular bodies formed by cell division in parent organism. Spores are generally very small and light; therefore, they are easily dispersed by wind, water and animals. Some have thick resistant walls which enable them to survive even in unfavorable conditions.

During the growth of the fungus Rhizopus, small rounded, bulb-like structures develop at the top of erect hyphae. Such structures are called sporangia. Insides each sporangium, the nucleus divides several times. Each nucleus gets surrounded by a small amount of cytoplasm to become a spore. A large number of spores are formed inside each sporangium. After some time, the sporangium bursts and spores are released in the air. When these spores land on food or soil, they germinate into new individuals under favorable condition.

Asexual reproduction in plants (Vegetative Propagation)

Vegetative Propagation is the process of asexual reproduction in plants where parts of the body such as root, stem, leaf, etc. develop into a new self-supporting plant. It is the simplest method of reproduction in plant.

In certain plants like sweet potato, Dahlia, etc. have buds develop in such roots When these bud-bearing tuberous root are put into soil, new plants are produced through vegetative propagation.

The grass stem grows along the surface of the soil and produces root at intervals form where a new plant grows. Such stem are called runners.

Underground stems like potato, onion, ginger, turmeric, etc. are also the source of vegetative reproduction. The eyes (buds) present in potato, bulb of onion and rhizome of ginger give rise to aerial shoot under favorable condition.

The fleshy leaves of certain plants like Bryophyllum bear adventitious buds inside the notch present along the margin of leaves. When these bud- containing leaves fall on soil, each bud gives rise to an independent plant.