Reproduction

Reproduction is defined as the biological process by which organisms give rise to their own kind. Reproduction may occur in two ways:

1. Asexual reproduction
2. Sexual reproduction

Bacterial and protozoan offspring may be produced by single individuals. This is termed asexual reproduction. Certain animals and many plants reproduce asexually. When two individuals are involved in reproduction, it is termed sexual reproduction. In sexual reproduction, male gamete fuses with female gamete to mark the beginning of a new individual. This is a more common mode of reproduction in plants and animals.

Asexual Reproduction

Reproduction by single individuals takes place in many ways in lower organisms like bacteria and protozoa and some algae. In plants, asexual reproduction is by vegetative propagation. Animals like sponges and hydra reproduce both asexually and sexually. Asexual reproduction is of various types:

Binary fission

A cell may divide to give rise to two individuals and lose its own identity as in binary fission that takes place in amoeba and bacteria.

Budding

In budding, a bud forms from the body of the mother cell and remains attached to it. The parental nucleus elongates and then divides into two, one of which moves into the bud. Example: yeast.

Spore Formation

The cytoplasm and nucleus of algae such as Chlamydomonas divide successively to form 4 to 8 spores. Spores are also formed for reproduction in fungus, moss and fern. Spores are single cells which upon their release from the parent plant develop into new individuals.

Vegetative Propagation

In nature, new plants may arise from root, stem or leaves (from the vegetative parts of the plant). This form of asexual reproduction in plants is termed vegetative propagation. New plants may be formed from roots or stem or leaves or when a stem grows to a distance and then enters soil and strikes roots to form a new plant.

Sexual Reproduction in Plants

The reproductive organ of flowering plants is the flower. Stamens (Androecium) which produce pollen are the male part. Pollen grains contains male sex cells. There may be several stamens in each flower. Each stamen (Androecium) has two parts. The upper part is known as anther which bears pollen. It is held on the lower part called filament.

The pistil (Gynoecium) is the female part and its basal part is the ovary carrying eggs or ovules or female sex cells. The parts of the pistil are the stigma, style and ovary.

In most plants, each flower bears both male and female parts. They are termed bisexual. In some plants there are male flowers with only androecium and female flowers bearing only gynoecium. They are unisexual.

Pollination and Fertilisation

For fusion of the nuclei, pollen and ovule are brought together by several agencies like the wind, water, and insects. This transfer of pollen grain from anther to the stigma of a flower is called pollination. Self pollination is when pollen of a flower falls on its own stigma and fertilizes the ovule. In Cross pollination, pollen from one flower falls on the stigma of another flower of a different plant of the same species and then fertilizes the ovule of that flower. Agents like wind, water or insects help to transfer pollen from one flower to another.

For fertilization or fusion of nuclei of pollen and ovule, pollen is brought by any pollinating agent on the stigma of the pistil. Each pollen grain forms a pollen tube and pollen grain nucleus reaches the ovule as pollen tube pushes through the pistil. The fertilized ovules develop into seeds which are capable of germinating into seedlings and new plants.

Sexual Reproduction in Animals

In all animals, the female produces eggs and the male produces sperms. An egg and a sperm fuse to form the zygote which then develops into the embryo and the embryo into a full-fledged individual. 

The development may occur partially or completely, inside the egg. Such animals that lay eggs include fish, frog, reptiles and birds and are thus called oviparous. In mammals such as cats, dogs, cows and humans, the baby develops inside the mother’s womb. They are termed viviparous.

In tapeworms and earthworms both female and male sex organs are in the same individual. Such individuals are termed hermaphrodite. Sexes are separate in all other animals, and the male individual has male organs like testes, etc. while the female has ovaries etc. Humans also belong to the animal kingdom. They are mammals and hence viviparous.

Reproduction in Humans

The period from infancy to reproductive maturity in humans includes childhood and adolescence.

Adolescence in human beings

The body undergoes natural changes as one grows into the reproductive period of life. These changes begin around the age of 10 to 11 and last till 18 to 19 years of age. This stage of life is called adolescence.

The time period when changes occur in humans make them capable of reproduction, is called puberty. In both the females and males, sex organs mature during adolescence.

Menstruation

In human females, reproductive phase begins at puberty and lasts till the age of 45 to 50 years. A girl is born with fixed number of ova (eggs). However, these begin to mature only at puberty. One ovum matures at a time and is released from the ovary once in 28 to 30 days. The egg (ovum) travels down the fallopian tube to reach the uterus.

The first menstrual bleeding is called menarche. Stoppage of menstruation at an age usually between 45 years and 55 years is termed menopause.

Reproductive Organs in Humans

Reproduction in humans require two individuals - a male and a female.

Male Reproductive Organs

• A pair of testes: Generate Sperms
• Two sperm ducts or vasa diferentia: Each arises from the testis and passes up into the body to join together and form the ejaculatory duct
• One ejaculatory duct: Common duct for passage of urine and sperms
• One Penis: Muscular organ which helps to transfer sperms into female body

Female Reproductive System

The female reproductive system is located in the lower abdomen. The ovary and oviduct are commonly found in all female animals and the uterus in those who do not lay eggs but give birth to young.

• A pair of ovaries: Produce ova
• Two fallopian tubes: The oviducts through which eggs pass from the ovaries into uterus
• One uterus: The womb in which the embryo develops
• One cervix: The opening of uterus
• One vagina: Female opening

Fertilization and Embryonic Development

The nuclei of sperm and egg fuse inside the egg, forming the zygote. This fusion is termed fertilisation and takes place in the fallopian tube (the oviduct). 

The fertilised egg or zygote begins to divide repeatedly and upon reaching a stage containing cells and a cavity called blastocoel, gets embedded in the thickened uterine wall in which many blood capillaries have formed.

In case fertilisation does not occur the egg disintegrates. The thickened wall of uterus along with capillaries breakdown leading to bleeding or menstrual flow (menstruation). In human female menstruation occurs every 28 to 30 days.

The embryo, now called foetus develops into a full fledged individual in 280 days inside the uterus and is born under the influence of a hormone from posterior pituitary, called oxytocin.

RTI and STD

Reproductive Tract Infections (RTIs) refer to infections of reproductive organs. These illnesses may occur due to poor genital hygiene, for example, poor menstrual hygiene among girls. RTIs include the illnesses that are transmitted from one person to another during sexual contact and are known as Sexually Transmitted Diseases (STDs).

Sexually Transmitted Diseases (STD)

These infections may be transmitted during vaginal or anal intercourses, or genital skin contact. Gonorrhea, syphilis, herpes, chlamydia, warts and chancroid are common STDs. Human Immuno-deficiency Virus (HIV) can also be transmitted through sexual contact.

HIV/AIDS

Human Immunodeficiency Virus (HIV) causes Acquired Immuno-Deficiency Syndrome (AIDS). HIV is a retro virus, i.e., its genetic material is RNA. It destroys vital cells of the immune system making the body vulnerable to several infectious agents.

It infects T-lymphocytes and makes thousands of copies of the virus. HIV-infected individual may remain asymptomatic for 10-15 years. Gradually, the number of T-helper cells of the immune system decrease in number to a low when the victim loses resistance against other diseases. This is the stage of full-blown AIDS.

HIV may be transmitted from one infected individual to another individual by the following mechanisms:

• Unprotected sexual intercourse
• Infected blood
• Infected syringes and needles
• Infected mother to her baby in utero (in the womb), during child birth and through breast milk