Check Short Notes: Reproduction in Organisms from Biology section on e akhabaar
- Reproduction is the biological process in which an individual gives rise to an offspring similar to itself.
Types of Reproduction:
Based on whether there is one or two organisms taking part in the process of reproduction
- ASEXUAL REPRODUCTION
- SEXUAL REPRODUCTION
Asexual Reproduction:
- Usually followed by organisms with relatively simpler organizations.
- Off springs produced by single parent.
- With/without involvement of gamete formation.
- Off springs produced are genetically and morphologically similar to each other and to the parent, i.e. they are clones.
Modes of asexual reproduction:
1.Binary fission
Parent cell divides into 2 daughter cells.
E.g.. Amoeba, Paramecium
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2.BUDDING
Parent cell divides to form large cell and small cell, called bud, attached to the large cell; the bud gets separated and grows into an adult. E.g., Yeast
- In fungi and algae specialized asexual reproductive units are formed
Zoospores in many algae and certain aquatic fungi, Conidia (Penicillium), Buds (Hydra), Gemmules (Sponge).
- In higher plants, vegetative propagation is followed using units such as rhizome, sucker, offset, bulb, tuber etc. – known as vegetative propagules.
Examples of Vegetative Propagation:
WATER HYACINTH (Terror of Bengal)
- One of the most invasive weeds
- Grows wherever there is standing water
- Drains oxygen from water- leads to death of fishes.
- Introduced in India because of its pretty flowers & shape of leaves
- Vegetative propagation occurs at a phenomenal rate
II SEXUAL REPRODUCTION:
- Involves formation of male and female gamete by two individuals of the opposite sex.
- Offspring produced by fusion of male and female gametes not identical to each other or to the parents.
- All sexually reproducing organisms share a similar pattern of reproduction.
DIFFERENT PHASES IN SEXUAL REPRODUCTION:
1. JUVENILE PHASE
It is a period of growth and maturity of an organism before it can reproduce sexually.
2. REPRODUCTIVE PHASE
Period in which organism is capable of reproducing sexually
- Easily seen in higher plants when they come to flower.
- Some plants flower seasonally & some through out the year
- Few plants exhibit unusual flowering phenomenon
For e.g.. 1. Bamboo species flower only once in their lifetime. 2. Srobilanthus kunthiana (neelakurinji) found in hilly areas of Kerala, Karnataka & Tamil Nadu flower once in 12 years the latest being in September 2006.
- In animals sexual reproduction is usually seasonal
- In placental mammals there is occurrence of cyclical changes in the activities of ovaries & accessory ducts as well as hormones.
- In non primate mammals like dogs, rats etc these cyclical changes during reproduction is called oestrus cycle – no bleeding occurs.
- In primate mammals like apes & humans, these cycles are called menstrual cycles.
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3. SENESCENT PHASE
- It is the end of reproductive phase.
- Old age ultimately leads to death
EVENTS IN SEXUAL REPRODUCTION:
1. PRE-FERTILIZATION EVENTS:
a) GAMETOGENESIS
- The gametes are usually haploid
- Gametes called homogametes / isogametes when both have same appearance.
- When gametes are different, they are called heterogametes; male gamete known as antherozoid /sperm, female is called ovum / egg.
- Bisexual organisms called as homothallic / monoecious whereas unisexual organisms called heterothallic / dioecious.
b) GAMETE TRANSFER
- In most of the organisms, male gametes are motile & female gametes are stationary.
- In algae, bryophytes & pteridophytes water is the medium for gamete transfer.
- Pollination is the method of gamete transfer in higher plants as pollen grains are carriers of male gametes.
2. SYNGAMY / FERTILIZATION:
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- Syngamy results in formation of a diploid zygote.
- In certain animals, female gamete develops into the adult organism without fertilization; such a process is called parthenogenesis – seen in rotifers, lizards turkeys etc.
- Fertilisation may be external or internal
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EXTERNAL FERTILIZATION Large numbers of gametes are released in the surrounding medium. |
INTERNAL FERTILIZATION Numbers of ova produced are less, but large numbers of male gametes are released and they travel towards the ovum. |
3. POST FERTILIZATION:
- Events in sexual reproduction after the formation of zygote
- Zygote – vital link that ensures continuity of species between organisms of one generation and the next.
- Development of zygote depends on
– the type of life cycle of the organisms.
– the environment it is exposed to.
- Zygote develops into embryo.
Embryogenesis involves:
– cell division to increase the number of cells
– cell differentiation for the formation of different kinds of tissues
- Based on whether the development of zygote occurs inside or outside the body of the female parent, animals are categorized into oviparous and viviparous.
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OVIPAROUS These animals lay eggs. The fertilized eggs have a calcareous shell to protect them from harsh environment. |
VIVIPAROUS These animals give birth to young ones Embryo protected inside the mother’s body. |
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