How do Organisms Reproduce - I

The thread-like structures on which sporangia develop are called hyphae. Their scientific name is sporangiophore.

Disease-free plants can be produced by tissue culture. In this technique, we can take few healthy cells and selectively multiply them to form new plants under sterile and aseptic conditions. But, in grafting or layering or cutting, the chances of having diseased plants are there because the cells cannot be selectively chosen in these and at any time, the pathogens may attack them.

Vegetative propagation in bryophyllum occurs through leaves. 

Buds grow on the parent body and gradually form a new individual. Hence, (3), i.e. budding, is correct. In fission and regeneration, new individuals separate out and there is no parent organism. In spore formation, spores develop on the parent body but they form new individuals after they fall on suitable substratum away from the parent body.

Vegetative propagation means formation of new plants from vegetative parts.

Spore formation is a common mode of reproduction in fungi, like bread moulds, i.e. rhizopus. Leishmania and paramecium reproduce by binary fission, while planaria forms new individuals by regeneration method.

We know that apple, pear and peach have seeds. It is only banana that is seedless and has lost the capacity to produce seeds. In case of apple, the ability to produce seeds is there but they may not be viable.

Binary (Bi means two) fission produces only two daughter cells and occurs repeatedly, generation after generation, in short time intervals. This fast reproduction is possible only under favourable conditions of temperature, food and humidity. But under unfavourable conditions like high or low temperature, low humidity or lack of food, multiple fission occurs.

On being cut, the two parts of hydra can regenerate the other part because it is a simple organism. Bud also helps it in reproduction, but on being cut, regeneration ability plays the role. The two parts cannot be two individuals unless they regenerate. Lastly, hydra does not produce spores as it reproduces by budding.

Regeneration is carried out by specialized cells, which can proliferate at a fast rate. Also, they can form all the different cells that are required to form the body part that the organism needs to regenerate. Broken cells may not proliferate while “all cells” cannot have the above mentioned specialized features. Differentiated cells take up a fixed role and may not be able to produce all the cells required in regeneration.

To develop a whole body from a fragment is possible by proliferation (i.e. repeated division and consequent growth) of cells. This is called regeneration. On the contrary, in budding, a complete organism forms a new individual from a bud, and in fission, the cells divide into two. Fusion is said to occur when two cells combine to mix their traits (like in bacteria) and it is uncommon.

Spores are the microscopic structures formed during the life cycle of many fungi. Each spore, upon falling on suitable substratum (medium on which an organism depends for deriving its food), grows into a complete organism, which can later divide and form a complete colony. Spore-bearing structures are called sporangia and they are present on stalks called sporangiophores.
Capsules refer to dry and indehiscent fruits. When they split, seeds fall out from them.
Cysts are hard covered bodies formed under unfavourable conditions in bacteria and they produce daughter cells, and not spores. Similarly, buds also do not bear spores as buds are bulging outgrowths of the parent body which, after maturation, detaches itself from the parent body and develops into a new individual. Buds and spores are two different means of asexual reproduction.

Leishmania has a specific organisation of body. It has a whip-like flagella on one end. Due to this, when its fission occurs, it divides the body such that both get the flagella (duplicated). This is possible in binary fission, but not in multiple fission, which will require the division to occur in different planes.

Plasmodium grows rapidly inside a human body and for this, it divides by multiple fission because binary fission will produce less number of organisms. Since it is a unicellular organism, so regeneration and spore formation do not occur in Plasmodium.

Sexual reproduction involves two parents and their gametes fuse to form a new individual. Thus, formation of gametes is a part of sexual reproduction. Fission, regeneration and spore formation are all asexual methods of reproduction.

Ornamental plants are very delicate and some of their features make them valuable. It is desired that during their propagation, there should be no change in those traits and also they should remain free from any disease. Both these requirements are met by tissue culture technique. In addition, it helps to produce a large-scale commercial yield. Budding, fragmentation and regeneration cannot serve all these purposes together because they are slow and cannot ensure disease free progeny.

Asexual reproduction involves only one parent and its cells divide by mitosis (without any change in chromosomes number). Thus, there is no change in genetic makeup of new cells, and the offsprings are identical to parents. Since this method of reproduction involves simple cell division (without any change in number or traits of chromosomes), so it is faster than sexual method. Thus, here only option (3) is incorrect.

Multiple fission, regeneration and spore-formation, all require the formation of a large number of new cells by repeated division of initial or few cells. But in binary fission, only one division occurs and produces two daughter cells. Thus, binary fission is the fastest mode of reproduction. In fact, cells are known to produce daughter cells in twenty minutes of time from their own birth by binary fission.

At the cellular level, reproduction refers to the formation of new cells. It can occur, if a new cell gets nucleus and nucleus contains DNA molecules. DNA is the genetic material, which is passed from one generation to another.
Thus, for reproduction, DNA material is to be duplicated. Hence, DNA is the biochemical basis of reproduction. On the contrary options (1), (2) and (3), which refer to different cells like RBC, WBC and neuron have no role in the process of reproduction.

The leaves of Bryophyllum plant help in vegetative propagation. The notches of these leaves contain vegetative buds, which can grow to produce roots and small shoots that later make a new plant. Its roots, stem or flowers have no such vegetative buds to take part in the reproduction process.

'Bread molds' are a type of fungi and reproduce by spore formation. In asexual reproduction, the mold makes spores inside a sporangium. When the spores are ready to leave and spread more molds, the sporangium breaks open and lets the spores float out. In sexual reproduction, its hyphae touch the hyphae of another Rhizopus mycelium. When they fuse, they make round balls called zygospores. After some time, the zygospore makes another sporangium, which then makes spores. During this sexual process, genetic recombination takes place, as with all eukaryotes. 

Higher plants have complex structures and they have specialised parts to perform asexual (vegetative) reproduction. For example, leaves, stem, roots, etc. This formation of new organism from vegetative parts of a plant is called vegetative propagation. On the contrary, spore formation, budding and regeneration are reproductive methods found in lower organisms or plants.

An unorganized mass of cells is called callus. When it is treated with suitable hormones, it forms tiny root and shoot, which are called rootlet and shootlet, respectively. At this stage, it is called a plantlet.

For the survival of individual life, reproduction is not essential. An organism will die without digestion, respiration and excretion, but it can live if it does not reproduce. Reproduction is essential for the survival of species, and not an individual.

Binary fission produces two daughter cells, while huge number of cells is produced by multiple fission. Thus, option (1) is correct.

Spores are formed inside the sporangia and they are released when the wall breaks. The wall usually breaks on coming in contact with water and seldom on shaking. For release of spores, the lower hyphae or the parent fungus do not break their walls. Thus, (1) is correct.

Species need to reproduce one generation after the other, and also to adjust with the changing environment and other conditions to survive. Reproduction alone cannot help them to adapt. Hence, variations are very useful for survival of species. Nutrition is necessary for survival of an individual and cannot insure survival of species. Option (3), i.e. 'continuation' is not a scientific term.

Regeneration refers to formation of the entire organism from a part and for this, the cells of that part need to convert into the form in which they can divide and proliferate. This is a specialized function. Since, only one organism is involved, it is not a sexual mode of reproduction. Hence, only (3) is wrong.

All the information about an organism, i.e. its morphology, colour, traits etc. depends upon the nature of proteins in its cells. The proteins are synthesized in the cells under the directions of DNA. Thus, DNA contains all the information. RBC's, WBC's and platelets are merely components of blood and their formation and role is also guided by DNA.

Invitro, in Latin, means “in glass”. It refers to biological process occurring usually under experimental conditions, outside the cell or organism body. This holds good for tissue culture and it also gives large-scale commercial yield. On the contrary, natural selection, budding and sporulation are natural, i.e. in-vivo methods and cannot be used for large-scale production.

A planaria is a simple, multicellular organism and it can regenerate its lost body parts.

Asexual reproduction involves only division of cells to form new individuals. This requires only one parent. Since two organisms are not involved, the role of gametes or their fusion is out of the logic. Also, reproduction cannot occur without a parent because a new organism is derived from the old (and not by magic). Hence, (3) is correct.

A bud is formed by proliferation of cells of the parent body. On maturity, it separates from parent by a constriction at the base. Since only one parent is involved, it is an asexual method. Clearly, here option (1) is incorrect because bud develops on parent and is not independent of it.

Under usual conditions, hydra reproduces by an asexual method, i.e. budding. Fission is limited to unicellular organisms while spores are formed in fungi. Thus, only (2) is correct.

Reproduction can occur through one parent (asexual) or through two parents (sexual).
Further, some sexual reproducing organisms may contain both sexes in one (e.g. earthworm), i.e. they can be bisexual, or the sexes may be separate (e.g. humans), i.e. they are unisexual.
Thus, it may be concluded that unisexual and bisexual modes are subtypes of sexual reproduction.
But broadly, the reproduction is classified into sexual and asexual types only.

Tentacles are permanent parts of hydra. The bulge like projection is the bud and it is formed in both, hydra and yeast. Spores or bulbs are not formed in either of them. Hence, (4) is correct.

Continuity of species means that generation after generation, the species continue to survive and this is ensured by reproduction. Nutrition can ensure life of an organism but cannot form a new organism. Variation and speciation are related processes of reproduction and refer to the changes that organisms develop during reproduction but they themselves cannot maintain continuity of species without reproduction.

In asexual reproduction, the cells of parent organisms divide by mitosis to form identical cells. There is no change in DNA. Hence, no variations occur. Also, gametes do not play any role because this will bring new re-combinations of traits and it occurs only in sexual reproduction. Thus, (1), (3) and (4) are all incorrect about asexual reproduction.
But as we know, spore formation is one of the methods of asexual reproduction. We can say that (2) is the most suitable answer.

Longitudinal or transverse divisions are features of binary fission and not related to budding. Bud appears as an outgrowth on the parent body and later forms an individual, which separates afterwards from the parent. Spores are not formed during any stage in budding method. Thus, only option (3) is correct.

In tissue culture, the selected cells from shoot tip etc. are allowed to proliferate and form new plants. This process is done under lab conditions and is not a natural method. Since it involves cells from only one plant, it is essentially asexual. There is no formation of spores and instead tiny plantlets are produced. Thus, only option (3) is correct.