LIFE SPAN: Life span is defined as the period from birth to natural death of an organism.

  • It is not correlated with the size and body complexity of the organism. (i.e., Crow and parrot are of almost same size but their life span is drastically different, Crow = 15 years & Parrot = 140 years)
  • Death of every individual organism is certainty, i.e., no individual is immortal. (Exception: single-celled organisms).

Reproduction : It is the biological process in which an organism gives rise to offsprings similar to itself.

  • It enables the continuity of the species.
  • Every organism has evolved its own reproducing mechanism.
  • Many factors collectively decide the reproduction process of the organism such as its habitat, its physiology etc.

Types of reproduction:
(On the basis of presence & absence of gametes or participation of one or two organism)

  1. Asexual reproduction
  2. Sexual reproduction

Comparison Between Asexual & Sexual reproduction

Asexual Reproduction

  • A mode of reproduction in which offsprings are produced by a single parent without the formation of gametes.
  •  Morphologically & genetically similar offsprings i.e., all the progeny are exact copies of each other and their parents.
  • This mode of reproduction is common among single celled organisms and plants having lower body complexity.
  • It is also known as somatogenic reproduction.

TYPES OF ASEXUAL REPRODUCTION

  1. Binary fission
  2. Budding
  3. Fragmentation
  4. Regeneration
  5. Special reproductive structure
  6. Vegetative reproduction

BINARY FISSION

  • Simple cell division
  •  A single cell divides into two halves and each rapidly grows to give rise to two new individuals.
  •  This mode of reproduction can be seen in Amoeba, Paramoecium, Monera, Protists etc.
Binary fission in Amoeba

BUDDING

  • In this method of reproduction unequal small buds are produced attached to the parent organism which later grows and gets separated to form a new organism.
  •  It is a type of unequal division.
  •  This mode of reproduction can be seen in Yeasts (fungus) & Hydra.
A bud of Hydra
(Diagrammatic representation of the process of budding in Hydra)
Budding in Yeast

FRAGMENTATION

  • The organisms simply break up into smaller pieces or fragments which gradually grow into new individuals.
  • Ex-Spirogyra (algae)
Fragmentation in Spirogyra (Algae)

REGENERATION

  • Some organisms have the ability to give rise to new individual organisms from their body parts.
  • If they’re somehow cut or broken all the pieces regenerate into new organism.
  • Ex- Hydra, Planaria, Starfish, etc.

SPECIAL REPRODUCTIVE STRUCTURES

  1. Sporulation: In some organisms during favourable conditions the formation of zoospore takes place to reproduce asexually.
    The protoplast of the cell divides to form pyramid shaped, anteriorly bi-flagellated zoospores.
    In the later stages the parent cell wall breaks down to liberate these zoospores into water where they get enlarged to become adult.
    Ex- Chlamydomonas, Ulothrix

2. Conidia formation
Conidia (a specialised reproductive structure) are produced exogenously at the tip of branched or unbranched conidiophores.
Ex- Penicillium (blue moulds), Aspergillus (black moulds)

3. Gemma cup
A single cell or mass of modified cells that detaches from parents and develop into a new individual.
Ex- Marchentia

VEGETATIVE PROPAGATION

A type of asexual reproduction in which new plants are produced from vegetative parts of plants without the fusion of gametes.

  • This mode of asexual reproduction is observed in higher plants.
  • Vegetative propagation is carried out by two means
    a. Natural: by stem, root, leaf, bulbil etc.
    b. Artificial: by cutting,layering, grafting, tissue culture etc

Means of vegetative propagation

  1. Roots (in sweet potato, Dahlia, Asparagus etc)
  2. Underground stem
    a. Rhizome (in banana, turmeric, ginger etc.)
    b. Bulbs (in onion, garlic etc.)
    c. Tubers (In potato)
  3. Creeping stems (in grasses)
  4. Leaves (in Bryophyllum)

Sexual Reproduction

  • Formation and fusion of gametes to form zygote
  • Zygote develops to form new organism
  • Common among higher plants & organisms
  • Offsprings are not genetically identical to their parents.

CHARACTERISTICS

  • Both uniparental or biparental
  • Offsprings are not identical
  • Involves meiosis & syngamy
  • Slow & complex process

PHASES OF LIFE CYCLE

JUVENILE PHASE: This phase is known as vegetative phase or pre-reproductive phase.

  • In this time period organisms undergo growth to attain reproductive maturity.
  • The duration of juvenile phase varies from organisms to organisms.

REPRODUCTIVE PHASE:

  • In this phase the development and maturation of reproductive organs take place.
  • Production of offspring might be similar or dissimilar to the parental generation
  • Duration varies from organisms to organism.

SENESCENT PHASE

  • It is also known as post reproductive phase.
  • Structural & functional deterioration of body takes place due to accumulation of waste metabolites.
  • This phase ultimately leads to death

Phase transition is dictated by hormones.
Interaction between hormones and environmental factors regulate the reproductive process and associated behavior of an organism.

Events in Sexual Reproduction

  1. Pre-fertilization events
  2. Fertilization
  3. Post-fertilization events

Pre-Fertilization Events

  1. Gametogenesis
  2. Gamete Transfer
  1. GAMETOGENESIS: The process of formation of gametes (both male & female) is referred to as Gametogenesis.

Gametes: Gametes are defined as haploid reproductive cells that fuse during sexual reproduction to form a zygote.


[ Figure: (a) Isogametes of Cladophora (algae), (b & c) Heterogametes in Fucas (algae) & humans]
  • Isogametes/Homogametes à when both the gametes are so similar in appearance that they are not categorized into male or female gamete.
  • Heterogametes à In majority of sexually reproducing the male and female gametes are morphologically distinct types.

In such organisms the male gametes are called antherozoid or sperm and the female gamete is called egg or ovum.

TERMS:

  • Bisexual: Both male & female reproductive structures on the same plants. (Also known as homothallic & monoecious)
  • Unisexual: Male & female reproductive structure on different plant body. (Also known as heterothallic or dioecious)
  • Staminate: Bearing stamen; Unisexual male flower.
  • Pistillate: Bearing pistil; Unisexual female flower.
  • Hermaphrodites: Bisexual animals having both male & female reproductive organs. Ex- Earthworm
This image has an empty alt attribute; its file name is image-26.png

[ Unisexual condition in cockroaches i.e., both male & female reproductive organs in different individual]



[Monoecious condition in Chara, both male and female sex organ in same plant body, homothallic condition]

[Dioecious condition in Marchantia, Both male and female sex organ on different plant body; heterothallic condition]

Cell Division during gamete formation:

  • Gametes are invariably haploid i.e., the parent body producing the gamete might be haploid or diploid but the gametes produced are strictly haploid (contains one set of chromosomes, n)
  • A haploid parent (Monera, fungi, algae and bryophytes) produces gametes by mitotic division whereas in diploid organisms, gametes are formed by meiosis (reductional division). In such organisms meiocytes, or gamete mother cells undergo meiosis to give rise to gametes
  1.  GAMETE TRANSFER:
    After the process of gametogenesis, the male and female gametes need to be brought together to initiate fertilization.

To facilitate the gamete transfer, in most of the organisms (with a few exceptions), male gametes are motile.

However, a medium is needed through which the male gametes move.

  • Zooidogamy : Simple plants like algae, bryophytes and pteridophytes use water as a medium to transfer gametes.
  • Siphonogamy : In seeded plants pollen grains produced in the anthers carry male gametes to the stigma through the pollen tubes.

There is an evident loss of male gametes during the transfer process, to compensate the loss the production of male gametes is thousand manifolds greater than that of the female gametes.

FERTILIZATION

  • Most vital event of the sexual reproduction.
  • Fertilization or syngamy is the fusion of haploid male and female gametes to form a diploid zygote.
  • Zygote is the connecting link between the two organisms.

POST-FERTILIZATION EVENTS

  • Events in the sexual reproduction cycle, after the formation of zygote is summed up as post-fertilization events.
  • It includes the process of embryogenesis i.e., development of an embryo from zygote by cell division and differentiation.
  • In animals, the embryogenesis starts soon after fertilization.
  • In flowering plants, after fertilization the ovary develops into fruits and the ovule develops into seeds. The mature seeds store the embryo inside itself which has the ability to produce new plants.
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