How do organisms reproduce

 

CHAPTER 7

How do Organisms Reproduce?

Mechanisms by which organisms reproduce? 

Organisms reproduce to ensure the continuation of their species. 

The mechanisms of reproduction can be broadly categorized into 

asexual reproduction and 

sexual reproduction:

1. Asexual Reproduction

Involves a single parent, and offspring are genetically identical to the parent (clones).

Mechanisms:

Binary Fission: Organism splits into two identical cells (e.g., bacteria, amoeba).

Budding: A small bud grows on the parent and detaches to form a new organism (e.g., yeast, hydra).

Fragmentation: Parent breaks into fragments, each developing into a new organism (e.g., starfish, spirogyra).

Spore Formation: Spores are produced and dispersed to grow into new individuals (e.g., fungi, mosses).

Vegetative Propagation: New plants grow from parts like roots, stems, or leaves (e.g., potatoes, bryophyllum).

 Sexual Reproduction

Involves two parents, and offspring inherit genetic material from both.

Mechanisms:

Gamete Formation: Male gamete(sperm)

 and female gamete (egg) 

gametes are produced.

Fertilization: Fusion of gametes to form a zygote.

External Fertilization: Outside the body (e.g., fish, amphibians).

Internal Fertilization: Inside the body (e.g., mammals, birds).

Why do organisms reproduce? 

Organisms reproduce to ensure the continuation of their species and prevent extinction. The key reasons for reproduction are:

1. Survival of the Species:

Reproduction ensures that new individuals are created, maintaining the population of a species over generations.

2. Transfer of Genetic Information:

Reproduction allows the passage of genetic material (DNA) from parents to offspring, preserving the traits and characteristics of the species.

3. Adaptation and Evolution:

Sexual reproduction introduces genetic variation, which helps species adapt to changing environments and evolve over time.

Asexual reproduction ensures rapid population growth in favorable conditions.

4. Maintaining Ecosystem Balance:

Reproduction helps maintain the food chain and ecological balance by sustaining populations of various organisms.

In short, reproduction is essential for the continuity, diversity, and adaptability of life on Earth.

The essential life processes such as nutrition. respiration, or excretion are essential to maintain the life of an individual orgainsm. 

Reproduction is not necessary to maintain the life of an individual organism. 

On the other hand, if an individual organism is going to create more individuals, a lot of its energy will be spent in the process. So why should an individual organism waste energy on a process it does not need to stay alive? 

Whatever the answer to this question, it is obvious that we notice organisms because they reproduce. 

If there were to be only one, non- reproducing member of a particular kind, it is doubtful that we would have noticed its existence. 

It is the large numbers of organisms belonging to a single species that bring them to our notice.

How do we know that two different individual organisms belong to the same species? Usually, we say this because they look similar to each other. Thus, reproducing organisms create new individuals that look very much like themselves.

7.1 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.

 

The chromosomes in the nucleus of a cell contain information for inheritance of features from parents to next generation in the form of DNA (Deoxyribo Nucleic 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.

Therefore, a basic event in reproduction is the creation of a DNA copy. 

Cells use chemical reactions to build copies of their DNA. This creates two copies of the DNA in a reproducing cell, and they will need to be separated from each other. 

However, keeping one copy of DNA In the original cell and simply pushing the other one out would not work, because the copy pushed out would not have any organised cellular structure for maintaining life processes. 

Therefore, DNA copying is accompanied by the creation of an additional cellular apparatus, and then the DNA copies separate, each with its own cellular apparatus.

Effectively, a cell divides to give rise to two cells.

These two cells are of course similar, but are they likely to be absolutely identical?

The answer to this question will depend on how accurately the copying reactions involved occur. 

No bio-chemical reaction is absolutely reliable. 

Therefore, it is only to be expected that the process of copying the DNA will have some variations each time. 

As a result, the DNA copies generated will be similar, but may not be identical to the original. 

Some of these variations might be so drastic that the new DNA copy cannot work with the cellular apparatus it inherits. Such a newborn cell will simply die. On the other hand, there could still be many other variations in the DNA copies that would not lead to such a drastic outcome. Thus, the surviving cells are similar to, but subtly different from each other. This inbuilt tendency for variation during reproduction is the basis for evolution, as we will discuss in the next chapter.

7.1.1 The Importance of Variation

Populations of organisms fill well-defined places, or niches, in the ecosystem, using their ability to reproduce. The consistency of DNA copying during reproduction is important for the maintenance of body design features that allow the organism to use that particular niche. Reproduction is therefore linked to the stability of populations of species. 

However, niches can change because of reasons beyond the control of the organisms. Temperatures on earth can go up or down, water levels can vary, or there could be meteorite hits, to think of a few examples. 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 c yyhance for them to survive. 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.

The Importance of Variation

Variation is crucial in the process of reproduction as it helps species adapt to changes in their environment. These differences arise due to genetic recombination, mutations, and sexual reproduction. Variations ensure that some individuals in a population possess traits that help them survive and thrive under adverse conditions, such as diseases or climate changes.

For example, if all organisms were identical, a single unfavorable condition could wipe out the entire population. Variation provides resilience, aiding in the survival and evolution of species over time.

The importance of DNA copying in reproduction lies in the following key aspects:

1. Transfer of Genetic Information: DNA copying ensures that the genetic information is passed on from parent to offspring, maintaining the characteristics of the species.

2. Variation: While DNA copying is highly accurate, occasional minor changes (mutations) during the process introduce genetic variations. These variations are essential for the evolution and adaptation of species to changing environments.

3. Cell Functioning: DNA contains instructions for the functioning and development of cells. Copying ensures that new cells formed during reproduction have the necessary instructions to grow and perform their roles.

4. Species Continuity: By creating identical or slightly modified copies of DNA, reproduction ensures the survival and continuity of a species over generations.

7.2 MODES OF REPRODUCTION USED BY SINGLE ORGANISMS

Activity 7.1

Dissolve about 10 gm of sugar in 100 ml of water.

Take 20 mL of this solution in a test tube and add a pinch of yeast granules to it

Put a cotton plug on the mouth of the test tube and keep it in a warm place.

After 1 or 2 hours, put a small drop of yeast culture from the test tube on a slide and cover it with a coverslip.

Observe the slide under a microscope.

Activity 7.2

Wet a slice of bread, and keep it in a cool, moist and dark place. 

Observe the 

surface of the slice with a magnifying glass.

Record your observations for a week.

Compare and contrast the ways in which yeast grows in the first case, and how mould grows in the second.

Having discussed the context in which reproductive processes work. let us now examine how different organisms actually reproduce. The modes by which various organisms reproduce depend on the body design of the organisms.

7.2.1 Fission

For unicellular organisms, cell division, or fission, leads to the creation of new individuals. 

Many different patterns of fission have been observed. 

Many bacteria and protozoa simply split into two equal halves during cell division. 

In organisms such as Amoeba, the splitting of the two cells during division can take place in any plane.

Activity 7.3

Observe a permanent slide of Amoeba under a microscope. Similarly observe another permanent slide of Amoeba showing binary fission.

Now, compare the observations of both the slides.

Figure 7.1(a) Binary fission in Amoeba

However, some unicellular organisms show somewhat more organisation of their bodies, such as is seen in Leishmania (which cause kala-azar), which have a whip-like structure at one end of the cell. In such organisms, binary fission occurs in a definite orientation in relation to

Figure 7.1(b) Binary fission in Leishmania

How do Organisms Reproduce?

these structures. Other single-celled organisms, such as the malarial

parasite, Plasmodium, divide into many daughter cells simultaneously

by multiple fission.

Multiple fission in Plasmodium

 

Yeast, on the other hand, can put out small buds that separate and grow further, as we saw in Activity 7.1.

7.2.2 Fragmentation

Activity 7.4

Collect water from a lake or pond that appears dark green and contains filamentous structures.

Put one or two filaments on a slide.

Put a drop of glycerine on these filaments and cover it with a coverslip.

Observe the slide under a microscope.

Can you identify different tissues in the Spirogyra filaments?

In multi-cellular organisms with relatively simple body organisation, simple reproductive methods can still work. 

Spirogyra, for example, simply breaks up into smaller pieces upon maturation. 

These pieces or fragments grow into new individuals. Can we work out the reason for this, based on what we saw in Activity 7.4?

This is not true for all multi-cellular organisms. They cannot simply divide cell-by-cell. The reason is that many multi-cellular organisms, as we have seen, are not simply a random collection of cells. Specialised cells are organised as tissues, and tissues are organised into organs, which then have to be placed at definite positions in the body. In such a carefully organised situation, cell-by-cell division would be impractical. Multi-cellular organisms, therefore, need to use more complex ways of reproduction.

A basic strategy used in multi-cellular organisms is that different cell types perform different specialised functions. Following this general pattern, reproduction in such organisms is also the function of a specific cell type. 

How is reproduction to be achieved from a single cell type, if the organism itself consists of many cell types? The answer is that there must be a single cell type in the organism that is capable of growing, proliferating and making other cell types under the right circumstances.

7.2.3 Regeneration

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 (see Fig. 7.3). Regeneration is carried out by specialised cells. These cells proliferate and make large numbers of cells. From this mass of cells, different cells

undergo changes to become various cell types and tissues. These changes take place in an organised sequence referred to as development. However. regeneration is not the same as reproduction, since most organisms would not normally depend on being cut up to be able to reproduce.

Figure 7.3 Regeneration in Planaria

7.2.4 Budding

Organisms such as Hydra use regenerative cells for reproduction in the process of budding. In Hydra, a bud develops as an outgrowth due to repeated cell division at one specific site (Fig. 7.4). These buds develop into tiny individuals and when fully mature, detach from the parent body and become new independent individuals.

-Tentacles

Bud

Figure 7.4 Budding in Hydra

7.2.5 Vegetative Propagation

There are many plants in which parts like the root, stem and leaves develop into new plants under appropriate conditions. Unlike in most animals, plants can indeed use such a mode for reproduction. 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.

How do Organisms Reproduce?