Topics to be learn : Why do organisms reproduce? Do organisms create exact copies of themselves? Modes of reproduction used by single organisms Sexual reproduction

Why do organisms reproduce?

Reproduction involves making copies of the blueprint of body design. Basic event in reproduction is the creation of DNA copy. Reproduction is not necessary for an individual to live but necessary for perpetuation of its species.

Reproduction performs the following functions:

• By the process of reproduction: one generation of living organisms produces the next generation i.e., parents produce offspring like themselves and thus, preserve a species and maintain continuity of the race.
• Reproduction is also the means of increasing the population of a species.
• Reproduction plays an important role in evolution by transmitting favourable variations from parents to offspring.

Do organisms create exact copies of themselves?

• An organism reproduces either asexually or sexually, with the sexual process involving the fusing of gametes.
• During sexual reproduction, the DNA copying mechanism is not error free and the resultant errors are a source of variations in population of organisms. Hence no two individuals are absolutely alike in a population.

• DNA found in chromosomes in the nucleus of the cells has the information to create proteins which lead to body design of an organism. If the organisms are to make exact copies of themselves, the DNA should replicate to make an exact copy of itself.
• DNA replicates in the cell with the help of various enzymes and this is accompanied by division of the basic unit of every organism, i.e., the cell.
• In the case of asexual reproduction, the offspring differ slightly from their parents; in sexually reproducing species, these variations are more pronounced due to genetic recombinations during meiosis and fertilisation.

Importance of variations :

• The consistency of DNA replication during reproduction is essential for the preservation of body design characteristics that enable an organism to use a specific ecological niche.
• However, DNA replication is not a foolproof process since it can result in tiny differences that are either harmful or advantageous to the individual depending on the biological niche it occupies.
• The niches may alter as well, and any change in an individual that makes it suited for the niche would be beneficial to its survival. Variations are the building blocks of evolution.
• Variations are useful for the survival of species in changed environmental situations. Example : If there is a population of bacteria living in temperate waters and if water temperature increases by global warming, most of bacteria would die. But a few variants resistant to heat would survive and grow further. Variation is thus useful for the survival of species over long time.

Modes of reproduction used by single organisms :

Asexual Reproduction: The type of reproduction in which only single parent is involved to produce offsprings exactly similar to the parent. In this type of reproduction, meiosis, gamete formation and fusion of gamete does not take place.

Methods of asexual reproduction :

• fission,
• fragmentation,
• budding,
• spore formation,
• vegetative propagation.

Binary fission:

• It is the process of asexual reproduction found in unicellular organisms such as Amoeba.
• In this process, the parent cell divides to give rise to two daughter cells.
• Each daughter cell has a nucleus and gradually grows into an adult organism.
• During binary fission, the nuclear division (karyokinesis) takes place first followed by the division of the parent cell cytoplasm (cytokinesis). Thus, two daughter amoebae organisms are formed.

• Leishmania (which cause kala-azar), which have a whip-like structure at one end of the cell. In such organisms, binary fission occurs in a definite orientation in relation to these structures.
• Other single-celled organisms, such as the malarial parasite, Plasmodium, divide into many daughter cells simultaneously by multiple fission.

Multiple fission:

• This mode of asexual reproduction is found in Plasmodium.

• The nucleus of the parent cell undergoes repeated divisions to produce a number of daughter nuclei. These nuclei are arranged along the periphery of the parent cell. Later, the cytoplasm cleaves and a cell membrane develops around each nucleus to form a number of daughter cells or individuals inside a parent cell.
• Ultimately, under favourable conditions parent cell membrane ruptures and daughter individuals are released.

Know This : Amoeba : Amoeba is a unicellular microscopic organism, irregular in shape. The outermost covering is the cell membrane which extends to form the organs of locomotion called pseudopodia.

 Stages of asexual reproduction in Amoeba :

• Single-celled Amoeba reproduces by the asexual method known as binary fission. During this, Amoeba splits into two equal halves.
• The splitting of the two cells during binary fission can take place in any plane.
• During binary fission of Amoeba, nuclear division starts first, followed by appearance of a constriction in the cell membrane.
• As the nuclear division completes, constriction deepens and divides the parent cell into two halves, each containing one nucleus.
• Finally, two independent daughter Amoebae are formed.

Regeneration :

Planaria can be cut into any number of pieces and each piece grows into a complete Planaria. This development of pieces of an organism into an individual is known as regeneration.

• Regeneration is carried out by specialised cells. These cells proliferate and form large number of cells.
• In this mass of cells differentiation occurs to form various types of cells and tissues.
• These changes take place in an organised sequence referred to as development. It results in the formation of new Planaria.

Budding :

• Budding: It is the process of formation of an offspring from an outgrowth or bud of a parent. The bud may be unicellular or multicellular depending upon the nature of the parent individual.

• In Coelenterates and other multicellular animals, bud consists of a small number of cells surrounded by an epithelium.
• In Hydra it is usually formed in the lower half of the body.
• The buds separate from the parent soon after their formation or after some initial growth.
• In unicellular individuals like yeast many buds develop from single parent and these buds may not separate but remain connected to form a chain.

Budding in Yeast: A small protuberance appears on the upper part of an adult yeast cell. This gradually grows in size. The nucleus undergoes intra-nuclear division or karyochorisis to form two daughter nuclei. One nucleus passes on to the outgrowth. The budding may be repeated resulting in the formation of one or more chains of bud cells. Separated bud may lead a life like individual yeast.

Fragmentation :

Fragmentation: The mode of reproduction of simple multicellular organisation in which body simply breaks up into smaller pieces upon maturity and each piece or fragment grows into a new individual is called fragmentation.

• The relatively simple multicellular organisms which have a simple collection of cells are involved in fragmentation (a type of asexual reproduction).  Example: Spirogyra

Spirogyra : Spirogyra is a multicellular, filamentous, green alga covered by a mucilaginous sheath. All cells of Spirogyra filament are alike and there is no tissue differentiation.

• All multicellular organisms cannot divide cell-by-cell like Spirogyra because the higher multicellular organisms have differentiation of structure and function of cells, while in Spirogyra all the cells are similar and independent units.
• In multicellular organisms, each organ performs a specific function/functions. In such specialised tissues, cell by cell division (for reproduction) would not be possible.

 

Q. How reproduction achieved from a single cell type while the organism consists of many cell types ?

Vegetative propagation: In plants, various vegetative parts such as roots, stem and leaves can regenerate into full individual plant under suitable conditions. This mode of reproduction is known as vegetative propagation.

• Plants like sugarcane, roses, grapes, etc., are grown by vegetative propagation.
• The plants produced by this method are genetically similar to the parent plant.
• In Bryophyllum, the leaves can generate full plants.
• Plants like banana, oranges, roses and jasmine have lost the capacity to produce seeds, so they are also propagated by vegetative methods.

Advantages of reproduction through leaves :

• Bryophyllum plant reproduces through leaves. Many buds are produced in the notches along the margin of a single leaf.
• Each leaf-bud of Brjyophyllum falls on the ground and produces a new plant.
• The plants from the buds are exactly similar to the parent plant.
• A large number of young plants are produced by a single leaf of a plant that helps in survival of the species.

Methods of plant propagation : Three methods of plant propagation which are commonly used by man for growing garden plants are : (i) grafting (ii) cutting and (iii) layering.

To grow plants by cuttings rather than from seeds has the following advantages:

• It is the only method of reproduction in seedless plants.
• Plants raised by vegetative reproduction bear flowers earlier.
• It helps to preserve the type of character that a plant breeder desires.
• It is very easy and economical method for the multiplication of ornamental plants.

Spores formation :

Many fungi propagate by means of spores.

Spores on getting favourable conditions germinate to give rise to new individuals.

Spores formation in Rhizopus :

• Rhizopus has three types of hyphae-one which grows horizontally on the surface are called stolons or stoloniferous hyphae.
• Second type of hyphae arises in clusters from the undersurface of apparent nodes is called rhizoidal hyphae.
• The third type of hyphae grow erect in groups of 2-5 opposite rhizoidal hyphae and are called sporangiferous hyphae,

• The erect hyphae has a blob like structure called sporangia. Each nucleus present in the sporangia gets surrounded by a bit of cytoplasm to form spore.
• The spores are covered by thick walls. After maturation, each sporangium ruptures to release a large number of spores.

Advantage: The spores remain protected by thick walls until they come in contact with moist surface and begin to grow.

Know This : Tissue culture : Tissue culture is the process of creating new plants by removing tissue or isolating cells from a plant's growing tip. The cells are then placed in an artificial medium where they divide rapidly to form a small group of cells or callus. The callus is moved to another medium containing hormones for development and differentiation, and the plantlets are then planted in soil to mature. Many plants can be grown from a single parent in disease-free conditions using tissue culture. This method is widely employed with decorative plants.

  Sexual reproduction :

Sexual reproduction involves two types of gametes—male gamete and female gamete. Male and female gametes fuse (fertilize) to form a new cell called zygote which grows and develops into a new individual.

Why the Sexual Mode of Reproduction? :

Disadvantages of asexual reproduction :

• In asexual reproduction, very little variations occur within generations.
• Asexual reproduction is less significant for evolution of species.
• Asexual reproduction involves only a single individual. It does not require two sexes.
• In asexual reproduction offsprings are produced from one parent and are genetically identical to the parent.

Advantages of asexual reproduction over asexual reproduction :

Sexual reproduction involves two different individuals, male and female sexes.  The offspring is produced by the fusion of male and female gametes in sexual reproduction. In sexual reproduction genetic material from two parents is combined.

Sexually reproducing species is likely to have better chances of survival, because:

• In sexually reproducing organisms, natural selection might take place which may give advantage for survival.
• During sexual reproduction, there might be errors in the DNA copying. This leads to variations which might promote survival advantage. In asexually reproducing organisms, DNA copying is errorless as they tend to preserve their similarities and offsprings are exact copies of parents.

Know This : Meiosis : Meiosis or reductional division is a type of cell division that occurs in the reproductive cells of organisms to form gametes which have half the DNA component of the parent. When the male and female gametes fuse to form the zygote, the DNA is restored to the normal complement. This maintains the chromosome number over generations in an individual. Chromosomes : The chromosomes are identified by their length and staining patterns. For example two normal human karyotypes look alike, but might differ slightly in banding patterns.

Sexual Reproduction in Flowering Plants :

Structure of a typical flower :

• Flower is a modified shoot. It is attached to the stem by a stalk called pedicle.
• Swollen tip of the pedicel is called thalamus. On the thalamus four whorls of floral leaves are arranged.
• The outermost whorl is called calyx which is formed of green leaf like sepals  Second whorl is called corolla which is coloured and formed of petals.
• Third whorl is called androecium which is formed of stamens.
• Fourth (innermost) whorl is called gynoecium or pistils.
• Stamen has two parts—filament and anther. The anther produces pollens. Stamen is the male reproductive structure of the plant.

• Pistil has three parts—basal ovary, long style and stigma at the top. Pistil is the female reproductive structure of the plant.

Unisexual and bisexual flowers :

• When a flower contains either stamens or pistils (carpels), it is said to be unisexual. Examples: Papaya and watermelon.
• When a flower contains both stamens (male part) and pistils (female part), it is said to be bisexual. Examples: Hibiscus and mustard.
• Pollen grain produces male germ-cells (male gamete) in the plants. The ovule contains embryo sac in which development of female gamete (female germ cell or egg) takes place.

Pollinatilon : The transfer of pollen grains from the anther to the stigma of a flower is called pollinatilon. An agent like wind, water or animal is often required. It takes place before fertilisation.

Germination: The development of the embryo (future plant) contained in the seed into a seedling under favourable conditions is called seed germination.

• Seed : The seed has a brown coloured seed coat on the outside. The seed consists of two cotyledon. In between the cotyledons an embryo is present. The part of the embryo which develops into shoot is called plumule and the part which develops into root is called radicle. Radicle comes out of the seed first during germination.

Fertilisation:

• The fusion of male germ-cell (male gamete) with the female gamete (egg) is known as fertilization.
• It is achieved by the development of pollen tube which carries male gamete to the female gamete in the ovule.
• It leads to the formation of zygote, which develops into embryo.

Act of fertilisation carried out in angiosperms :

After pollination, pollen grains start germination on the stigma.

• The intine (inner wall) of the pollen grain develops into pollen tube which comes out through germ pore of the exine.
• This pollen tube pierces through tissues of the style and reaches up to ovule in the ovary. Pollen tube possesses male gametes or sperms.
• The tip of the pollen tube enters the ovule through micropyle and releases male gamete.
• This male gamete fuses with the female gamete present in the ovule and a new cell called zygote is formed.

Changes occur in a flower after fertilization :

• The petals, stamens, style and stigma of the flower fall off.
• Generally sepals dry up and hold the ovary.
• The ovules present in the ovary mature into seeds.
• The ovary matures into fruit. Sometimes the ovary wall may harden as in poppy. Sometimes, ovary wall becomes fleshy and succulent as in tomato or plums.

Reproduction in Human Beings :

Changes occur in the body of male and female human beings after attaining puberty:

• The males show the growth of pubic hair, beards and moustaches, cracking of voice, occasional penis enlargement and erection.
• The females show development of breasts, darkening of nipples, menstruation and growth of pubic hair. Menstruation starts and ovaries start to release egg.

(a) Male Reproductive System :

Reproductive organs of man : 

Male reproductive organs: Reproductive system of male human body consists of the following organs:

(i) Testes:

• A pair of oval shaped testes are present in the scrotum, which are located outside the abdominal cavity.
• They descend into the scrotum at the time of birth.
• The approximate size is 4 x 3 cm and each weighing about 12 g.
• Testes keeps the temperature lower than the normal body temperature, that is required for the formation of sperms.
• Testes produce sperms and male hormone testosterone.

(ii) Epididymis:

• It is coiled tube-like structure which is firmly attached to testis.
• It acts as storehouse of sperms.

(iii) Vas deferens:

• From each epididymus a tube of whitish colour arises. The tube is known as vas deferens.
• The sperms pass through it.

(iv) Urethra:

• It is a tube starting or arising at the urinary bladder.
• It passes through the copulatory organ (penis) to the tip of the glans penis.
• The urethra of male functions both for the passage of sperms and urine.

(v) Penis:

• The penis is the copulatory organ of male for the transfer of the sperms to the female reproductive tract.
• It is about 3" long in the flaccid condition.
• It is highly muscular organ having erectile tissue and vascular spaces.
• Under excitement these vascular spaces are filled with blood and provide additional rigidity. The enlarged tip of the penis is known as glans penis.
• It is the most sensitive part and is protected by a sheath of skin called the prepuce.

(vi) Seminal vesicle and Prostate gland:

• The seminal vesicles are a pair of small lobulated sacs situated at the base of urinary bladder above the prostate gland.
• They join each other to form ejaculatory duct.
• Seminal vesicles secrete a viscous fluid which forms most of the part of semen.
• This fluid also protects the sperms from the acids normally present in the urethra and female reproductive duct.
• Prostate gland is a bilobed gland which produces prostatic fluid.
• Along the path of the vas deferens, these glands add their secretions so that the sperms are now in a fluid which makes their transport easier and this fluid also provides nutrition to sperms.

Know This : Sperms : The sperms are tiny bodies that consist of mainly genetic material and a long tail that helps them to move towards the female germ-cell.

(b) Female Reproductive System :

The female reproductive organs and their functions :

(i) Ovaries:

• The paired ovaries lie on either side of the uterus or lower part of the abdominal cavity.
• Each ovary is 3 cm long, bean-shaped with the size of unshelled almond.
• The functions of the ovary are to produce eggs (ova) and hormones notably estrogens and progesterone.

(ii) Fallopian tubes (Oviducts):

• These are the two tubes attached to the uterus one on either side.
• Each tube is about 10-12 cm in length.
• The fallopian tubes carry egg (ova) from ovary to uterus.
• It is the site of fertilisation.

(iii) Uterus (Womb):

• It is a pear-shaped hollow muscular organ situated between the urinary bladder and the rectum.
• It receives the fallopian tubes on either side of uterus and is capable of undergoing great enlargement.
• Uterus contains developing young one and expels it by muscular contractions at birth. It is the site of development of young one.
• The uterus develops thick and spongy lining to receive fertilised egg. When the egg is not fertilised, uterine lining slowly breaks and comes out as blood and mucous.
• The lower portion of the uterus is narrow and called cervix.
• Uterus provides, nutrition for the development of foetus, and contributes in the formation of placenta.

(iv) Vagina:

• The uterus opens into vagina through the cervix.
• It is a tubular structure also known as birth canal.
• Vagina receives sperms from male and also serves as passage through which fully developed foetus comes out.

Menstruation :

• The first occurrence of menstruation in a woman is known as menarche.
• The uterus develops thick and spongy lining to receive fertilised egg.
• When the egg is not fertilised, uterine lining slowly breaks and comes out as blood and mucous.
• This cycle takes place roughly every month and is known as menstrual cycle (menstruation) Fertilisation :
• The fusion of male gamete (sperm) with the female gamete (egg) is known as fertilization. It leads to the formation of zygote, which develops into embryo.

What happens after the sperms enter the human female body? :

• The sperms enter through the vaginal passage during sexual intercourse.
• They travel upwards and reach the oviduct where they may encounter the egg.
• The sperm may fuse with the egg and form a zygote.
• The zygote gets implanted in the lining of the uterus and starts developing into foetus.
• The mother’s body is designed to undertake the development of the child for which the uterus prepares itself every month to receive and nurture the growing child.

Placenta : A special umbilical cord that develops in the fourth week is called placenta. It establishes an intimate connection between the foetal membrane and the uterine wall.

Structure of placenta: Disc like structure embedded in the uterine wall. It has villi on the embryo’s side and blood supply on the mother’s side which surround the villi. The functions of placenta:

• The exchange of materials between the mother’s blood and the blood of foetus takes place through placenta.
• It serves as nutritive, respiratory and excretory organs of the foetus.

(c) What happens when the Egg is not Fertilised? :

• The ovary releases one egg every month and the uterus prepares for the plantation of the fertilised egg or zygote by thickening of its walls.
• If the egg is not fertilised, the uterine lining slowly breaks down and comes out through the vagina as blood and mucous.
• This cycle takes place roughly every month and is known as menstruation and lasts for about two to eight days.

Know This : Menopause : It is the cessation of menstruation, i.e., menstrual cycle. Generally, it occurs after the age of 45-50 years. This indicates the end of the fertility phase in a woman. Eggs : Eggs are produced in female reproductive organs and are known as the female gametes. Eggs are non-motile.

Role of hormones secreted by the gonads :

• Regulate the formation of sperm and ovum (gametogenesis).
• Maintain the structure and function of accessory sex organs.
• Help in developing secondary sexual characters such as cracking of voice, growth of facial hair in males; development of mammary glands in females.

 (d) Reproductive Health :

Methods of Contraception :

The methods used for regulation of child birth are.

• Barrier method: In this method physical devices like condom, cervical cap and diaphragm is used to prevent sperms to reach up to the ovum.
• Chemical method: In this method specific drugs are used by females. These drugs may be (a) oral pills or (b) vaginal pills. Oral pills mainly contain hormones and are called oral contraceptives. They disturb hormonal balance so that eggs are not released and fertilised.
• Intra uterine (contraceptive devices (IUCDs): IUCD like copper-T is placed safely in the uterus by a skilled doctor. IUCDs prevent implantation of the fertilised ovum inside the uterus. This technique is not meant for males.
• Surgical method: This method is applicable to both males and females. In males, a small portion of vas deferens and in female a small portion of fallopian tube is surgically removed or ligated (tied). It is known as vasectomy in males which prevents release of sperms from the testes. In females, it is called tubectomy.

The use of contraception method helps:

• To avoid frequent pregnancies which have very adverse effect on the body and mind of the mother. Only a healthy mother can take care of her family properly.
• In controlling the size of the family and pace in the birth of children.
• It helps in developing children properly and maintaining the living standards. These techniques have direct impact on the health and prosperity of the family as they regulate birth control. Mother’s health and economy of the family also improves considerably.

Sexually transmitted diseases (STD) : 

The sexual act is very intimate connection of bodies and thus many diseases can be sexually transmitted. These include bacterial infections like gonorrhoea and syphilis and viral infection like warts and AIDS (Acquired Immuno Deficiency Syndrome).

Symptoms:

• Lesions, irritation or inflammation in the mucous membrane of urino-genital tract.
• Loss of body immunity or defensive system to fight against infections. Prevention of STDs: Using condom (a covering) for the penis during sex helps to prevent transmission of many of these infections to some extent.

AIDS (Acquired Immuno Deficiency Syndrome) : 

• AIDS is a lethal and fatal disease caused by the HIV virus.
• The virus enters the body mainly through sexual contact with infected person, blood transfusion and by an infected mother to the foetus.
• The virus destroys the White Blood Cells (WBCs) in humans and makes the body very susceptible to other diseases.

Human population explosion - Causes and Control :

In our country the population is increasing exponentially with equivalent of population of Australia being added every year This puts strain on the natural resources of the country and is major cause of poverty in the country.

Causes :

• Decline in the child mortality and death rate.
• Increase in average life span of individuals.

Control:

• Using books, posters, and other forms of mass media to inform the public about the drawbacks of big families and the benefits of small families.
• Contraception prevents unwanted pregnancies.
• Encouraging people to utilise contraception or surgical methods to plan their families through family planning facilities or other channels. They must be convinced that these practises don't impair a person's potential.

Know This : Reproduction boosts population. Population size is determined by the rate of birth and death. The size of the population is unimportant if society is unequal. Reproduction is not always a personal choice. Outside pressure from friends and relatives or from the government Carl Influence an individuals decision.