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Reproduction

Introduction

     Reproduction is defined as the production of individuals of the same species, that is the next generation of the species. While it is one of the fundamental characteristics of living things, it is not an essential life process. An individual can live without reproducing, but, a species cannot survive without reproduction.

Methods of Reproduction

     There are basically two types of reproduction - asexual and sexual. This classification is based on the number of parents. Asexual reproduction involves only one parent and the offspring is genetically similar to the parent.

Asexual reproduction in Unicellular organism

     Asexual reproduction is the production of new individuals by mitotic divisions from a single parent. It does not involve the fusion of gametes, also called sex cells. There are many types of asexual reproduction, all producing individuals that are genetically identical to the parent.

Vegetative propogation

     Regeneration of new plants from the vegetative parts of the parent plant is called vegetative propagation or vegetative reproduction. Vegetative propagation is done with the help of vegetative parts such as roots, stem or leaves. These parts may also be variously modified for vegetative propagation.

Artificial Vegetative Propagation or Cloning

     Vegetative propagation produces the next generation that is genetically identical to the parent. Such an organism that is genetically identical to the parent is called a clone. In case of plants with advantageous characteristics, the characteristics can be preserved by producing clones. This is particularly useful to agriculturists and horticulturists in order to get the best crop and uniform yield every time.

Parthenogenesis and Tissue Culture

     Parthenogenesis is a form of reproduction in which the ovum develops into a new individual without fertilisation. Natural parthenogenesis has been observed in many lower animals (it is characteristic of the rotifers), especially insects, e.g., the aphid. In many social insects, such as the honeybee and the ant, the unfertilized eggs give rise to the male drones and the fertilized eggs to the female workers and queens.

Advantages and Disadvantages of Vegetative Propagation

     The offsprings are genetically identical and therefore advantageous traits can be preserved. Only one parent is required which eliminates the need for special mechanisms such as pollination, etc.

Sexual Reproduction

     Sexual reproduction mostly occurs in higher multicellular plants and animals. However, it is also seen in lower organisms like the bacteria, Spirogyra (an alga) and Paramoecium (a protozoan).

Sexual Reproduction in Plants

     The plants that sexually reproduce have the reproductive structures called the flowers. The flower is a condensed shoot with the nodes present very close to each other. The different parts of the plant are attached to the nodes. All the structures present at one node are collectively called the whorl. The first or the outermost two whorls are called the non-reproductive whorls. They are the calyx and corolla. The inner two whorls are androecium and gynoecium, the reproductive whorls.

Sexual Reproduction in Animals

     Sexual reproduction is seen in nearly all animals. In animals reproduction also involves production of gametes that are haploid cells. In unicellular organisms like the protozoans, the gamete-producing individuals are called gametocytes. The gametes may be similar - isogametes as in Monocystis or dissimilar - heterogametes as in Plasmodium.

Reproduction in Humans

     The human beings are bisexual with two separate sexes - male and female. The individuals of the two sexes show distinctive features called the secondary sexual characteristics. Some of the male secondary sexual characteristics are facial hair, cracking of voice, etc. female secondary sexual characteristics include development of breasts, broadening of hips, etc. Such distinguishing features are present in all the animals. These characteristics serve to identify and attract sex partners.

Reproductive System of Male

     The male reproductive system comprises of a pair of testes that are present in a thin-walled sac called the scrotum. The scrotum is contained within the abdominal cavity in the embryonic stage. Shortly before birth, they come down and remain outside throughout life. This is because the testes cannot produce sperms at the body temperature.

Reproductive Organs of Female

     The female reproductive system consists of a pair of ovaries, a pair of oviducts, uterus, vagina and vulva. The main functions of the female reproductive system are to produce eggs, receive the sperms, provide the site for fertilisation, implantation of the growing embryo and development of the foetus. It also produces hormones that control the various stages of ovulation and maintenance of pregnancy.

Different Phases of Menstruation

     At about the age of 10 to 13 years, the ovaries of females are stimulated by the follicle stimulating hormone (FSH) of the pituitary. This is called the onset of puberty and is accompanied by release of hormones oestrogen and progesterone. These hormones control the production of ova or eggs and appearance of secondary sexual characteristics. Unlike males where sperms can be produced through out the life of man, in females the reproductive phase only lasts till the age of 45-50years. This phase is characterised by the presence of menstrual cycle.

Fertilisation in Human

     Fertilisation can be defined as the fusion of the sperm nucleus with the egg nucleus to form a diploid cell known as zygote.

Reproductive Diseases

     The diseases/disorders affecting the reproductive system are of many types. Some are due to malfunctioning gonads, others are due to pathogens.

Population Control

     Increasing population is a serious issue, particularly in developing countries. It is necessary for every generation to produce more off springs because many individuals do not survive to reach the reproductive age due to natural causes. However, man has upset this equation as he has been successful in bringing down the mortality rate. But a simultaneous increase in the birth rate is also essential to maintain the balance in nature.

Summary

     Reproduction is defined as the production of individuals of the same species, that is the next generation of the species. There are basically two types of reproduction - asexual and sexual.



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