Reproduction In Organisms Class 12 Notes

Reproduction is a biological process in which an organism give rise to young ones (offsprings). There is a continous cycle of birth, growth and death.

TYPES OF REPRODUCTION

reproduction occurs in two ways namely asexual reproduction and sexual reproduction.

Asexual Reproduction

When single parent produces offsprings without gamete formation, similar to itself i.e., offsprings produced are the exact copies of their parents, the reproduction is called as asexual reproduction.

Characteristics of Asexual Reproduction

Only one parent is involved.
The new organisms are produced only by somatic cells and hence, also called as somatogenic reproduction.
It occurs by mitotic division.
Formation and fusion of gametes do not occur.

Sexual Reproduction

When two parents of opposite sex, participate in the reproductive process and involve the fusion of male and female gametes, it is called sexual reproduction.

Characteristics of Sexual Reproduction

It involves two participants of opposite sex-male and female.
in this, meiotic division occurs during gamete formation and mitosis during development.
The offsprings produced are genetically different from parents.

Events in Sexual Reproductions

The events that take place during the sexual reproduction can be grouped into three categories,

pre-fertilisation
fertilisation
post-fertilisation

Pre-Fertilisation

All the events of sexual reproduction that occur before the fusion of gametes are included in this category. Two main pre-fertilisaton events are gametogenesis (formation of gametes) and copulation or gamete transfer, (close association of male and female for the fusion of gametes).

Fertilisation

The process of fusion of male gametes with the female gametes is called fertilisation or syngamy. It results in the formation of a diploid zygote.

It is mainly of two types

External Fertlisation

When process of fertilisation takes place outside the body of an organism.

EXAMPLE:- in aquatic organisms like algae, fishes and amphibians.

Internal Fertilisation

When process of fertilisation takes place inside the body of an organism.

EXAMPLE :-In most terrestrial organisms like fungi, higher animal and majority of plants such as bryophytes, pteridophytes, gymnosperms and angiosperms.

Post-Fertilisation Events

During the sexual reproduction, the events which occur after the fusion of gametes are called post-fertilisation events. These events include the formation of a zygote and embryogenesis.

The process of development of embryo from the zygote.

Based on whether the development of the zygote takes place outside or inside the body of the female parent, animals are divided into two categories.

(i) Oviparous

(ii) Viviparous

(i) Oviparous

Those animals in which the development of zygote takes place outside the female parent are called oviparous.

They lay fertilised eggs covered with hard calcareous shell in a safe environment

e.g., reptiles and birds. After a period of incubation, the young ones hatch out from the egg.

(ii) Viviparous

Those animals in which the Everyday Science development of zygote takes place inside the body of the female parent are called viviparous. After attaining a certain stage of growth, the young ones are delivered out of the female’s body,

EXAMPLE :- Human, monkeys and all other mammals

Reproduction in Animals

Animals reproduce by both asexual and sexual means.

Asexual Reproduction

It is the primary means of reproduction among the protists, cnidarians and tunicates. The process of asexual reproduction can occur through following methods

Regeneration

It is the formation of whole body of an organism from the small fragment of its body, e.g., planaria Hydra, etc.

Fission

The parent body is divided into two or more parts, which become new individual e.g. planarians, protozoans, etc.

Budding

Small projection or outgrowths in protozoans and sponges. Projection is called bud later bud develops into new organisms e.g. yeast and coelenterates.

Fragmentation

The parent body breaks into two or more fragments. Each fragment becomes new organism e.g. sponges and echinoderms .

Strobilisation

In this, the ring-like constrictions are developed and organisms look like a pile of minute saucers e.g., Aurelia.

Spore Formation

The propagule which germinates to new individual e.g., zoospore conidia, etc.

Gemmules

These are the asexual reproductive structure present in several sponges. These are oval in shape.

Sexual Reproduction

In animals, the sexual reproduction occurs by the fertilisation of haploid sperm and haploid egg, generating a diploid offspring.In most individuals the female produces eggs, and the male produce sperms.

In other individual, (i.e., monoecious) such as earthworm and many snail, single individual produce both sperm and egg. These individuals are called as hermophrodite.

The union of sperm and egg occurs in variety of ways, depending on the mobility and the breeding environment of individual. Sexual reproduction is of two types

Reproduction in Humans

Humans are viviparous, unisexual and have specialised cells that take part in sexual reproduction, called gametes (sperms in male and ova in females). These gametes are produced by organs called gonads.

Sperms and egg cell fuse to produce a zygote (fertilised egg) that results into a new multicellular organism by further development (mitotic division) in it.

NOTE :- Puberty is the time phase in males and females when they become sexually mature. The puberty age in girls begins at 10-11 yrs and completes at age of 15-17 yrs whereas, in boys, it begins at the age of 11-12 yrs and completes at 16-17 yrs.
Signs of puberty in males are hair growth in pubic area and armpits, muscle growth, voice deepening, facial hair development and size increment of penis.
Whereas, the signs of puberty that appear in girls are breast development, hair growth in the pubic area and armpits and onset of menstruation cycle. When a person reaches a puberty, certain hormones in the body affect the sweat glands especially those in the armpits that cause odour.

Male Reproductive System

Glands(ग्रंथियों)*

Testes

Paired
Primary sex organs; posterior to the penis within the scrotum <—–LOCATION
Produce spermatozoa (gametes) and testosterone (male sex hormone). <—–FUNCTION

Seminal vesicle

Paired
Club-shaped glands posterior to the prostate, attached to the ejaculatory ducts. <—–LOCATION
Secrete alkaline fluid containing nutrients fructose and prostaglandins. <—–FUNCTION

Cowper’s gland

Paired
Pea-sized glands inferior to the prostate; empty into the membranous urethra. <—–LOCATION
Secrete fluid that lubricates urethra and end of penis <—–FUNCTION

Prostate gland

Unpaired
Walnut-sized gland at the base of the urinary bladder, surrounding the prostatic urethra. <—–LOCATION
Secretes alkaline fluid that helps neutralise acidic environment of the vagina. <—–FUNCTION

Ducts (नलिकाओं)*

Epididymis

Paired
Mass of tubule attached to the posterior surface of the testes. <—–LOCATION
Site of sperm maturation; store spermatozoa <—–FUNCTION

Vas deferens

Paired
Ducts extending from the epididymis to the ejaculatory ducts. <—–LOCATION
Store spermatozoa; transport spermatozoa during ejaculation. <—–FUNCTION

Ejaculatory duct

Paired
Short ducts between the ductus deferentia and the prostatic urethra. <—–LOCATION
Receive spermatozoa and additives to produce seminal fluid. <—–FUNCTION

Supporting structures

Scrotum

Unpaired
Pouch of skin, posterior to the Penis <—–LOCATION
Encloses and protects testes. <—–FUNCTION

Penis

Unpaired
Pendant organ anterior to the scrotum and attached to the pubis, external genitalia <—–LOCATION
Carry urine and seminal fluid to outside of body: organ of coitus. <—–FUNCTION

Female Reproductive System

Glands(ग्रंथियों)*

Ovaries

Paired
Primary sex organs; upper pelvic cavity on both lateral sides of uterus <—–LOCATION
Production of ova (gametes) and female sex hormones pestrogen and progesterone. <—–FUNCTION

Vestibular glands

Paired
Subcutaneous within the wall of the vaginal opening. <—–LOCATION
Secrete lubricating fluid into the vestibule and vaginal opening during coitus. <—–FUNCTION

Mammary glands

Paired
Composed of lobes within the breasts. <—–LOCATION
Produce and secrete milk for nourishment of an infant. <—–FUNCTION

Fallopian tube

Paired
Open-ended tubes that extend from the ovaries to the uterus. <—–LOCATION
Convey ova toward uterus; site of fertilisation; convey developing blastocyst to uterus. <—–FUNCTION

Ducts(नलिकाओं)*

Uterus

Unpaired
Hollow-shaped like an inverted pear; maintained in position within the pelvic cavity bu muscles and ligaments.<—–LOCATION
Site of implantation; sustains life of embroys and foetus during pregnancy’ plays active role in parturition. <—–FUNCTION

Vagina

Unpaired
Hollow-positioned between the urinary bladder and urethra anteriorly and the rectum posteriorly; external genitalia. <—–LOCATION
Conveys uterine secretion to outside of body; receives erect penis and semen uring coitus; passage way for foetus during parturition. <—–FUNCTION

Labia majora

Unpaired
Two longitudinal folds of skin that extend from the perineum; separated longitudinally by the pudendal cleft. <—–LOCATION
From margins of pudendal cleft. enclose and protect other external reproductive organs. <—–FUNCTION

Supporting structures

Labia minora

Unpaired
Two longitudinal folds of skin medial to the labia majora; separated longitudinally by the vaginal vestibula. <—–LOCATION
From margins of vestibule protect openings of vagina and urethra. <—–FUNCTION

Clitoris

Unpaired
Rounded projection at the upper part of the pundendal cleft, sheathed by a prepuce.<—–LOCATION
Provides feeling of pleasure during sexual stimulation. <—–FUNCTION

Menstrual Cycle

It is the sequence of changes that start in female body at the age of puberty (13 to 15 yrs) in the uterus and ovary for the purpose of sexual reproduction.

The start of menstrual cycle is known as menarche and it continues up to the age of 45-50 years. After that it ceases (stops) and this phase is known as menopause. The cyclic activity proceeds after every 28/29 days from the previous cycle.

Mechanism Involved with Human Reproductive System

Gametogenesis

formation of male and female sex cells or gametes, i.e., sperms and ova in the male and female gonads (testes and ovaries) respectively. It is divided into two main types

1.Spermatogenesis
2. Oogenesis

1.Spermatogenesis

The process of formation of a sperm is called spermatogenesis. It niferous tubules of the testes. A spermatogonium (sperm mother occurs in the seminiferous tubules of the test cell) produces four functional spermatozoa.

2. Oogenesis

It is the process of formation of a matur female gamete (ovum). It occurs in the ovaries (female gonads).

It consists of three phases

Multiplication
Growth Phase
Maturation Phase

Fertilisation

It is the fusion of a haploid male gamete (spermatozoan) and a haploid female gamete (ovum ) to form a diploid cell, the zygote. It is an internal process that occurs in the ampulla region of the female’s fallopian tube

Embryonic Development

During embryonic development, different organs are formed from the three basic germ ayers namely ectoderm, mesoderm and endoderm.

Parturition

It is the act of expelling the fully formed young one from the mother uterus at the end of gestation period. This process is induced by both neural and hormonal system. Hormones are secreted by the endocrine glands of the mother.

Oxytocin promotes the contraction of the uterine muscles. It is also called birth hormone, while relaxin increases the flexibility of the pubis ligaments and helps in the dilation of uterine cervix during labour pains (pains of child birth).

Lactation

Production of milk in the mammary glands is called lactation. It starts towards the end of pregnancy and after the birth of the young one. The first milk produced after the birth of baby is called colostrum.

It is rich in proteins, calories and antibodies (IgA). Milk synthesis is under the control of prolactin hormone, while its release is controlled by oxytocin hormone released by pituitary gland.