Reproduction is the formation of new similar young living organisms by the grown up individuals of a species or race. It is meant for the perpetuation of the race/ species because individuals are bound to die after a life span. Reproduction provides group immortality. Four processes are basic to reproduction— DNA replication, cells division, the formation of reproductive units and development of a new individual. Animal reproduction is of two types, asexual and sexual.
Reproduction is the inherent property of the living organisms to continue their race Sby producing offsprings. Reproduction is of two types: (i) Asexual reproduction, (ii) Sexual reproduction
Asexual Reproduction System
The development of new individuals without the fusion of the male and female gamates is known as asexual reproduction. The asexual reproduction usually includes mitotic division of the body (somatic) cells. Therefore it is also known as somatogenic reproduction. The sexual reproduction is common only in lower plants and animals and it is of following types:
- Binary fission: In the binary fission, the body splits in such a way that two equal and identical halves are produced. It is most common in protozoans, bacteria, and some lower metazoans.
- Budding: In certain multicellular animals, e.g. hydra and fungi the body gives out a small outgrowth known as the bud, the bud is supported by the parent body and it ultimately develops into a new individual.
- Gemmule formation: In certain metazoan animals, e.g. fresh water sponges and in some bryophytes such as Marchantia, the asexual reproduction is carried on by certain peculiar asexual bodies known as gemmules, which is composed of a group of undifferentiated cells which contain stored food material.
- Spore formation: In many unicellular plants, fungi etc. the asexual reproduction by spore formation is very common under unfavorable conditions. In this process the condensation of protoplasm due to reduction in the water content results. These spores under favorable conditions absorbs water and germinate to form a new plant.
- Vegetative reproduction: A fragment or part of the plant grows to produce new plants. Fragmentation is very common in algae and fungi. In higher plants the vegetative propagation takes place in many ways under the following ways:
- Sucker stem as in mint banana which run horizontally under ground can be separated and grown individually and each is able to develop into separate plant. The vegetative multiplication in the certain storage fleshy leaf like Bryophyllum has also been found very useful.
- Tubers like potato, when cut into pieces with a bud in each piece and sown each grow into individual tuber plants.
- Bulbs are also propagated by separating the young bulb eg. Crinum or adventintious ends of the scale leaves eg. Urginea.
It is a mode of multiplication in which the new individuals or young ones are formed through the process of formation and fusion of gametes. Haploid (gametes)- diploid (individual) attraction occurs. Gametogenesis involves meiosis. Fusion of gametes or fertilization restores the chromosome number. The offspring is produced by growth of the fusion product of gametes called zygote. Sexual reproduction is commonly bi-parental.
- Exogamy: Sexual reproduction involving gametes from different parents (bi-parental).
- Unisexual: Organism in which the two sexes occur in different individuals, e.g., humans, mammals, birds, lizards.
- Bisexual/Hermaphrodite: Organism in which the two types of sex organs (mal and female) occur in the same individual, e.g., Earthworm.
Sexual reproduction in plants
This is a very common type of reproduction in the angiospermic plants. Meiosis (reduction division) occurs in the male and female parts of a flower to form male (pollen grains) and female (ovum) gametes. In order to produce seeds (to reproduce) male and female gametes fuse together to form zygote that divides mitotically and develops into a seed. The process of transference of pollen grains from the another of a flower to the stigma of the same or of different flower is called pollination. It is of two types:
If the pollen grains are transferred to the stigma of the same flower or between two flowers borne by the same parent then it is self pollination or autogamy.
If the pollen-grains are transferred to the stigma of a different flower borne by a different plant then it is called cross-pollination or allogamy. All unisexual flowers are cross pollinated. However, the bisexual flowers also show the following characteristics to favor cross-pollination.
Primary Sex Organs: Gonads which form gametes are called primary sex organs — testis (plural testes) in males and ovary (plural ovaries) in females. Testis produces sperms and secretes testosterone (formation and maintenance of secondary sex organs, accessory male glands, and external sex characters). Ovary produces ova. Maturing Graffian follicles secrete estrogens for development and maintenance of secondary sex organs, accessory or external sex characters and part of menstrual cycles. Progesterone produced by ruptured Graffian follicles or corpus luteum controls a part of menstrual cycle, implantation, and development of placenta.
Secondary Sex Organs: Sex organs, glands, and ducts which do not produce gametes but are otherwise essential for sexual reproduction are known as secondary sex organs. In human male reproductive system, the secondary sex organs are vasa efferentia, epididymes, vasa deferentia, ejaculatory ducts, seminal vesicles, urethra, prostate glands, Cowper’s glands and penisin. Secondary sex organs of a human female include fallopian tubes, uterus, vagina, external genitalia, Bartholin glands and mammary glands.
Accessory/External Sex Characters: They are traits which do not have any direct role in reproduction but provide specific features and structures to the two sexes. The important external/ accessory sex characters of the human male are beard, mustaches, body hair on shoulder and chest, pubic hair on both lateral and vertical directions, comparatively more height with more muscular body, larynx apparent externally, voice low pitched with breathing more by means of diaphragm. The important accessory sex characters of human females are high pitched voice, breast, broader pelvis, lateral pubic hair, rounded body contous with more subcutaneous fat in thighs buttocks and face, and a sternal breathing.
Puberty: Beginning of sexual maturity or ability to reproduce is known as puberty. Primary sex organs being functioning. Secondary sex organs develop fully under the influence of sex hormones produced by primary sex organs. Growth is rapid. It is accompanied by the slow development of accessory/ external sexual characters. Puberty occurs at the age of 10-14 years in girls and 13-15 years in boys. Characteristics of Human Reproduction
- Human beings are nonseasonal breeders.
- There is no oestrus/ heat.
- In human females, the ability to produce young ones begins at menarche (beginning of menses) and ends at menopause (stoppage of menses).
- In human females, the reproductive phase has 28 days repeated the menstrual cycle.
- Fertilization is internal.
- There is vivipary. i.e., giving birth to young ones.
- The fetus develops inside the uterus and is nourished by a joint special structure called placenta.
- Infants can be fed on mother’s milk.
- Parental care is very well developed.
- Male Genital
Below the bladder and between the legs the plum sized testes and expandable sausage-like penis exist which make and deliver sperm into a woman.
- Sperm and male sex hormones are made in two plum sized glands called the testes.
- The testes lie in a sac called the scrotum that hangs outside the body, between the legs. This arrangement maintains sperm at a temperature slightly below body temperature, which is essential for the production of healthy sperm.
- Each testis has a long highly coiled tube attached to it known as an epididymis. Immature sperm passes out of the testes into the epididymis. As sperm make their way through these tubes, they mature and learn to swim. Maturation takes between one and three weeks, after which they are either ejaculated out of the body or reabsorbed into the body.
- Every day a man makes between 50 and 500 million sperm.
- Sperm and urine both leave the body through a tube called the urethra that runs down the middle of the penis.
- Semen is a milky white, sticky mixture of sperm and fluid secretions. It provides nutrients to energize sperm and acts as a transport medium to carry sperm through the penis, out of the body and into a woman’s vagina.
- During ejaculation, sperm is propelled from the epididymis along a duct called the vas deferens. This carries sperm upwards, out of the scrotum, towards the penis.
- En route, fluids from the seminal vesicles (the small red glands at the end of the vas deferens) and the prostate gland dilute the sperm. These fluids provide the optimal environment for sperm to swim and survive in the acidic conditions of the female reproductive tract.
- During ejaculation, a man produces between two and five milliliters of semen that contains between 50 and 130 million sperm per milliliter.
- The tiny bulbourethral glands secrete a thick, clear mucus that drains into the urethra. It cleans the urethra before ejaculation and acts as a lubricant during sex.
Behind the bladder there is uterus, or womb, the size of an upside-down pear which produces eggs, have sex, carry a developing baby and to give birth.
- The female genitals consist of two glands called ovaries that are responsible for producing eggs and female sex hormones.
- Once women reach sexual maturity, they experience a hormone regulated monthly fertility cycle known as a menstrual cycle. These hormones control the maturation and release of an egg from one of the ovaries every month, and they affect the lining of the womb making it ready to receive a fertilized egg.
- Mature eggs are released from the ovary and move along the fallopian tube to the uterus. If an egg is fertilized, it tries to embed itself in the uterus wall, ready to develop into a foetus. If no egg becomes implanted, the uterus lining is shed, and this results in a flow of menstrual blood, known as the ‘period’.
- When a baby girl is born, she already carries all the eggs that she will ever produce through her life. A boy, however, does not carry sperm at birth, and only begins to produce sperm when he reaches sexual maturity.
It is a series of cyclic changes that occur in the reproductive tract of human females and other primates with a periodicity of 28 days, right from menarche to menopause. It is characterised by menses or loss of blood for a few dyas. Menstrual cycles consists of the following phases.
It is the phase of menstrual flow menses which continues for 3-5 days and involves the discharge of blood (a total of 50-100 ml). Menstrual phase is also called funeral of unfertilized egg or shedding tears of lost ovum. The first day of menstrual phase is also considered to be first day menstrual cycle.
Anterior pituitary secretes FSH which stimulates follicular cells of Graafian follicle to secrete estrogen
Recovery Phase. It last for 2 days and brings about repair of ruptured blood vessels and mucous lining or endometrium of reproductive tract.
Proliferative Phase. The endometrial lining begins to thicken, especially that of uterus. There is development of blood capillaries, elongation, and coiling of uterine glands, greater activity of uterine muscles thickening and development of more cilia in the epithelial lining of fallopian tubes.
Production of FSH decreases while that of LH increases. Presence of both FSH and LH causes ovulation. The ovum is drawn into the fallopian tube. It is viable for two days when fertilization can occur. Ovulation takes place between 10-14 days (fertility period 10-16 days of menstrual cycle).
It operates under the influence of progesterone produced by corpus luteum (empty proliferated Graafian follicle and supervised by LH of the anterior pituitary. Endomitrial lining thickens and its glands become secretary. Uterine movements are reduced. The stage is meant for receiving fertilized ovum. In the absence of fertilization, corpus lutem degenerates. LH level falls. Progesterone level is reduced. Reduced level of both progesterone and estrogen causes menses.
It is a series of cyclic changes that occur in the reproductive tract of nonprimate mammalian females with variable periodicity in different animals. There is a period of estrous or heat near the time of ovulation due to rising level of estrogen. It lasts for only 18 hours in cow. The female receives the male only during estrous or heat. There is generally specific period of oestrous (= estrous, except for domesticated mammals), may be two (spring and autumn for bitches), called breeding seasons. In case of no fertilization, the endometrial breakdown is not accompanied by rupturing of blood vessels. Hence, there is no bleeding.