Reproduction in Organisms
Reproduction:
(1) Reproduction is
defined as a biological process in which every living organism give rise to new
organisms similar to themselves.
(2) Basic features of
reproduction:
(i)
Replication of DNA
(ii) Cell division (only mitotic, or both mitotic and meiotic)
(iii)
Formation of reproductive bodies or units.
(iv)
Development of reproductive bodies into offsprings.
Difference between Asexual and
Sexual Reproduction
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S.No.
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Asexual Reproduction
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Sexual Reproduction
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1)
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Offsprings are produced by single parents.
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Involvement of single or two individual.
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2)
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New individual develops from one cell or a vegetative body
part of one parent.
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New individual develops from zygote or fusion product of two
gametes, which may or may not be produced by two parents.
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3)
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Offsprings are genetically identical to their parents.
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The offsprings are not genetically identical with their
parents.
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4)
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Involves only mitotic divisions.
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Involves meiosis at one or the other stage.
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5)
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It does not require the formation of sex organs.
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Formation of sex organs is a pre-requisite for sexual
reproduction.
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6)
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It does not introduce variability. Hence has no evolutionary
importance.
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It introduces variability and is, hence of evolutionary importance.
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Asexual Reproduction
Asexual reproduction in plants
In flowering plants
there are two main types of asexual reproduction:
(a) Agamospermy – The formation of embryo without fertilization and meiotic
division. There are three different types of agamospermy:
(i) Diplospory
(ii) Adventive
embryony
(iii) Apospory
(b) Vegetative propagation –
(i) Plants belonging to this category propagates
by a part of their body other than a seed. The structural unit that is employed
in place of seed for the propagation of new plants is called propagule.
(ii) In angiosperms any parts of the plants –
roots, stemsand leaves can be used for vegetative propagation.
(5) Following are the
asexual reproductive structures:
(a) Zoospores: aquatic
fungi, Chlamydomonas
(b) Conidia: Penicillium
(c) Bud: Hydra
(d) Gemmules: sponges
(6) Below given are
the vegetative propagation units in plant: (Vegetative propagules)
Runner, rhizome,
sucker, tuber, offset, bulb
Sexual reproduction:
Sexual reproduction
involves fusion of male and female gametes by the process of fertilization.
Features of sexual
reproduction:
(a) The period between
birth and sexual maturity is called juvenile phase.
(b) Juvenile phase is
also known as vegetative phase in plant.
(c) Oestrus
cycle: The cyclical changes
during reproduction in non-primate mammals like cows, sheep, rats, deers, dogs,
tiger etc.
(d) Menstrual
cycle: The cyclical changes
during reproduction in primate mammals like monkeys, ape, and humans.
(e) Seasonal breeders: The reproductive cycle takes place only
in favorable seasons as in wild animals.
(f) Continuous
breeders: They are
reproductively active throughout their reproductive phase.
(g) Gametogenesis is the process of gamete formation.
(h) Isogametes are one of a pair of conjugating
gametes, exhibiting no differences in form, size, structure, or sex.
(i) Gametes produced
of two morphologically distinct types are called heterogametes.
(j) Male gamete is
called antherozoid or sperm and the female gamete is
called ovum or egg.
Sexuality in organism:
(a) Plants having only
one sex organ is called heterothallic or dioecious.
(b) Plants having both
male and female sex organ called homothallic or monoecious.
(c) In flowering
plants, the unisexual male flower is staminate (bearing stamens), while
the female is pistillate (bearing pistils).
(d) Animals having one
type of reproductive system are called unisexual.
(e) Animals having
both male and female reproductive system are called hermaphrodite or bisexual.
Cell division during gamete
formation:
(a) Gametes in all
heterogametic species are of two types namely male and female.
(b) Gametes are always
haploid irrespective of parent’s ploidy.
(c) Gametes are
produced by a haploid parent by mitotic division.
(d) Gametes are
produced by a deploid parent by meiotic division.
(e) In diploid
organisms specialized cells called meiocytes undergo meiosis to produce haploid
gametes.
Gamete transfer:
(a) Male and female
gamete must be physically brought together to facilitate fusion called fertilization.
(b) In most cases
female gametes are non-motile , male gametes are motile.
(c) In case of few
fungi and algae, both male and female gametes are motile.
(d) In most cases the
medium for gamete transfer is water.
(e) Number of male
gametes are produced is several thousand times the number of female gametes
produced to compensate the loss during transfer.
Fertilization:
(a) The process of
fusion of male and female gamete is called fertilization or syngamy.
(b) The female gamete
undergoes development to form new organism without fertilization. This
phenomenon is called parthenogenesis. It
is a modification of sexual reproduction.
(c) Gametic fusion
taking place outside the body i.e. water is called external fertilization.
(d) There must be
synchrony of gamete release. Large number of gametes is released to enhance the
chance of fertilization.
(e) A major
disadvantage is that the offsprings are extremely vulnerable to predators.
(f) Fertilization
takes place inside the body is called internal fertilization.
Zygote:
(a) Formation of
zygote after fertilization is found in all sexually reproducing organisms.
(b) In case of external fertilization, zygote is formed
usually in water.
(c) In case of internal fertilization, zygote is formed inside the body of the organism.
(d) Zygote of fungi
and algae develops a thick wall that is resistant to desiccation and damage.
(e) Organism with
haplontic life cycle, zygote undergoes meiosis to produce haploid spores.
Embryogenesis:
(a) The process of
development of zygote into an embryo is called embryogenesis.
(b) Zygote undergoes
cell division (mitosis) and cell differentiation.
(c) Oviparous are the
animals which lay eggs and development takes place inside egg.
(d) Viviparous animals
give birth to the young ones. The development takes place inside the body of
the female.
(e) In plants:
(i) Zygote developed
into embryo.
(ii) Ovule developed
into seed
(iii) Integument of
the ovule developed into seed coat.
(iv) Ovary developed
into fruit.
(v) Ovary wall
developed into pericarp.
