CLASS 12 Biology CHAPTER 1 REPRODUCTION IN ORGANISMS (NCERT Notes)

CLASS 12 Biology                                                                                                  CHAPTER 1                                                                                              REPRODUCTION IN ORGANISMS (NCERT Notes)

The period from birth to the natural death of an organism is called lifespan. Life spans of organisms are not necessarily correlated with their sizes; the sizes of crows and parrots are not very different yet their life spans show a wide difference. Similarly ,a mango tree has a much shorter life span as compared to a peepal tree. No individual is immortal, except single-celled organisms.

Reproduction is defined as a biological process in which an organism gives rise to young ones (offspring) similar to itself. The offspring grow, mature and in turn produce new offspring. Thus, there is a cycle of birth ,growth and death. Reproduction enables the continuity of the species, generation after generation.                                                                                                                                          

The organism’s habitat, its internal physiology and several other factors are collectively responsible for how it reproduces.  Based on whether there is participation of one organism or two in the process of reproduction, it is of two types. When offspring is produced by a single parent with or without the involvement of gamete formation, the reproduction is a sexual. When two parents (opposite sex) participate in the reproductive process and also involve fusion of male and female gametes, it is called sexual reproduction.

ASEXUAL REPRODUCTION                                                                                                  

Single individual (parent) is capable of producing offspring. The offspring that are produced are not only identical to one another but are also exact copies of their parent. Clone is used to describe  morphologically and genetically similar individuals.                     

Asexual reproduction is common among single-celled organisms, and in plants and animals with relatively simple organisations. In Protists and Monerans, the organism or the parent cell divides into two to give rise to new individuals. In these organisms cell division is itself a mode of reproduction. Many single-celled organisms reproduce by binary fission ,where a cell divides into two halves and each rapidly grows into an adult (e.g., Amoeba, Paramecium).                                                                

In yeast, the division is unequal and small buds are produced that remain attached initially to the parent cell which ,eventually  gets separated and mature into new yeast organisms (cells).                                  
Under unfavourable condition the Amoeba withdraws its pseudopodia and secretes a three-layered hard covering or cyst around itself. This phenomenon is termed as encystation. When favourable conditions return, the encysted Amoeba divides by multiple fission and produces many minute amoeba or pseudopodiospores; the cyst wall bursts out, and the spores are liberated in the surrounding medium to grow up into many amoebae. This phenomenon is known as sporulation.                                  

Members of the Kingdom Fungi and simple plants such as algae reproduce through special asexual reproductive structures .The most common of these structures are zoospores that usually are microscopic motile structures.  Other common asexual reproductive structures are conidia (Penicillium), buds (Hydra) and gemmules (sponge). Adventitious buds arise from the notches present at margins of leaves of Bryophyllum.

In plants the term vegetative reproduction is used. The units of vegetative propagation such as runner (mint), rhizome (ginger, banana), sucker(banana) ,tuber (potato), offset( water hycinth) , bulb (onion) are all capable of giving rise to new offspring. These structures are called vegetative propagules.  Since the formation of these structures does not involve two parents, the process involved is asexual.                                                                                                                                                    

Organisms in which the body breaks into distinct pieces (fragments) each fragment grows into an adult capable of producing offspring (e.g., Hydra). This is also a mode of asexual reproduction called fragmentation.

The scourge of the water bodies or about the ‘terror of Bengal’. This is an aquatic plant ‘water hyacinth’ which is one of the most invasive weeds found growing wherever there is standing water. It drains oxygen from the water, which leads to death of fishes.

SEXUAL REPRODUCTION                                                                                                               Sexual reproduction involves formation of the male and female gametes, either by the same individual or by different individuals of the opposite sex. These gametes fuse to form the zygote which develops to form the new organism. Because of the fusion of male and female gametes sexual reproduction results in offspring that are not identical to the parents or amongst them selves. 
All organisms have to reach a certain stage of growth and maturity in their life, before they can reproduce sexually. That period of growth is called the juvenile phase. It is known as vegetative phase in plants. 

Few plants exhibit unusual flowering phenomenon such as bamboo species flower only once in their lifetime ,generally after 50-100 years, produce large number of fruits and die. Another plant , Strobilanthuskunthiana (neelakuranji), flowers once in 12 years. Plant flowered during September-October 2006. Its mass flowering transformed large tracks of hilly areas in Kerala, Karnataka and Tamil Nadu into blue stretches and attracted a large number of tourists.                                                                
In animals the juvenile phase is followed by morphological and physiological changes prior to active reproductive behaviour. Birds in captivity (as in poultry farms) can be made to lay eggs through out the year. Laying eggs is not related to reproduction but is a commercial exploitation for human welfare.
The females of placental mammals exhibit cyclical changes in the activities of ovaries and accessory ducts as well as hormones during the reproductive phase. And devided into two groups :-                      In non-primate mammals like cows, sheep ,rats, deers ,dogs ,tiger ,etc., such cyclical changes during reproduction are called oestrus cycle where as in primates(monkeys, apes ,and humans) it is called menstrual cycle.                                                                                                                              

Mammals those breeders only during favourable seasons in their reproductive phase are called  seasonal breeders. Example frog , snakes ,lizards.                                                                          Mammals those  are reproductively active throughout their reproductive phase are called continuous breeders. Example monkry , apes , human beings.                                                                                   
In both plants and animals, hormones are responsible for the transitions between the three phases. Interaction between hormones and certain environmental factors regulate the reproductive processes and the associated behavioural expressions of organisms.

Events in sexual reproduction :                                                                                                        Sexual reproduction is characterised by the fusion (or fertilisation ) of the male and female gametes and  the formation of zygote and embryogenesis.                                                                                  Sequential events may be grouped into three distinct stages :                                                              

Pre-fertilisation                                                                                                                                      Fertilisation                                                                                                                                              Post-fertilisation 

The two main pre-fertilisation events are gametogenesis and gamete transfer.          Gametogenes is refers to the process of formation of the two types of gametes – male and female. Gametes are haploid cells. In some algae the two gametes are so similar in appearance that it is not possible to categorise them into male and female gametes are called homogametes (isogametes).          In sexually reproducing organisms the gametes produced are of two morphologically distinct types  are called heterogametes. In such organisms the male gamete is called the antherozoid or sperm and the female gamete is called the egg or ovum.                                                                                  

Plants  having both male and female reproductive structures in the same plant are called bisexual plant. Plants having both male and female reproductive structures on different plants are called unisexual plant . In fungi and plants terms homothallic and monoecious are used to denote the bisexual condition and heterothallic and dioecious are used to describe unisexual condition.                  

In flowering plants the unisexual male flower is staminate bearing stamens while the female is pistillate or bearing pistils.                                                                                                                      
Flowering plants in which both male and female flowers may be present on the same individual are called monoecious plant. Some examples of monoecious plants are cucurbits and coconuts.                Flowering plants in which both male and female flowers may be present on separate individuals  are called dioecious. Some examples of dioecious plants are papaya and date palm.                              

Animals in which individuals of a species is either male or female are called unisexual . Cockroach is an example of a unisexual species. Animals in which individuals of a  species which possess both the reproductive organs are called bisexual. Earthworms, sponge, tapeworm and leech, typical examples of bisexual animals that possess both male and female reproductive organs, are  hermaphrodites. 

Cell division during gamete formation:                                                                                        Gametes in all heterogametic species are of two types namely ,male and female. Gametes are haploid though the parent plant body from which they arise may bee it her haploid or diploid. A haploid parent produces gametes by mitotic division. Organisms belonging to monera, fungi, algae and bryophytes have haploid plant body. But organisms belonging to pteridophytes ,gymnosperms, angiosperms and most of the animals including human beings, the parental body is diploid.                                             

In meiosis the reduction division has to be occur if a diploid body has to produce haploid gametes. In diploid organisms specialised cells called meiocytes (gamete mother cell) undergo meiosis. At the end of meiosis, only one set of chromosomes gets incorporated in to each gamete.

Gamete Transfer                                                                                                                                 
After their formation, male and female gametes must be physically brought together to facilitate fusion (fertilisation). In majority of organisms male gamete is motile and the female gamete is stationary. Exceptions are a few fungi and algae in which both types of gametes are motile .There is a need for a medium through which the male gametes move. Plants like algae, bryophytes and pteridophytes, water is the medium through which this gamete transfer takes place. Large number of the male gametes however, fail to reach the female gametes. To compensate this loss of male gametes during transport, the number of male gametes produced is several thousand times the number of female gametes produced.                                                                                                                                                      
In seed plants pollen grains are the carriers of male gametes and ovule have the egg. Pollen grains produced in anthers have to be transferred to the stigma before it can lead to fertilisation.                        

In bisexual self-fertilising plants e. g., peas transfer of pollen grains to the stigma is relatively easy as anthers and stigma are located close to each other; pollen grains soon after they are shed, come in contact with the stigma. In cross pollinating plants (including dioecious plants), a specialised event called pollination facilitates transfer of pollen grains to the stigma. Pollen grains germinate on the stigma and the pollen tubes carrying the male gametes reach the ovule and discharge male gametes near the egg.  In dioecious plant male and female gametes are formed in different individuals.

Fertilisation                                                                                                                                                  The most vital event of sexual reproduction is the fusion of gametes. This process is called as syngamy (fertilisation) results in the formation of a diploid zygote. 

Some organisms like rotifers ,honeybees and even some lizards and birds (turkey), the female gamete undergoes development to form new organisms without fertilisation. This phenomenon is called parthenogenesis.                                    

In most aquatic organisms  majority of algae and fishes as well as amphibians, syngamy occurs in the external medium (water) ,i.e. ,outside the body of the organism. This type of gametic fusion is called external fertilisation.                                                                                                                                   
In the bony fishes and frogs where a large number of offspring are produced. A major disadvantage is that the offspring are extremely vulnerable to predators threatening their survival up to adulthood.        Terrestrial organisms  such as fungi, higher animals  reptiles like birds, mammals and  majority of plants (bryophytes ,pteridophytes, gymnosperms and angiosperms) ,syngamy occurs inside the body of the organism this process is called internal fertilisation. In all these organisms, egg is formed inside the female body where they fuse with the male gamete. Organisms exhibiting internal fertilisation, the male gamete is motile and has to reach the egg in order to fuse with it. In these even though the number of sperms produced is very large, there is a significant reduction in the number of eggs produced. In seed plants, however, the non-motile male gametes are carried to female gamete by pollen tubes.

Post-fertilisation Events                                                                                                                  Formation of the diploid zygote is universal in all sexually reproducing organisms. Organisms with external fertilisation zygote is formed in the external medium (usually water),where as in those exhibiting internal fertilisation, zygote is formed inside the body of the organism.                            

Development of the zygote depends on the type of life cycle the organism has and the environment it is exposed to. In organisms belonging to fungi and algae ,zygote develops a thick wall that is resistant to dessication and damage. It undergoes a period of rest before germination.  In organisms with haplontic life cycle  zygote divides by meiosis to form haploid spores that grow into haploid individuals.   

Zygote is the vital link that ensures continuity of species between organisms of one generation and the next.                                                                                                                                          

Embryogenesis refers to the process of development of embryo from the zygote. During embryogenesis zygote undergoes cell division (mitosis) and cell differentiation. While cell divisions increase the number of cells in the developing embryo; cell differentiation helps groups of cells to undergo certain modifications to form specialised tissues and organs to form an organism.          

Animals are categorised into oviparous and viviparous based on whether the development of the zygote take place outside the body of the female parent or inside, i. e .,whether they lay fertilised/ unfertilised eggs or give birth to young ones.                                                                                            
In oviparous animals like reptiles and birds, the fertilised eggs covered by hard calcareous shell are laid in a safe place in the environment; after a period of incubation young ones hatch out. In viviparous animals (majority of mammals including human beings), the zygote develops into a young one inside the body of the female organism. After attaining a certain stage of growth, the young ones are delivered out of the body of the female organism. Because of proper embryonic care and protection, the chances of survival of young ones is greater in viviparous organisms.

In flowering plants, the zygote is formed inside the ovule. After fertilisation the sepals, petals and stamens of the flower wither and fall off. Can you name a plant in which the sepals remain attached? The pistil however, remains attached to the plant.  The zygote develops  into the embryo and the ovules develop into the seed. The ovary develops into the fruit which develops a thick wall called pericarp that is protective in function.