How do organisms reproduce?

Class 10 Chapter 6: How do Organisms Reproduce 

 Introduction

• The ability to produce a new organism is known as reproduction.
• There are two types of reproduction:
  • Asexual reproduction, and
  • Sexual reproduction.

Do organisms create exact copies of themselves?

• Reproduction at its most basic level involves making copies of the blueprints of body design. 
• The chromosomes in the nucleus of a cell contain information for the inheritance of features from parents to the next generation in the form of DNA (Deoxyribose Nucleic Acid) molecules. 
• The DNA in the cell nucleus is the information source for making proteins. If the information is changed, different proteins will be made. Different proteins will eventually lead to altered body designs.
• The cells use chemical reactions to build copies of their DNA. This creates two copies of the DNA in a reproducing cell, and they will need to be separated from each other.
• To ensure that, a cell divides itself into two cells, each cell containing its own DNA and cellular apparatus. 
• Now, both these cells will be identical, but we cannot say that the copying of the DNA will be 100% identical. 
• There might be some variations in the chemical reaction which means organisms cannot create exact copies of themselves. 

Different types of Reproduction

• There are mainly two types of reproduction- asexual reproduction and sexual reproduction.

Asexual Reproduction

It is a mode of reproduction involving a single parent. There are different modes of asexual reproduction-

• Fission is defined as the splitting of the organisms into two halves and each half gives rise to a new organism.

• For example, Amoeba, and bacteria. The most common type of fission is binary fission.

• Binary Fission: It is a division of organisms such as bacteria into two cells.

• Binary fission can be irregular (division can take place in any plane), longitudinal (division occurs longitudinally), transverse (division occurs transversely) and can also be oblique (division occurs obliquely).

Fig.1. Binary fission in Amoeba

• Fragmentation is another mode of asexual reproduction in which an organism breaks into pieces and each piece gives rise to a new organism.

• For example, Spirogyra, Planaria.

Fig.2. Fragmentation in Planaria

Regeneration

• Regeneration is the ability to form new organisms from body parts. Cut or broken parts generate a new organism.

• Regeneration takes place with the help of a special class of cells called regenerative cells.

• Regenerative cells have the capability to differentiate into various other types of cells of the body like skin cells, muscle cells, etc.

• For example, Hydra, Planarians can reproduce through regeneration.

Fig. 3. Regeneration in Hydra

• Budding is defined as an outgrowth from the body of the organism.

• Outgrowth is due to accumulation of regenerative cells at one place.

• This outgrowth then detaches from the body and forms a new independent organism.

• For example, Hydra and Yeast.

Fig .4. Budding in Hydra

Vegetative Propagation/Reproduction

• In this form of asexual reproduction, stem, root and leaves are used to form plants when provided with suitable conditions.
• Layering and cutting are the two common methods used for vegetative propagation. For example, banana, rose, jasmine etc.  
• The plant produced through vegetative propagation is genetically identical to the parent plant.
• Cutting involves the rooting of the severed piece of the plant.
• Layering involved rooting the piece of the plant and then severing it.
• Grafting occurs when two plant parts are joined together such as stem and root.
• The stem of the plant to be grafted is known as the scion, and the root is called the stock.

Fig.5. Vegetative Propagation

• Fungi reproduce by forming microscopic spores in their fruiting body.
• Examples are bread mould (Rhizopus) and mushrooms.
• Spores when fall on a suitable substrate like bread, and with proper growing conditions (moister, temperature, etc.), starts growing and produces network like structure called hyphae.
• Hyphae produces an upward filament like structure which has a sac like top called sporangium.
• Sporangium bears thousands of microscopic spores and upon maturity it ruptures to release all the spores in the air.

Fig.6. Spore formation in Rhizopus

Sexual Reproduction

• Sexual reproduction involved two different parent organisms involving a female and a male parent.

Significance of Sexual Reproduction

• Sexual reproduction is the source of variation. 
• The mixing of two organisms gives rise to new recombinants or variants.
• Sexual reproduction involves the mating of germ cells also known as gametes.
• These gametes are haploid, that is, they have a half set of chromosomes.
• These gametes are formed through the process of meiosis.
• When male gametes and female gametes each with a haploid set of chromosomes combine, they will form a diploid zygote.
• Zygotes undergo repeated divisions to form a new organism.
• In humans, the male gamete is small and motile whereas the female gamete is large and non-motile.

Sexual Reproduction in Flowering Plants

• The flower is the reproductive structure found in angiosperms (Flowering Plants).
• The flower consists of sepals, petals, stamens and pistils.

Fig.7. Structure of flower

• A flower is said to be unisexual if it contains either stamens or pistils whereas if both stamens and pistils are present on the same flower, it is known as bisexual.
  • Papaya and watermelon are unisexual whereas Hibiscus and mustard are bisexual.
• Pistil/carpel is the female reproductive structure that consists of a swollen basal ovary, middle elongated style and terminal stigma. The ovary contains ovules and each ovule bears an egg cell.
• Stamen is the male reproductive part and it consists of the anther and the filament. The anther contains pollen grains that fuse with a female gamete, that is, egg cells. Fusion leads to zygote formation which forms seeds. Mature seed germinates to form a new plant.
• Transfer of pollen grains from the anther to the stigma of the flower is known as pollination.
• When the pollen and the stigma are of the same flower, it is known as self-pollination.
• When pollen from one flower lands on the stigma of another flower it is known as cross-pollination.

Gametogenesis

• The formation of gametes is known as gametogenesis.
• The male gamete is pollen grains whereas the female gamete is present inside the ovary.
• The ovary contains an ovule.
• The ovule contains the female gametophyte.
• Ovule also consists of outer layers known as integuments, nucellus and female gametophyte.
• Male and female gametes are haploid.
• There are two types of gametes- homogametes and heterogametes.
• When male and female gametes cannot be differentiated morphologically. They are known as homogametes. For example, gametes in Cladophora and algae.
• When male and female gametes can be differentiated morphologically, they are known as heterogametes.

Post Fertilisation Events

• The most important post-fertilization structure is the zygote, embryo and seeds.
• The zygote divides to form a ball of cells called an embryo.
• Zygote first forms a pro-embryo which later converts into a mature embryo.
• Seeds are the result of sexual reproduction.
• Ovules mature into seeds whereas the ovary develops into fruits.

Double Fertilisation

• In flowering plants, one sperm fertilises the egg cell, whereas the other sperm fuses with the two polar nuclei forming the endosperm.
• This is known as double fertilisation as two fertilisation events are taking place.
• The zygote divides to form 7 celled and 8 nucleated embryo sacs.
• Out of these 7 cells and 8 nuclei, there are two synergids with egg cells, 3 antipodals and two polar nuclei.
• Two polar nuclei fuse with one sperm and the other sperm fuse with the egg cell to form the zygote.

Fig.8. Double fertilisation

Reproduction in Human Beings

• Humans reproduce sexually.
• When girls and boys attain puberty (reproductively active) there occur lots of changes in their bodies.
• The development of hair in armpits and genital regions is common to both males and females.
• Girls have increased breast size, darkening of skin and tips of nipples etc.
• Some changes such as thick hairy facial growth, voice changes occur in males.

Male Reproductive System

• The male reproductive system comprises a pair of the testis, glands, accessory ducts, and male genitalia.
• The testis is the site where male gametes or germ cells are produced.
• They are located outside the abdominal cavity in a sac-like structure known as the scrotum.
• This is to maintain the lower temperature required for the formation of sperm.
• Testis produces the male hormone testosterone needed for the development of secondary sexual characteristics in males such as the formation of beard and moustaches and also in the development of sperm.
• Vas deferens is a duct that transports sperm to the urethra, which is a common passage for both urine and sperm ejaculation.
• Prostate glands and seminal vesicles are also found in males to nourish and for easy transport of sperm in the female genital tract.
• Cowper’s gland produces mucus-like fluid that neutralises the acidity of the female vagina.
• All these secretions along with sperm form the semen.

Fig.9. Male Reproductive System

• The formation of male gametes or sperm in testes is known as spermatogenesis.
• Sperms are haploid in nature.
• Seminiferous tubules are the site for spermatogenesis.
• Testis produces a male hormone known as testosterone needed for the male secondary sexual characteristics as well as for spermatogenesis.

Female Reproductive System

• The female reproductive system consists of a pair of ovaries, uterus, cervix, vagina, and external genitalia.
• Female eggs or ova are produced in the ovaries.
• The formation of ova in the ovaries is known as oogenesis.
• The ovary produces female hormones such as oestrogen and progesterone.
• These hormones are needed for female sexual development as well as for pregnancy.
• The fallopian tube carries ova from the ovary to the womb.
• Two oviducts joined to form the uterus. Uterus then opens into Vagina via the cervix.

Fig.10. Female Reproductive System

• Sperm enter into the female vagina at the time of sexual intercourse.
• Then the sperm reaches the fallopian tube where it fuses the ova to form a zygote. This is known as fertilisation.
• Then the zygote divides to form an embryo.
• The embryo gets implanted into the uterus.
• The embryo development occurs in the uterus to form the foetus.
• Mother supplies nutrition to the growing foetus via the placenta.
• The placenta helps in the exchange of nutrients, gases and the removal of excretory products from the developing body of foetus.
• The development of a child inside the womb of the mother takes place for about 9 months.
• Then the rhythmic uterine contraction leads to the delivery of the baby outside the female body.

Fig.11. Human Placenta (feotus developing inside uterus)

• If the egg is not fertilised, the uterine lining is shed off in the form of fluid known as menstrual fluid.
• The discharge occurs from the vagina as blood and mucus. This is known as menstruation. It lasts for about 2 to 8 days.

Reproductive Health

• It is defined as the state of wellbeing in terms of safe sex, reproductive fitness as well as the absence of any reproductive diseases.
• Unsafe sex leads to different diseases which are known as sexually transmitted diseases.
• Some of the sexually transmitted diseases are as follows-
  • Gonorrhoea is caused by bacteria.
  • Syphilis is caused by bacteria.
  • AIDS (acquired immunodeficiency syndrome) is caused by a virus HIV (human immunodeficiency virus).

In-vitro Fertilisation (IVF)

• IVF is an infertility treatment method.
• In this case, the egg is fertilised with the sperm outside the female body.
• Ovum/egg is removed from the female body and is allowed to fertilise with sperm outside the body in in-vitro conditions.

Birth Control Methods

• The sexual act has the potential to lead to pregnancy, which can put major demands on the body and mind of the female.
• For a country like India where the population is increasing continuously, there is a need for birth control methods to avoid unwanted pregnancies.
• Condoms (on penis) and diaphragms are mechanical barrier methods for birth control. They prevent the binding of sperm to the ovum.
• The chemical method of birth control includes oral pills and vaginal pills.
  • These hormonal pills do not allow release of the egg and fertilisation cannot occur.
  • Pills can change the hormonal balance of the body and hence can have side effects also.
• Intrauterine contraceptive devices (Copper-T) are also there to prevent the implantation of embryos in the uterus.
  • They can also cause side effects due to irritation of the uterus.
• Surgical methods include vasectomy in males and tubectomy in females.
• Vasectomy is done by cutting the Vas deferens and then tying them up.
• Tubectomy involves cutting and tying a small portion of oviducts.
• Surgeries can also be used to remove unwanted pregnancies.
  • This can be misused by people who do not want a particular child.
  • This may also lead to sex selective abortions.
  • For healthy society male-female sex ratio must be maintained.

Fig. 12. Birth Control Methods