How do Organisms Reproduce?

"Creation of new organisms from the existing organism of the same species is known as reproduction."

1.0Introduction

Reproduction is one of the important process by which every living organism makes a copy like itself. It is the means of multiplication and perpetuation of the species.

In reproduction, lot of energy goes waste to produce new individual, but it is essential for the survival of a species or for the continuity of life on the earth because the older individuals of each species undergo senescence and die.

• In previous classes, we learnt that the chromosomes in the nucleus of a cell have information for inheritance of features from parents to the next generation in the form of DNA molecules.

Reproduction at its basic level (cellular reproduction) is involved in making similar or dissimilar body designs through the genetic material (DNA) present in the chromosomes of its nucleus. DNA is the source of information for making proteins. Any change in the information leads to production of different proteins, which ultimately lead to altered body designs.

Reproduction is not an essential life process, means it is not necessary to reproduce in order to survive, one can survive without undergoing reproduction also, yet it is a unique property of living beings. Importance of variation: Basic event in reproduction is production of DNA copies in a reproducing cell. The process is called DNA replication. Cells use chemical reactions to build copies of their DNA. When the cell divides into two, each new cell gets a copy of each DNA or chromosome along with the whole cellular apparatus.

Complete accuracy in DNA copying leads to two exactly identical cells but any error in duplication can lead to dissimilar cells or variations.

Variations are differences in structure, physiology and other traits found in individuals of species. If there were a population of bacteria living in temperate regions and if the water temperature were to be increased by global warming, most of these bacteria would die but the few variants resistant to heat would survive and grow further. Variation is, thus useful for the survival of species over time.

Why is simply copying DNA in a dividing cell not enough to maintain continuity of life?

• Explanation: Simply copying DNA in a dividing cell is not enough to maintain continuity of life because an organized cellular structure (with genetic material) is required to maintain life processes. So, DNA copying must be accompanied by the creation of an additional cellular apparatus otherwise simply copying DNA is not enough to maintain continuity of life.

• Some variations in new cell might be so drastic that the new DNA copy cannot work with the cellular apparatus it inherits. These newborn cells will simply die.

2.0Significance of reproduction

For maintenance of continuity of a species: Since every organism dies after a certain age so by reproduction new individuals are born which replace them. For adding variation for evolution: By reproduction new variations are added as you can see the members of a family having variation in colour, shape, size, intelligence, body capacity etc, some are better suitable than other which are selected by nature and others die and are removed. For maintenance of population size: By reproduction new members are added which maintains the population size.

Why is variation beneficial to the species but not necessary for the individual?

• Explanation: Variation is beneficial for the species as variations make species more adapted to survive with the changing conditions. A particular variation in an individual may not be suitable for any given condition but when condition changes such variation may save it from being dead and leads to formation of a new population with suitable character. It is not necessary for the individual.

Types of reproduction: There are two types of reproduction in living organisms -

• Asexual reproduction
• Sexual reproduction

1. Asexual reproduction: Production of offsprings by a single parent without the formation and fusion of gametes is called asexual reproduction. It is a primitive type of reproduction in which offsprings are produced by a cell or any vegetative organ of an organism. In this type of reproduction, offsprings are genetically identical to their parents.

2. Sexual reproduction: It is a type of reproduction in which two different sex cells (gametes) take part. The gametes often come from two different individuals called male and female, so they bring some variations in the next generations. All sexually reproducing organisms have two sets of chromosomes called homologous chromosomes. In such set one set of chromosomes come from each parent. The number of chromosomes is reduced to one set by a kind of cell division called meiosis in which four cells are produced each with only one set of chromosomes, which later develops into gametes. Thus, when two such sex cells (gametes) fuse they restore the number of chromosomes of that species.

• Following way we see that the number of chromosomes remain same 46 generation after generation.
  

Sexual reproduction involves following steps - a. Formation of gametes (by meiotic cell division) b. Fusion of gametes (Formation of zygote) c. Development of zygote to new individual

How are number of chromosomes maintained in sexually reproducing organisms? Explanation: Two gametes take part in sexual reproduction which, on fusion give rise to a zygote. If each gamete is having a full set of chromosomes their number will double each time. But during gamete formation, a different kind of cell division takes place in which number of chromosomes in gametes is reduced to half of original number. So when such gamete fuses the number of chromosomes restores to the full set.

Modes of asexual reproduction: These are fission, budding, regeneration, fragmentation, spore formation and vegetative propagation.

(1) Fission: It is a kind of asexual reproduction in which unicellular organisms create two or more new individuals. It can be of two types.

(a) Binary fission: One cell split into two equal halves, e.g., many bacteria and protozoa like Amoeba, Paramecium and Leishmania. In Amoeba fission can take place in any plane.

In 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. i.e. longitudinal binary fission.

(b) Multiple fission: One cell divides in repeated fashion into many daughter cells simultaneously. e.g., Plasmodium (Malarial parasite) and Amoeba in unfavourable conditions.

Why is it said that Amoeba never dies a natural death?

• Explanation: Amoeba never dies a natural death because every time they reproduce by asexual means the parent cell is divided into daughter cells and thus, they never get old to die.

(2) Budding: Process in which an outgrowth (bud) is formed on the body of parent organism which then detaches when fully mature and becomes a new independent organism. e.g., Yeast and Hydra.

Hydra uses regenerative cells for reproduction in the process of budding. In Hydra, a bud develops as an outgrowth due to repeated cell division at one specific site. These buds develop into tiny individuals and when fully mature, detach from the parent body and become new independent individuals.

What are tentacles?

• Explanation Tentacles are the extension of the body layer of Hydra. It encircles the opening of mouth. It helps in capturing the prey.

(3) Regeneration: The process of getting back a full organism from the body parts of the parent individual is called regeneration. Regeneration is carried out by specialised cells. These cells proliferate and make large number of cells. These changes in organised sequence is called as development.

Hydra and Planaria can be cut into any number of pieces and each piece grows the missing parts and form a complete organism.

Regeneration is the ability of an organism to replace its lost body part. This ability to regenerate is due to presence of stem cells. Higher animals have limited power of regeneration (tail in wall lizard, arm in star fish). Regeneration is not same as reproduction. Most organisms would not normally depend on being cutup to be able to reproduce.

(4) Fragmentation: In this process an organism breaks up into two or more fragments and each fragment develops into an adult organism. e.g. Spirogyra. Spirogyra, simply breaks up into smaller pieces upon maturation which grow into new individuals.

How is reproduction to be achieved from a single cell type, if the organism itself consists of many cell types?

• Explanation: There must be a single cell type in the organisms that is capable of growing, proliferating and making other cell types under the right circumstances.

(5) Spore formation: It takes place in fungi (multi-cellular organisms). The thread-like structures that develop on the bread are the hyphae of the bread mould (Rhizopus). The tiny blob-on-a-stick structures(sporangia) are involved in reproduction. Sporangia contains cells or spores that develops into new Rhizopus individuals.

The spores are covered by thick walls that protect them until they come into contact with another moist surface and can begin to grow.

Spores are light weight, so they are easily carried away by air and water current to new locations and have a tough covering over them which protect them from harsh conditions of environment like dryness and high temperature.

(6) Vegetative propagation: This is an asexual method of reproduction in plants where vegetative parts like root, stem and leaves give rise to new plants in appropriate conditions.

Natural vegetative propagation:

Plants reproduce without the help of human beings. By leaves: Leaves of some plants produce adventitious buds with adventitious roots on their margin, which get detached and develop into new plants e.g. Bryophyllum, Kalanchoe.

By stem: Underground modified stems possess buds that produce aerial shoots under favourable conditions when planted in soil. e.g. Potato, Ginger, Onion.

By roots: Modified tuberous roots when planted in soil, the buds present on the roots grow into leafy shoots above ground. E.g. sweet potato, Dahlia.

Artificial vegetative propagation:

These methods are man-made and developed by plant growers to prepare plants with desirable characters. These are of four types: Cutting: In this method, a small part of plant (stem, root or leaf) is cut and buried partly in the moist soil then cutting develops roots and grows into a new plant. Stem cuttings are most commonly used for artificial vegetative propagation. e.g. Rose, Sugarcane, Potato, Cactus.

Grafting: Two dicotyledonous plants of closely related varieties are joined together so that they live as one plant. The plant of which roots remain in the soil is called as stock. Cutting part of a plant that is grafted on the other rooted plant is called scion. e.g. Mango, Apple, Lemon. Grafting is not possible in monocot plants. Cambium activity is essential for the union of stock and scion.

Layering: This is the most common method of propagating herbaceous plants. In this method the lower branch of the parent plant is bent and buried in the moist soil with the growing tip of the branch remains above the soil surface. The portion of the branch which is in contact with the soil develops adventitious roots and this rooted branch is called layer.

Layer is then detached from the parent plant and grows into a new plant. e.g. Citrus, Grapevine, Jasmine and Hibiscus.

Tissue culture or Micropropagation: Cells or tissue are isolated from the growing tip of plant called explant. The explant develops into undifferentiated mass of cells called callus in the proper culture medium. The callus is transferred to another medium containing hormones for growth and differentiation, that forms plantlet. The plantlets are transplanted into pot or soil to form mature plant. This technique is known as micropropagation. Using tissue culture, many plants can be grown from one parent in disease-free conditions. This technique is commonly used for ornamental plants. e.g. Orchids, Chrysanthemum.

Advantages of vegetative propagation (i) It is a rapid, cheap and easy method of reproduction for the multiplication of plants. (ii) Disease free plants can be produced. (iii) Superior quality fruits or flowers can be produced by grafting. (iv) Genetically identical plants are produced. (v) Plants raised by vegetative propagation can bear flowers and fruits earlier than those produce from seeds. (vi) Propagation is also possible for plants that have lost the capacity to produce seeds. e.g. banana, rose and jasmine.

Advantages of asexual reproduction (i) It is only method of reproduction in most unicellular organisms. (ii) The parental properties are preserved. (iii) It is rapid method as it does not require any sexual maturation, production of gametes, transfer of gamete and their fusion.

Why is vegetative propagation favoured for growing some kinds of plants?

• Explanation: There are several reasons for which vegetative propagation is favoured - (i) Either they do not produce seeds or their seeds are not viable. (ii) Their qualities are good and we want to preserve those qualities. (iii) Only male or female plant is available. (iv) Fast growth

Differences between asexual and sexual reproduction

Sexual reproduction in flowering plants:

Structure of flower: Sexual reproduction takes place within specialized reproductive organs, called flowers in angiosperms (flowering plants).

Flower is a modified condensed reproductive shoot on which all the floral appendages are inserted. A typical flower has four whorls arranged on thalamus.

• Calyx
• Corolla Essential organs or reproductive organs
• Androecium
• Gynoecium
• Calyx: It is the outermost whorl consisting of sepals. Sepals are green and leaf like structure. Calyx protects the flower bud before it opens.
• Corolla: It is the second whorl, inner to calyx, consisting of petals. Petals are generally large, coloured and showy. Corolla attracts insects for pollination.
• Androecium: It is the third whorl, inner to corolla, consisting of male reproductive parts called stamens. Each stamen has two parts, filament and anther. Anther is bilobed structure present at the tip of filament. Each anther produces pollen grains that are yellowish in colour. Each pollen grain produces two male gametes/germ cells.
• Gynoecium: It is the fourth and innermost whorl consisting of carpels. Carpel is present in the centre of flower. Each carpel has three parts that is Ovary, Style and Stigma. Ovary is a swollen basal part of carpel that contains one or several ovules. Each ovule contains an embryo sac that bears an egg (female gamete). Style is the middleelongated part of the carpel and has sticky part called stigma above it which receives pollen grains.

The flower may be unisexual (papaya, watermelon, date palm, mulberry, gourd) when it contains either stamens or pistil or bisexual (Hibiscus, mustard) when it contains both stamens and pistil. If both sepals and petals are coloured and cannot be distinguished from each other, then their whorl is known as perianth. e.g. Tulip and Lily. Calyx and corolla are non-essential parts of the flower because they are not directly involved in reproduction.

3.0Pollination:

Process in which pollen grains are transferred from the ripe anther to the stigma. It is of two types:

(i) Self-pollination: It is the transfer of pollen grains from an anther to the stigma of the same plant. If it is in the same flower, it is called autogamy (e.g. Pea) and if it is between flowers of the same plant then it is called geitonogamy (e.g. Maize).

(ii) Cross pollination: It is the transfer of pollen grains from anther to the stigma of different plants of the same species, it is also called xenogamy. (e.g. Mango).

Agencies of pollination: Transfer of pollen from one flower to another is achieved by agents like wind, water or animals.

What is the significance of bright colours of flowers?

• Explanation: The bright colour of flowers is meant to attract insects which help in pollination.

Fertilization in Plants: Fertilization is the process of fusion of the male and female gametes to form a diploid zygote which takes place in the embryo sac present in the ovule.

After pollination, pollen grains germinate on the stigma by producing pollen tube. It then penetrates the stigma and passes through the style. The nucleus in the pollen tube divides into two male gametes/germ cells. Pollen tube enters the ovule through a narrow pore called micropyle. It releases two male gametes in embryo sac.

One male gamete fuses with egg cell and second male gamete fuses with the two polar nuclei.

• One male gamete + Egg cell $\underset{\text{ Syngamy }}{\overset{\text{ Sygote }}{\to }}$ Zygote
• Second male gamete + Two polar nuclei $\underset{\text{ (Primary Endosperm Nucleus) }}{\overset{\text{ Triploid nucleus }}{\to }}$ Syngamy + Triple fusion $=$ Double fertilization.

A fully developed typical angiospermic embryo sac consists of seven cells and eight nucleus stage. 3 antipodal cells, 2 synergids, central cell containing 2 polar nuclei (which fuse to form diploid secondary nucleus) and 1 egg cell.

Differences between pollination and fertilization

4.0Post fertilization changes in the flower

The petals and stamen wither off, however the calyx may be present in some cases. Style and stigma degenerate. Ovary develops into fruit and ovule grows into seed. The seed contains the future plant or embryo which develops into a seedling under appropriate conditions. This process is called as germination.

True fruits are those which are formed from ovary of a flower e.g. Guava, grapes. While if any other part of flower like thalamus or other modifies into fruit, it is called false fruit e.g. apple, pineapple.

Seed: A typical seed is formed from the ovule of a flower. It includes two basic parts an embryo and a seed coat. Embryo is the fertilized egg formed by repeated division of zygote. Seed contains one cotyledon or seed leaf in monocots and two cotyledons in dicots. In angiosperms usually there is store of nutrients for the embryo called endosperm. The seed coat develops from the integuments, originally surrounding the ovule. The seed coat is either bitegmic or unitegmic. Bitegmic seeds have two integuments outer testa and inner tegmen while unitegmic seed have only one integument.

Some plants, like wheat and marigold, produce seeds only once in a year and die out, while others continue to produce seeds for many years such as mango, apple, etc.

Seed remains inactive for sometimes, this resting period is called dormancy. Dormancy protects seed from all unfavourable conditions. On germination first the radicle comes out which later develops into root and then plumule which later develops into shoot.

Human Reproductive System: Human beings become reproductively active with some signs, these signs are visible from a certain age called the age of puberty.

Puberty: The age at which the gametes and sex hormones are produced and the boy and girl become sexually mature is called puberty.

Generally female pubertal age is 10-12 years, male pubertal age is $13-14$ years.

Secondary sexual characters

Sexual characteristics are physical or behavioural traits of an organism which are indicative of its biological sex. These can include sex organ (primary sexual character) used for reproduction and secondary sexual characteristics which distinguish the sexes of a species, but which are not directly part of the reproductive system. The reproductive organs in humans are testes and ovaries which produce the gametes, sperm in male and ova in female.

Secondary Sexual Characters in Male

(i) Widening of shoulders (ii) Deepening of voice (iii) Appearance of beard and moustaches (iv) Growth of sex organs (Testes and penis)

Secondary Sexual Characters in Female

(i) Widening of pelvis and hips (ii) High pitch voice (iii) Initiation of menstrual cycle (iv) Growth of mammary glands (breasts) (v) Maturation of secondary sex organs like fallopian tubes, uterus

5.0Human male reproductive system

The male reproductive system consists of portions which produce the germ cells and sex hormones and other portions that deliver the germ-cells to the site of fertilisation. (1) Scrotum: The scrotum is a pouch of skin that encloses the testes. These arise from the lower abdominal wall and act as a thermoregulator, maintaining the testes at a temperature $2^{\circ }\mathrm{C}$ lower than that of the body which is required for normal development and maturation of sperms. (2) Testes: They are the primary sex organs. A pair of testes are located outside the abdominal cavity in scrotum. They produce the germ cells or sperms. They also secrete male sex hormone testosterone into the blood that brings about development of secondary sexual characters in boys at time of puberty. (3) Spermatic ducts: The sperms formed are delivered through the vas deferens which unites with a tube, urethra, coming from the urinary bladder. The urethra thus forms a common passage for both the sperms and urine. Hence urethra is also known as urinogenital tract. (4) Sex glands: Along the path of the vas deferens, glands like the prostate gland and the seminal vesicle add their secretions and mucus so that the sperms are now in a fluid that is collectively called seminal fluid or semen which makes their transport easier and this fluid also provides nutrition. (5) Penis: It is the male genital muscular organ carrying the duct (urethra) for the transfer of sperm during copulation into the female body.

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.

6.0Human Female Reproductive System

Human Female Reproductive System (1) Ovaries: The ovaries are the primary sex organs of the female that lie in the lower part of the abdomen. The ovaries like the testes have both exocrine function that is production of ova and endocrine role that is secretion of female sex hormone, estrogen and progesterone. When a girl child is born, the ovaries already contain thousands of immature eggs. On reaching puberty, some of these starts maturing. One egg is produced every month by one of the ovaries. (2) Fallopian tube (oviducts): A fallopian tube is a long muscular tube. It conveys the egg from the ovary to the uterus and also provides the appropriate environment for its fertilization. Oviducts are the sites of fertilization of male and female gametes. (3) Uterus: The uterus is a large, highly elastic sac specialized for the development of the embryo. The two oviducts unite to open into uterus. The uterus opens into the vagina through the cervix. (4) Vagina: It is a large, elastic, muscular tube. It is adapted for receiving the semen during copulation, allowing menstrual flow and serving as birth canal during parturition.

Embryo development and nourishment: The sperms enter through the vaginal passage during sexual intercourse. They travel upwards and reach the oviduct where they may encounter the egg. The fertilised egg (zygote) starts dividing and form a ball of cells or embryo. The embryo gets attached to the lining endometrium of uterus, this is known as implantation where the embryo continues to grow and develop organs to become foetus. The mother's body is designed to undertake the development of the child. Hence the uterus prepares itself every month to receive and nurture growing embryo. The lining thickens and is richly supplied with blood to nourish the growing embryo.

The embryo gets nutrition from the mother's blood with the help of a special tissue called placenta. Placenta is a fully formed reddish brown disc embedded in the uterine wall, that serves to bring the foetal and maternal blood close enough to permit the exchange of materials between the two. It contains villi. On the mother's side are blood spaces, which surround the villi. This provides a large surface area for glucose and oxygen to pass from the mother to the embryo.

The developing embryo will also generate waste substances which can be removed by transferring them into the mother's blood through the placenta.

The development of the child inside the mother's body takes approximately nine months that is 280 days known as gestation period. The child is born as a result of forceful muscular contractions in the uterus, called labour pain.

What happens when the egg is not fertilised?

If the egg is not fertilised, it lives for about one day. Since the ovary releases one egg every month, the uterus also prepares itself every month to receive a fertilised egg. Thus, its lining becomes thick and spongy. This would be required for nourishing the embryo if fertilisation had taken place. If fertilization does not occur, the released egg, and the thickened lining of the uterus along with its blood vessels are shed off. This causes bleeding in women which is called menstruation. It usually lasts for about two to eight days.

Advantages of sexual reproduction

(i) More genetic variations within a species. Genetic variations can lead to evolutionary advancement. (ii) More variations assist with survival. It increases the chance that at least some offsprings of a parent survive. Note: The type of fertilization which occurs inside the body of female is called internal fertilization and fertilization which occurs outside the body of female is called external fertilization. External fertilization takes place in many invertebrates, fish and amphibians and internal fertilization takes place in all reptiles, birds and mammals.

Hermaphrodite: The animals having both male and female reproductive organs are called hermaphrodite animals. e.g. Earthworms.

7.0Reproductive Health

Sexually Transmitted Diseases (STDs) There are many infectious diseases which are spread by sexual contact, called Sexually Transmitted Diseases (STDs) e.g. AIDS, Hepatitis, genital warts etc. STDs occur mostly in the individuals who are involved in sexual activities with many partners.

Methods of prevention of STDs:

(i) The people should be educated about various STDs. (ii) Avoid multiple sexual partners. (iii) No sex without proper precaution. (iv) Finding sources of infection and treating them.

Methods of Contraception:

The prevention of pregnancy in women is called contraception.

• Planned control of population: (i) By educating people about the advantages of small family. (ii) Raising the age of marriage can help in reducing population growth. (iii) By family planning.
• Natural method Intercourse is safe for a week before and week after menstruation.
• Mechanical methods (i) It includes use of condoms which are the rubber or plastic sheets put on the penis before coital activity. (ii) Use of diaphragms or cervical caps fitted in vagina of female to check the entry of sperms into the uterus and also helps in avoiding conception. (iii) Use of IUCD i.e., Intra Uterine Contraceptive Devices like copper T and loops fitted in the uterus, help to prevent fertilization and implantation of embryo. They can cause side effects due to irritation or infection of uterus.

• Chemical methods (i) It consists of using some chemicals which are spermicidal. They may be in form of tablets, jellies, paste and creams introduced in the vagina before coital activity. (ii) Another chemical method is the use of oral contraceptive (OC) pills which inhibit the secretion of FSH (Follicle Stimulating Hormone) and LH (Leutinising Hormone) from the anterior lobe of pituitary gland and thus inhibiting ovulation from the ovary. These contraceptives change the hormonal balance so that egg cell is not released (inhibit ovulation) and hence prevent fertilization.
• Surgical methods (i) Tubectomy involves cutting of fallopian tubes in females and Vasectomy involves cutting of vas deferens of each side in males. If the vas deferens in the male is blocked, sperm transfer will be prevented. If the fallopian tube in the female is blocked, the egg will not be able to reach the uterus. In both cases fertilisation will not take place.
  
  Tubectomy in Females
  
  Vasectomy in males (ii) Surgical removal of ovaries is known as ovariectomy and removal of testes is known as castration. (iii) Another surgical method is MTP i.e. Medical Termination of Pregnancy or abortion. (iv) Other method is tubal ligation in which fallopian tubes are blocked by an instrument called laproscope.

Note:

• In tubal ligation, the fallopian tubes are blocked but eggs continue to be produced because the ovaries are intact. Eggs fail to pass into the uterus and sperms fail to reach the eggs for fertilization.
• Amniocentesis is a prenatal diagnostic technique to determine the genetic disorders in the foetus. Amniocentesis is being misused to kill the normal female foetuses as it can help to detect the sex of foetus also. Determination of sex by amniocentesis has been banned.

8.0Biology Diagrams made Easy

Leaf of Bryophyllum with buds in the margin

9.0Chapter At a Glance

10.0SOME BASIC TERMS

• DNA: Deoxyribonucleic acid
• Scion: Plant part that is grafted on the other rooted plant in grafting.
• Stock: Plant part which root remain in the soil in grafting.
• Explant: Cell or tissue which is isolated from the growing tip.
• Micropropagation: Tissue culture
• Calyx: Group of sepals
• Corolla: Group of petals
• Hydrophily: Pollination by water
• Anemophily: Pollination by wind
• Entomophily: Pollination by insects
• Ornithophily: Pollination by birds
• Chiropterophily: Pollination by bats
• Syngamy: Fusion of egg cell and one male gamete in plants
• Castration: Surgical removal of testes
• Amniocentesis: Diagnostic technique to determine the genetic disorder in foetus.