How do Organisms Reproduce?

 How do Organisms Reproduce?

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.

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 the 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.

The Importance of DNA Copying In Reproduction

The 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.

Double strand helix structure for DNA

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 biochemical 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 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.

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 chance 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.

Work in progress 

Sexual Reproduction

Sexual reproduction takes place by the combination of special reproductive cells called 'sex cells'. The sex cells are commonly known as gametes. Please note that sex cells or gametes are also called germ cells. 

Types of gametes 

👉  Male gametes

👉 Female gametes

In sexual reproduction, a male gamete fuses with a female gamete to form a new cell called a 'zygote'.  This zygote then grows and develops into a new organism in due course of time. 

Sexual Reproduction in Flowering Plants

Longitudinal section of flower

👉The reproductive parts of angiosperms are located in the flower.

👉 Angiosperms are flowering plants that produce seeds enclosed within a fruit. They are the most diverse and widespread group of plants on Earth.

👉 The 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.

It is made of two parts, anther & filaments.

👉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.