REPRODUCTION IN ORGANISMS

 REPRODUCTION IN ORGANISMS

 Life Span
 Period from birth till natural death.
Every organism live only for a certain period of time. Reproduction –Producing young-ones of their kind, generation after generation. Types of reproduction: Asexual reproduction :single parent capable of producing offspring. 

Sexual reproduction : two parents are involved in producing offspring.

Modes of asexual reproduction

 Binary fission: parent body divides into two halves, genetically identical to parent.
Amoeba: It is simple or irregular. Paramecium: Transverse binary fission.

 Multiple fission: parent body divides into many daughter organisms : Plasmodium.

 Budding: daughter organisms grow from small buds arising in parent body.
Exogenous budding: out side the body eg. Hydra, Yeast. 
Budding in Yeast Endogenous budding : inside the body eg. Gemmule in sponge.
 Conidia: non-motile, exogenous spores in chains eg. Fungi.
 Zoospores: microscopic motile structures eg. Algae. 
In plants : term vegetative reproduction frequently used instead of asexual reproduction, units of vegetative propagation called vegetative propagules.
 All organisms show remarkable similarity. Vast difference in their reproductive structure. Similar pattern or phases in their life cycles.
 Juvenile phase: The phase of growth before reproductive maturity.
 Reproductive phase: Reproductive maturity.
 Senescent phase: Phase between reproductive maturity and death.
 The main events of sexual cycle are:
i. Prefertilisation events: a.Gametogenesis :

 The process of formation of male and female gametes by meiosis (cell-division).
 Homogamete (Isogamete): - gametes similar eg. Algae
 Heterogamete(an-isogamete): - morphologically dissimilar gamete ,male gamete (antherozoid or sperm) ,female gamete (egg or ovum) eg. Human.

 Sexuality in organisms : In plants Bisexual term is used for 
Homothallic and Monoecious plants
Both male and female reproductive structures in same plant 
eg. Higher plants, cucurbits and coconut.
 Unisexual term used for Heterothallic and Dioecious plants
Male and female reproductive-structure on different plants. Flowering plants – male flower–stamina flower and female flower–pistillate flower eg. papaya and date-palm.



Animals – 

Bisexual term is used for Hermaphrodite animals-eg. Earth-worm, Tape-worm, Leech, Sponge. 
Uni sexual animals have male & female sexes in separate individuals-
e.g. insects, frogs, human beings

Cell division during gamete formation: 
Haploid-parent (n) produces haploid gametes (n) by mitotic division, eg. Monera,fungi, algae and bryophytes. Diploid parent (2n) produces haploid gametes(n) by meiosis division (possess only one set of chromosomes )and such specialized parent cell is called 
meiocyte or gamete mother cell.

b) Gamete transfer:- to facilitate fusion.

 Male gametes mostly motile and female non-motile, exception few fungi and in algae both gametes are motile in some cases

 Water medium for gamete transfer- in lower plants. Large number of male gametes produced to compensate loss

 Higher plants, pollen-grains are transferred by pollination.

 Fertilization: Fusion of male and female gametes diploid zygote.

 Parthenogenesis.-development into new organism without fertilisation eg. Rotifers, honey-bees, some lizard, bird(turkey).


Fertilisation Two types- external and internal . 

External fertilisation- outside the body of organism in external- medium (water) eg. majority of algae, fishes, amphibians. 

Advantage- show great synchrony between the sexes –

1. Release of large number of gametes into surrounding medium 
2. Large number of offspring produced.

 Disadvantage- offspring vulnerable to predators, natural disasters. 

Internal fertilisation- fusion occurs inside female body eg. majority of plants and animals. Egg non-motile and formed inside female body. Male gamete motile, produced in large numbers to reach egg and fuse with it. In seed plants, non- motile male gamete carried to female gamete by pollen-tube.


Post -fertilisation events- formation of zygote.

a. Zygote.One celled , diploid, vital link between two generations 

External fertilization –zygote formed in external medium water eg. Frog, 

Internal fertilization –zygote formed inside the body eg. Human beings. Development of zygote depends on type of life cycle and environment. Some develop thick wall 
( prevent damage and desiccation) & undergo period of rest eg. Algae, fungi. Haplontic life cycle- zygote (2n) divides by meiosis to form haploid (n) spores. 

Diplontic life-cycle- zygote (2n) divides mitotically, develops into embryo (2n). Oviparous animals lay eggs out-side the female body. 
Eggs can be fertilized/ unfertilized. Fertilized eggs covered which hard calcareous shell, laid in safe place in the environment. Unfertilised eggs laid in water. Example- fishes, frogs, reptiles, birds Viviparous animals bear and rear the embryo inside female body, give birth to young-ones. Advantage- proper embryonic care, protection, survival chances of young-ones greater. Example- cows, whales, human beings 
Embryogenesis: development of embryo from zygote by cell division (mitosis) and cell differentiation.
 Cell- division increases the number of cells in the developing embryo
Cell differentiation - groups of cells undergo certain modifications for the formation of different kinds of tissues and organs. In flowering plants- zygote formed inside ovule  
Parthenogenesis: Female gamete develops into new organism. Seedless fruits formed by parthenogenesis Clone: A group of individuals of the same species that are morphologically and genetically similar to each other & their parents

Very short answer type(1 mark)

1- What is meiocyte?
Ans: Specialized cells in diploid organism, i.e., gamete mother cell which undergo meiosis.

2- Name the kind of reproduction in bees by which drones are produced?
Ans: Parthenogenesis.

3- What is special in flowering bamboo?
Ans: Bamboo species flower only once in their life-times generally after 50-100 years.

4- What is meant by homothallic?
Ans: The term homothallic refers to bisexual or hermaphrodite condition.

5- Why are the date palms referred to as dioecious ?
Ans: In date-palms, the male and female flowers are present in different plants.

6- If the meiocyte of an onion plant contains 32 chromosomes, work out the number of
chromosome in the endosperm and embryo? 
Ans: Hint: endosperm is triploid.

7- Name two acellular organisms which reproduces sexually.
Ans: Paramoecium, Plasmodium



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