Reproduction in Organisms
1. The time period for
which an organism is functional, from birth to natural death, is called its lifespan. During its lifespan, an
organism grows, develops, attains maturity and gives rise to new individuals of
the same type.
This process of
making new individuals is called reproduction.
2. Reproduction:
It is a biological process in which
an organism gives rise to young ones (offspring) similar to itself. It involves
passing the genetic material to the next generation and ensures the continuation of
species.
3. Reproduction is of two types:
i.
Asexual reproduction occurs in organisms of relatively
simple organisations, e.g., Fungi, algae and a few invertebrates.
It is
characterised by
a)
Involvement
of only one parent.
b)
No
formation or fusion of gametes.
c)
Offsprings
are identical to the parent cell and also to each other.
Genetically
and morphologically identical offspring, produced by the parent through asexual
reproduction is referred to as clones.
ii.
Sexual reproduction involves the fusion of male and
female gametes.
4. Asexual Reproduction in
Protistan and Moneran
 |
Asexual Reproductive Structure- (a) Zoospores of Chlamydomonas (b) Conidia of Penicillium (c) Buds in Hydra (d) Gemmules in Sponge |
- Fission: Parent cell makes one or more copies
of its DNA by mitosis and then undergoes cytokine (cytoplasmic division) to
make two or more daughter cells.
-
Binary fission: Parent cell divides into two halves
and each half grows into a and new individual, e.g., Bacteria Amoeba.
- Multiple fission: Parent cell makes multiple copies of DNA and then many daughter cells, e.g., Amoeba, under unfavourable conditions makes multiple spores inside the parent cell and makes a protective covering around the parent body. Only under favourable conditions, these spores will come out and grow into new amoebae.
- Budding: Nucleus makes a copy and cell
divide unequally to produce a small projection (bud) on aside. This
eventually gets separated and grows cell, e.g., Yeast.
5. Asexual Reproductive as new yeast Structures in Fungi and Simple
Plants (algae)
Zoospores are flagellated, motile spores produced inside a parent cell called zoosporangia. Parent nucleus divides repeatedly to form many daughter nuclei that later develop a cell membrane around them. The parent cell habitat, bursts open to release zoospores in the aquatic e.g., In fungi, Phycomycetes and algae Chlamydomonas.
Conidia are non-motile spores produced
singly or in chains by a constriction at the tip or laterally on special hyphal
branches called conidiophores. These are exogenous spores that disperse by wind
and germinate habitat by producing germ tubes, in suitable e.g., Penicillium.
Buds form as a small outgrowth on the body
of the parent containing the copy of the parental nucleus. It may split away
from the parent body and grow up as a new individual, e.g., Hydra.
Gemmules are endogenous buds that are simply
a copy of the parent nucleus, with a small amount of cytoplasm and protective
covering around. During favourable conditions, each bud comes out and grows as
an individual, e.g., Freshwater sponges.
6. Sexual Reproduction Generally Involves:
- Two
prospective parents.
- Gamete
formation by meiosis.
- Fusion
of gametes (fertilisation).
- Genetic and morphological variation of offsprings from the parents.
Gamete fusion leads
to the formation of the zygote that is the first cell of the new individual. Most of
the higher animals and plants reproduce sexually.
7. Life Cycle of an Organism Involves Three Phases:
- Juvenile phase: The stage of growth and maturity in
life before reproducing sexually.
- Reproductive phase: The stage where one can reproduce
sexually as is able to give birth to offsprings.
- Senescent phase: The end of reproductive phase till
death. If refers to old age.
8. Events of Sexual Reproduction are:
I. Pre-fertilisation: Gametogenesis or formation of
gametes.
ii.
Fertilisation: Syngamy or fusion of gametes to
form zygote.
iii.
Post-fertilisation: Embryogenesis through division and
differentiation of the zygote.
9. Bisexual or Unisexual Organisms:
Bisexual animals have both male and female reproductive organs in the same individual, e.g., earthworm, tapeworm, leech, sponge, etc. They are also called hermaphrodites.
Unisexual animals have either male or female organs in one individual, e.g., humans. reproductive
Bisexual plants have both male (stamen) and female (pistil) parts on the same flower, e.g., in rose, lily, jasmine, sweet peas, etc. They are also called homothallic or monoecious.
Unisexual plants have
flowers with only male or only female parts. Unisexual male flower will have
stamens and unisexual female flower will have only pistil. They are also called
heterothallic or dioecious.
10.Gametes:
Types of Gametes
(a) Isogametes of Cladophora (an alga)(b) Heterogametes of Fucus (an alga)(c) Heterogametes of Homo sapiens (human beings)
These
are the cells that make sexual reproduction possible. The male gamete is known as sperm or antherozoids. The female gamete is known as an egg or ovum.
- If
male and female gametes are similar in appearance, they are called homogametes or isogametes.
- If
male and female gametes are different in appearance, they are called heterogametes.
- In
most of the sexually reproducing species including humans, gametes look
drastically different. The female gamete is much bigger in size and non-motile,
whereas male gamete is very tiny and is generally motile.
11. Cell Division During Gametogenesis:
Gametes are produced by meiotic cell division of the diploid mother gamete cell (meiocyte 2n) to reduce the chromosome number to half. This means that each gamete cell has only one set of chromosomes (n) which are not paired any more.
- In haploid organisms, gametes are produced by mitosis as the chromosomes already exist as a single, non-paired set.
- Transfer of male gamete is an important event in sexual reproduction and different ways are used by the organisms to accomplish this.
- Biotic ways of gamete transfer make use of animals, whereas abiotic ways are those that employ water or wind.
12. Fertilisation:
Fusion between male and female gametes is called syngamy or fertilisation. This results in the formation of a diploid zygote that will further develop and differentiate to form the new individual.
13. Types of Fertilisation
Syngamy may occur inside the female body
(internal fertilisation) that ensures safe development or outside the female
body (external fertilisation). This makes the embryo vulnerable to predators or
changing environmental conditions.
Parthenogenesis is a phenomenon, where female gamete
undergoes development to form new organisms without fertilisation with a male
gamete. It is a way by which a female alone can produce offspring without the
need for a male. In complex animals and plants it's a way to clone themselves.
Its disadvantage is low genetic diversity and susceptibility to harmful
mutations that can persist through generations.
14. Embryogenes:
It is
the process, wherein the zygote divides repeatedly and differentiates to form
embryo, eventually resulting in a new organism.
Changes After Fertilisation
In flowering plants-
a.
the
ovary develops into fruit.
b.
ovules
mature into seeds.
The embryo is
present inside the mature seed that will germinate to give rise to the new
plant, under proper environmental conditions.
15. Oviparous:
Oviparous animals
lay eggs, e.g., reptiles and birds. Eggs are covered by a hard calcareous
shell.
16. Viviparous
Viviparous animals give birth to young, e.g., majority of mammals. In flowering plants, zygote is formed inside the ovule. The zygote develops into the embryo and the ovules develop into the seed.
