Friday, December 25, 2020

Reproduction in Organisms | Plus 2 Botany | Exam Capsule Notes

REPRODUCTION IN ORGANISMS: 
CHAPTER AT A GLANCE

ASEXUAL REPRODUCTION: TYPES 

a. Fission: Parent cell divides into two or more individuals. E.g. Protists and Monerans. 


Fission is 2 types:
  • Binary fission: Division of parent cell into 2. E.g. Amoeba, Paramecium.
  • Multiple fission: Division of parent cell into many individuals. E.g. Plasmodium, Amoeba.
Under unfavourable condition, Amoeba → encystation (formation of cyst) → favourable conditions → sporulation by multiple fission → pseudopodiospores → cyst wall bursts → spores liberate → many amoebae.

b. Budding: A bud grows in the parent body. After maturation, it is detached from parent body to form new individual. E.g. Hydra, Sponge, Yeast etc. 


c. Fragmentation: The body breaks into fragments and each fragment grows into adult. E.g. Hydra.

d. Vegetative propagation: Production of offspring from vegetative propagules in plants.

    E.g. for vegetative propagules (units of vegetative propagation): 



Other asexual reproductive structures:


SEXUAL REPRODUCTION 

Juvenile phase (vegetative phase in plants): The period of growth to reach in maturity for sexual reproduction.

In higher plants, the flowering indicates the end of vegetative phase.

Annual & biennial plants show clear cut vegetative, reproductive & senescent phases.

Unusual flowering: E.g. Bamboo flowers only once in their lifetime, Strobilanthus kunthiana flowers once in 12 years.

Based on breeding season, mammals are 2 types:
  • Seasonal breeders: exhibit reproductive cycles only during favourable seasons.
  • Continuous breeders: reproductively active throughout their reproductive phase.
Senescence (old age): It is the last phase of lifespan and end of reproductive phase.

EVENTS IN SEXUAL REPRODUCTION 

I. Pre-fertilisation Events (gametogenesis & gamete transfer) 

1. Gametogenesis: Formation of male and female gametes. Gametes are 2 types:
  • Homogametes (isogametes): Similar gametes. E.g. Some algae like Cladophora.
  • Heterogametes: Male & female gametes are different. Male gamete is antherozoid (sperm) and female gamete is egg (ovum). E.g. Fucus (an alga), Human etc.

Sexuality in organisms:

  • Bisexual: Male & female sex organs in same individual.
  • Bisexual plants: E.g. Hibiscus, Pisum.
  • Bisexual animals (hermaphrodites): E.g. Earthworms, leech, sponge, tapeworm, etc.
  • Monoecious: Male & female flowers on same plant. E.g. Cucurbits, coconuts.
  • Unisexual: Male & female reproductive structures in different individuals.
  • Dioecious: Male & female flowers on different plants. E.g. papaya & date palm.
  • Fungi are homothallic (bisexual) or heterothallic (unisexual).
Cell division during gamete formation:

Many monerans, fungi, algae & bryophytes have haploid parental body. They produce haploid gametes by mitosis.

Pteridophytes, gymnosperms, angiosperms & animals have diploid parental body. They produce haploid gametes by meiosis of meiocytes (gamete mother cell).

2. Gamete Transfer:

Generally male gamete is motile and female gamete is stationary. In some fungi & algae, both are motile.

In simple plants (algae, bryophytes & pteridophytes), gamete transfer takes place through water medium. To compensate loss of male gametes during transport, large number of male gametes is produced.

In cross pollinating plants, pollination helps in transfer of pollen grains.

II. Fertilisation (syngamy) 

It is the fusion of gametes to form a diploid zygote.

In rotifers, honeybees, some lizards, birds (turkey) etc., female gamete develops to new organisms without fertilisation. This is called parthenogenesis.

Types of fertilization:

a. External fertilisation: 
  • Occurs in the external medium (water). E.g. many algae, bony fishes & amphibians. 
  • Disadvantage: Offspring are extremely vulnerable to predators.
b. Internal fertilisation: 
  • Occurs inside the body. E.g. terrestrial organisms, belonging to fungi, animals & plants. 
  • In this, non-motile egg is formed inside the female body to where motile male gamete reaches and fuses.
III. Post-fertilisation Events 

Zygote
  • Vital link between two generations.
  • Sexually reproducing organisms begin life as a zygote.
  • In fungi & algae, zygote forms a thick wall to resist damage & desiccation. It undergoes a rest before germination.
  • In haplontic life cycle, zygote divides by meiosis into haploid spores that grow into haploid individuals.
Embryogenesis
  • It is the development of embryo from zygote.
  • Zygote undergoes mitosis and cell differentiation.
  • Cell divisions increase the number of cells in the embryo.
  • Cell differentiation causes the modifications of cells into various tissues and organs to form an organism.
    Based on place of zygote development, animals are 2 types:
  • Oviparous: Animals lay fertilized/unfertilized eggs. E.g. In reptiles & birds, fertilized eggs with calcareous shell are laid. After incubation, young ones hatch out.
  • Viviparous: Zygote develops to young one inside female body. Later, young ones are delivered out. E.g. mammals.
It shows proper care & protection. So young ones have greater chances of survival. 

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