Introduction to Reproduction in Organisms | Biology Class 12 - NEET PDF Download

What is Reproduction?

• Reproduction is a biological process of the formation of new offspring from pre-existing organisms. 
• Reproduction becomes a vital process without which species cannot survive for long. 
• It ensures continuity of species generation after generation as older individuals undergo senescence and ultimately they die.

Purpose of Reproduction 

1. Continuity of Species: Reproduction maintains the continuity of species. 
2. Population Organization: Reproduction maintains the population of young, adult and aged persons.
3. Variations: Reproduction introduces variations in the organisms. Useful variations are essential for adaptations and evolution.
4. Life: Life exists on earth due to reproduction in organisms.

Basic Features of Reproduction

(i) Synthesis of RNA, proteins, and biochemicals necessary for new cell/tissue production

(ii) DNA replication to transmit genetic information to newly formed cells

(iii) Cell division to produce daughter cells for growth and specialized cell production

(iv) Growth of cells to develop mature reproductive cells/structures

(v) Formation of reproductive units (gametes or spores) through sexual or asexual reproduction

(vi) Formation of new individuals from reproductive units through sexual or asexual reproduction mechanisms

Lifespan

Reproduction is essential for maintaining the continuity of species, an organism's lifespan plays a critical role in achieving this goal.

 What is lifespan?

• The period from birth to the natural death of an organism represents its lifespan. 
• The lifespan of organisms varies from a few days (Butterfly-1 to 2 weeks) to thousands of years (Banyan tree).

The lifespan of different organisms

How Reproduction and Lifespan related?

In general, organisms have a limited lifespan, which means they can only reproduce for a certain period. As an organism ages, its reproductive capacity gradually declines, and eventually, it becomes unable to reproduce altogether. So, the timing and frequency of reproduction are crucial for the survival and propagation of a species.

Modes of Reproduction

Based on whether there are one or two organisms taking part in the process of reproduction.

1. Asexual Reproduction: When the offspring is produced by single parents with or without the involvement of gamete formation, the reproduction is called Asexual reproduction.

There are different types of Asexual reproduction. It can take various forms, including budding, fragmentation, and parthenogenesis:

(a) Budding: Budding is an asexual mode of reproduction where a new organism develops from an outgrowth or bud on the parent organism. The bud grows in size and eventually detaches from the parent to become a new, independent organism.

(b) Fragmentation: Fragmentation is another form of asexual reproduction where an organism breaks into two or more fragments, each of which can grow into a new organism. This process is common in some invertebrates like flatworms and sea stars.

(c) Parthenogenesis: Parthenogenesis is a form of asexual reproduction where an egg develops into a new individual without fertilization. This process is common in some insects, reptiles, and fishes. In parthenogenesis, the offspring inherits genetic material only from the mother.

2. Sexual Reproduction: When two parents (opposite sex) participate in the reproductive process and also involve fusion of male and female gametes, it is called Sexual reproduction.

The process of sexual reproduction can be divided into three main stages: pre-fertilization, fertilization, and post-fertilization.

(a) Pre-fertilization includes the formation and transfer of gametes (sperm and egg cells) in animals and also involves pollination in plants.

(b) Fertilization is the fusion of sperm and egg cells resulting in the formation of a zygote with a unique set of genetic traits.

(c) Post-fertilization involves the development of the zygote into an embryo, which develops into a fetus in animals and a mature plant in plants. This phase includes cell division, differentiation, and organ formation in animals and seed formation in plants.

Asexual Reproduction v/s Sexual Reproduction