What is reproduction
Reproduction is the ability of living organism to produce a new generation of living individuals similar to themselves.
Basic features of reproduction:
All organism reproduce.Modes of reproduction vary in different organism.However, all modes have certain common basic features.These are
(i)Reproduction of DNA.This is the molecular basis of reproduction.
(ii)Cell division,only mitotic,or both mitotic and meiotic.This is cytological basis of reproduction.
(iii)Formation of reproduction bodies or units.
(iv)Development of reproduction bodies into offspring.
Types of Reproduction:
These are two main types:
- Asexual reproduction (Non-gametic)
- Sexual reproduction (gametic)
Production of offspring by a single parent without the formation and fusion of gametes is called asexual reproduction.The young one receives all its genes from one parent.
Asexual reproduction is also known as agamogenesis or agamogeny. It involves only mitotic cell division,and also termed somatogenic reproduction. Asexual reproduction produces identical offspring commonly referred to as a clone. Today,the scientists have been able to produce clones of multicellular animals(e.g.,boar calf names as Frosty,and Film Dorset lamb named as the famous Dolly)artificially in the laboratory.
Asexual reproduction occurs in protozoans and some lower animals such as sponges,coelenterates,certain worms and tunicates.It is absent among the higher non-vertebrates and all vertebrates.
Types of Asexual reproduction:
- Binary fission:
- Multiple fission:
Binary fission is the division of the parent into two small,nearly equalized daughter individuals.During binary fission nuclear division or karyokinesis is always followed by division of cytoplasm or cytokinesis Examples:-Protozoans(Amoeba,Euglena etc) bacteria,and Planarians.
- Simple binary fission
- Transverse binary fission
- Longitudinal binary fission
Multiple fission is the division of the parent into many small daughter individuals simultaneously.Examples-Multiple fission occurs in many protozoans such as Plasmodium, Amoeba and Monocystis,Foraminifera.
It is the production of offspring usually by two parents,male and female. Involving four processes:
- Formation of special haploid cells,the gametes,by meiosis(Gametogenesis).
- Fusion of the gametes in pairs,forming diploid cells,the zygotes(Fertilization).
- Repeated mitotic divisions of zygotes to form embryos(Embryogenesis).
- Growth of Embryos into a new individual(Development).
- Sexual reproduction is also called syngenesis.
Sexual reproduction occurs nearly in all animals,including those which reproduce asexually.Some protozoans,such as Amoeba,Euglena lack sexual reproduction. In male and female,and the difference between them is determined genetically. In sexual reproduction offsprings resemble the parent.
Sexual reproduction is of two main types:-
It involves the complete and permanent fusion of two gametes from different or from the same parent to form a composite cell,the zygote.It is further of two kinds:
- Anisogamy or Heterogamy
It involves the fusion of two entire gametes to form a zygote. The fusion nucleus of zygote called synkaryon.It is further of two types with regard to the source of fusing gametes:
It involves self-fertilization.i.e.,the fusion of two gametes of the same parent.It is, thus uniparental.e.g.,Taenia.
It involves cross-fertilization.i.e.,the fusion of two gametes formed by different parents.It is,thus,biparental.e.g.,frog, rabbit and man.Syngamy is also of two kinds:
The fusing gametes are similar morphologically as in Monocystis(a protozoans). Such gametes are known as isogametes,and their fusion is termed isogamy. Although the isogametes are similar in structure,they have behaviorural differences.e.g., foraminifera,monocystis.
(ii)Anisogamy or heterogamy:
The fusing gametes are different in form,size,structure and behaviour as in frog and humans.Such gametes are known as anisogametes, or heterogametes,and their fusion is termed anisogamy,or heterogamy. e.g.,Plasmodium,vertebrates.
Some acellular protist animals(e.g.,Paramecium) exhibit sexual reproduction by forming male and female gamete nuclei,which they exchange through temporary cytoplasmic bridge; later,the cytoplasmic bridge disappears and the gamete nucleus of one individual fuses with that of the other to form zygote nuclei.This mode of sexual reproduction is known as conjugation.
Reproductive unit in sexual reproduction:
The reproductive units in sexual reproduction are specialised cells called gametes. The gametes are generally of two kinds:-
- Microgametes or Spermatozoa
- Macrogametes or Ova
Both are well developed for their role in reproduction.The male gametes are mostly minute and motile so that they may swim to the female gametes for fertilization.The female gametes are usually large,non motileand often have a store of food to nourish the developing embryo.
Maintenance of chromosome number:
The gametes are usually formed by meiotic divisions.Therefore,they are haploid.i.e.,have halved or reduced(n) number of chromosomes. In sexual reproduction.the male and female gametes fuse to form a single cell,the zygote formed by fusion of two haploid gametes in naturally diploid,i.e.,has double or normal number(2n) of chromosomes.